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Tectonostratigraphic history ofthe southern Foothills terrane.
Item Type text; Dissertation-Reproduction (electronic)
Authors Newton, Maury Claiborne, III.
Publisher The University of Arizona.
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Order Number 9028151
Tectonostratigraphic history of the southern Foothills terrane
Newton, Maury Claiborne, III, Ph.D.
The University of Arizona., 1990
Copyright ®1990 by Newton, Maury Claiborne, III. All rights reserved.
U·M·I 300 N. Zeeb Rd. Ann Arbor, MI 48106
..•. ;
-----~--------.. - .. _ ..... _ .......... - .. -•....... _-._---_. __ ..
TECTONOSTRATIGRAPHIC HISTORY OF
THE SOUTHERN FOOTHILLS TERRANE
by
Maury Claiborne Newton, III
Copyright@ Maury Claiborne Newton, III 1990
A Dissertation Submitted to the Faculty of the DEPARTMENT OF GEOSCIENCES
In Partial Fulfillment of the Requirements For the Degree of
DOCTOR OF PHILOSOPHY In the Graduate College
THE UNIVERSITY OF ARIZONA
199 0
1
THE UNIVERSITY OF ARIZONA GRADUATE COLLEGE
As members of the Final Examination Committee,
we certify that we have read the dissertation
prepared by Maury Claiborne Newton. III
entitled TECTONOSTRATIGRAPHIC HISTORY OF THE SOUTHERN
FOOTHILLS TERRANE
and recommend that it be accepted as fulfilling the disser-
tat ion requirement for the Degree of Doctor of Philosophy
.-£. Qr4 2,.A I 1'110
Date
A.H ~. \~
George E. Date
2
Final approval and acceptance of this dissertation is contingent upon the candidate's submission of the final copy of the dissertation to the Graduate College.
Christopher J. Eastoe
Spencer R. Titley
---.... _.--_ .. _-,..- .•..... - ., .. , .,.-.", ~ -....... __ .... __ .
.;
3
STATEMENT BY AUTHOR
This dissertation has been submitted in partial fulfillment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under the rules of the Library.
Brief quotations from this dissertation are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the copyright holdeI'.
SIGNED:
4
ACKNOWLEDGEMENTS ,
This dissertation has benefited from reviews by all the committee members, particularly Chris Eastoe. This work was generously supported by NEVCAN Exploration, Inc., through Paul Mattinen. Material support was provided by Westmin Resources, Ltd., through Garfield MacVeigh, by Kennecott Exploration company, through steve Craig, and by The University of Arizona, through Chris Eastoe. Some mapping of the Melones fault zone was performed while in the service of Freeport-McMoran Gold Company and Kennecott Exploration Company, and permission from these companies to present this mapping is greatly appreciated.
The author wishes to thank the following people and agencies for their contributions to this work. Some analyses were performed by and at the expense of the united states Geological Survey, through Robert Earhart. Irradiation for some neutron activation analyses was performed by and at the expense of the TRIGA nuclear reactor facility at The University of Arizona, through George Nelson. Some microprobe analyses were performed by Mae Gustin in the Lunar and Planetary Labora'i:ory at the University of Arizona. Neutron activation analyses were performed in the Lunar and Planetary Laboratory of the University of Arizona, with the. assistance of Delores Hill and Alan Hildebrand. Sample preparation was performed in the Department of Geosciences with equipment administered by Joaquin Ruiz, Jonathan Patchett, and Paul Damon.
Additional financial support for this work was provided by research grants to M.C. Newton, III, from the Geological Society of America, Sigma Xi-The Scientific Research Society, the Graduate College at The University of Arizona, and the Department of Geosciences at The University of Arizona. Stipends were provided by Chevron Corporation and The University of Arizona.
"Ignorance is preferable to error; and he is less remote from the truth who believes nothing, than he who believes what is wrong."
(Thomas Jefferson, 1781, in Notes on the state of virginia)
-----------------------------~.- .. -.... _ ... _ ........ .
5
TABLB 01' CONTBNTS
LIST OF ILLUSTRATIONS •••••••••••••••••••••••••••••••• 9
LIST OF TABLES ....................................... 13
ABSTRACT ••••••••••••••••••••••••••••••••••••••••••••• 14
CHAPTER ONE. The Triangular Element-ratio Diagram: Application to Discriminating Volcanic Rocks from Different Tectonic Settings and Different Differentiation Trends
INTRODUCTION 16
TRACE ELEMENT DISCRIMINATION OF BASIC ROCKS FROM DIFFERENT TECTONIC SETTINGS •••••••••••••••••• 16
The TVRT Diagram
The VTRT Diagram ............................. 16
21
Arc Rift Zones ••••••••••••••••••••••••••••••• 26
CALC-ALKALINE VERSUS THOLEIITIC DISCRIMINATION ••• 30
CHAPTER TWO. Nevadan Sinistral Transpression in the South~rn Part of the Foothills Terrane, Western Sierra Nevada, California
INTRODUCTION ..................................... STRATIGRAPHY .....................................
Penon Blanco Formation ....................... Gopher Ridge Formation .......................
41
46
46
47
Mariposa Formation ••••••••••••••••••••••••••• 47
STRUCTURAL GEOLOGY ••••••••••••••••••••••••••••••• 47
Flattening Foliation ••••••••••••••••••••••••• 47
Lineation ..... 0 • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 50
Folds ........................................ 50
:::S:.:h~e=a~r!:.......l:Z:!,;o~n~e::.s~ •••••••••••••••••••••••••••••••••• 52
--------------------------------------~.- .. -.... - .............. .
[ . .'
6
RelationGhip of structural Elements •••••••••• 56
Domain 1, ...••....•...................... 57
Domain lA, •.•.•..•.•..............•...... 59
Domain 2. ...•...•.••..........•.•..•..... 64
COMPARISON OF FOOTHILL'S STRUCTURE WITH TECTONIC MODELS •••••••••••••••••••••••••••••• 67
DISCUSSION ••.•••••••••..•••••••••••.••••••••••••• 74
Nevadan Orogeny •••••••••••••••••••••••••••••• 77
CHAPTER THREE. Mesozoic Tectonic Environments of Massive Sulfide Deposition in the Southern Part of the Foothills Terrane, Western Sierra Nevada, California
INTRODUCTION ••••••••••••••••••••••••••••••••••••• 84
ELEMENT IMMOBILITY ••••••••••••••••••••••••••••••• 86
STRATIGRAPHY •••••••••••••••••••••••••••••••••••• '" 93
Penon Blanco FOrmation ••••••••••••••••••••••• 95
Basaltic Lavas, Basalt Member, Lower Penon Blanco Formation.
Thin-bedded Pelites, Lower Penon
........ 97
Blanco Formation.. •••••••••••••••••••• 102
Intermediate Crystal Tuffs and Breccias, Middle Penon Blanco Formation. •••••••••••••••••••• 103
Volcaniclastic/epiclastic Turbidites, Upper Penon Blanco Formation. •••••••• 104
Gopher Ridge Formation ••••••••••••••••••••••• 106
Intermediate/mafic Breccias and Tuffs, Lower Gopher Ridge Formation. •••••••• 107
Rhyolitic Volcanic Rocks and Basalts, Upper Gopher Ridge Formation. •••••••• 113
contact with the Mariposa Formation. ••••• 116
--_ .. __ .. _ .. _ .. -_ ....... -.... -. .. __ ....•..
7
Mariposa Formation ••••••••••••••••••••••••••• 117
Mudstone and Tuff. Lower Mariposa Formation. 118
Mudstone. Mariposa FOrmation. ............ 120
Intrusive Rocks •••••••••••••••••••••••••••••• 122
Isolated Mafic/ultramafic Lenses in the Central Pelitic Belt ••••••••••••••••• 126
MASSIVE SULFIDE DEPOSITS ••••••••••••••••••••••••• 132
Cyprus-type cu (Cu-Zn) Deposits •••••••••••••• 132
Kuroko-type Zn-Cu-Pb Deposits •••••••••••••••• 133
Besshi-type Cu-Zn Deposits ••••••••••••••••••• 137
PETROTECTONIC ASSEMBLAGES •••••••••••••••••••••••• 140
Ocean Floor. Assemblage 1 •••••••••••••••••••• 141
Primitive Volcanic Arc. Assemblage 2
Extensional Volcanic Arc, Assemblage 3 •••••••
146
148
Arc Rift Zone, Assemblage 4 •••••••••••••••••• 151
Back-arc/interarc Basin. Assemblage 5 •••••••• 156
Forearc/proto-trench, Assemblage 6 ••••••••••• 162
DISCUSSION OF TECTONIC ENVIRONMENTS AND ORE DEPOSIT SETTINGS ••••••••••••••••••••• 165
SU~y •••••••••••••••••••••••••••••••••••••••••• 170
APPENDIX A - DATA SOURCES FOR THE TVRT DIAGRAM ••••••• 173
APPENDIX B - ROTATED FOLIATION IN DOMAIN 1A •••••••••• 178
APPENDIX C - INSTRUMENTAL NEUTRON ACTIVATION ANALYTICAL TECHNIQUES 0 •••••••••••••••••••••••
APPENDIX D - PETROGRAPHIC DESCRIPTIONS OF REPRESENTATIVE ANALYZED SAMPLES ••••••••••••••
.......... _ .... _ ...... "" ... .
184
185
8
.APPENDIX E - FOOTNOTES FOR CHAPTER THREE ••••••••••... 189
REFERENCES ........................................... 192
-----.... - .... _--.. _-,,--- ".-"""- ... -..... _._ .... -.----_ .....
9
LIST OW ILLUSTRATIONS
Figure 1. Plotted basic rocks from different tec-tonic settings in the TVRT diagram........... 17
Figure 2. simplified extended Coryell-Matsuda plots of young basic rocks from different tectonic settings.................. 19
Figure 3. The TVRT diagram of basic rocks from plate margin rift basins..................... 22
Figure 4. simplified extended coryell-Matsuda plots of basalts of the Green Tuff and arc rift zone basalts.................... 25
Figure 5. Plots of best-fit curves for several rock suites in Ti02lFeO* and MgO/FeO* vs. MgO/Al203 space........................... 34
Figure 6. Derivation of field boundaries in the TFMA diagram............................. 36
Figure 7. comparison of TFMA diagram, AFM dia-gram, and revised Jensen cation plot......... 37
Figure 8. Location map of the Foothills ter-rane, central California..................... 42
Figure 9. Geology of the southern part'of the Foothills terrane........................ 44
Figure 10. Stereogram of foliation and linea-tion for the entire field area............... 49
Figure 11. vertical cross-section along A-A'......... 53
Figure 12. Sheath folds within the southwest strand of the Bear Mountains fault zone...... 58
Figure 13. Stereogram of foliation and lineation in Domain 1A....................... 60
Figure 14. Diagram illustrating the relationships between fold hinges, finite stretching direction and line of slip........ 62
Figure 15. stereogram of foliation and lineation in Domain 2........................ 65
.... _ .• _ .. _._.,-.'rI!It. ," ., ...•.• , .. _" ,.- ........ -.-_ ...... - .
=
Figure 16. outline of the major structures along a portion of the Bear Moun-
10
tains fault zone............................. 68
Figure 17. Suggested plate motion between 135 and 140 Ma ago........................... 70
Figure 18. Convergence velocity vectors of the oceanic Farallon plate relative to the North American plate.................. 72
Figure 19. For a maximum principal stress direction of 88°, expected orientation of a sinistral transpressive shear zone with associated structures........ 73
Figure 20. c i vs. cl plots of basalts to rhyo-lites of the Gopher Ridge Formation.......... 91
Figure 21. Cl/cl vs. cl plots of basalts to rhyo-lites of the Gopher Ridge Formation.......... 92
Figure 22. Geology of the southern part of the Foothills terrane........................ 94
Figure 23. Stratigraphy and petrotectonic assemblages in the southern part of the Foothills terrane..................... 96
Figure 24." TFMA diagram of lower Penon " Blanco basalts and middle Penon Blanco basalts to andesites......................... 99
Figure 25. Coryell-Matsuda plots of basic to intermediate volcanic rocks of the Penon Blanco Formation....................... 101
Figure 26. TFMA diagram of lower Gopher Ridge basic to silicic volcanic rocks.............. 111
Figure 27. Coryell-Matsuda plots of basic to silicic volcanic rocks of the the lower Gopher Ridge Formation............. 112
Figure 28. Coryell-Matsuda plot of basic and silicic volcanic rocks of the upper Gopher Ridge Formation................. 115
--...... --.. -~- .. --._ . .., ...... .
I.:..'
Figure 29. coryell-Matsuda plot of basic to silicic hypabyssal rocks, northern
11
part of Guadalupe intrusive complex.......... 125
Figure 30. Plot of NaM4 vs. Alw of blue-green amphiboles from mylonitic ultramafic/ mafic blocks in melange, southwest strand of Bear Mountains fault zone.......... 127
Figure 31. TFMA and ol-di-hy plots of high-Mg basic rocks.......................... 129
Figure 32. coryell-Matsuda plot of high-r'!g basic rocks, lenses in the central pelitic belt......................... 131
Figure 33. simplified extended coryell-Matsuda plot of basic volcanic rocks of lower Penon Blanco Formation................. 142
Figure 34. simplified extended Coryell-Matsuda plots of young basic rocks from different tectonic settings •••••••••••••••••• 144
Figure 35. TVRT diagram of basic rocks of the Penon Blanco Formation and the lower Gopher Ridge Formation ••••••••••••••••• 145
Figure 36. simplified extended Coryell~Matsuda plot of basic volcanic rocks of the middle Penon Blanco Formation •••••••••••• 147
Figure 37. simplified extended Coryell-Matsuda plots of basic volcanic rocks of the lower Gopher Ridge Formation ••••••••••••• 150
Figure 38. VTRT diagram of samples from dif-ferent subduction-associated rift basin units in the southern Foothills terrane •••••• 154
Figure 39. simplified extended Coryell-Matsuda plot of high-AI basalts of the upper Gopher Ridge Formation ••••••••••••••••••••••• 155
Figure 40. Regular and simplified extended coryell-Matsuda plots of basalt near the Green Mountain deposit •••••••••••••• 159
------------------------_. __ .. _-- , .... _ .... _.-.. _ ... ' .. ' .
Figure 41. simplified extended Coryell-Matsuda plot of basic hypabyssal rocks, north-
12
ern part of Guadalupe intrusive complex...... 161
Figure 42. Simplified extended Coryell-Matsuda plots arid TVRT diagram of high-Mg basalts in the central pelitic belt.......... 163
------------------------~--- .. ---.-...... -.- .... -....... -.-- .....•.... ---..•...
LIST O~ TABLES
Table 1. Analyses of Tertiary basaltic andesites, arc rift zone, Sierra Madre
13
Occidental, Mexico........................... 23
Table 2. Metavolcanic rock analyses, Penon Blanco Formation, southern part of the Foothills terrane..................... 98
Table 3. Metavolcanic rock analyses, Gopher Ridge Formation, southern part of the Foothills terrane..................... 109
Table 4. Basic meta-igneous rock analyses, central pelitic belt, southern part of the Foothills terrane................ 124
14
ABSTRACT
As a tool in discriminating basic rocks from different
tectonic settings, a type of diagram was developed that
employs three ratios of trace elements. The diagram sepa
rates basic rocks formed in mid-ocean ridge, intra-plate,
and volcanic arc settings. It can be used to differentiate
basalts from marginal basin, forearc, and arc rift zone
settings. A second application of this type of diagram,
employing major elements, distinguishes tholeiitic, calc
alkaline, and boninitic series volcanic rocks.
The southern part of the Foothills terrane, western
Sierra Nevada, california, is composed chiefly of Jurassic
Triassic(?) metavolcanic and metasedimentary rocks of lower
greenschist grade. Major tectonism affecting the terrane,
associated with the Late Jurassic-Early cretaceous Nevadan
orogeny, was sinistral transpression with shearing along the
Bear Mountains and Melones fault zones. The line of slip in
high shear strain regions is approximated by the modal
stretching lineation, which is at a rake of approximately
45° SE to the general shear zone orientation, suggesting
sub-equal components of strike slip and dip slip. The sense
of shear from kinematic indicators is consistently east side
to the northwest.
15
The terrane hosts three types of syngenetic massive
sulfide deposits: Cyprus-type cu deposits, Kuroko-type
Zn-Cu-Pb deposits, and Besshi-type Cu-Zn deposits. The
Cyprus-type deposits lie at the top of a Triassic(?)
tholeiitic basalt sequence in the lower Penon Blanco Forma
tion. The deposits are part of an ophiolitic sequence that
appears to have formed in an open-ocean spreading center
environment.
Felsic lava facies host the Kuroko-type deposits at the
top of the Middle to Late Jurassic upper Gopher Ridge Forma
tion, a dominantly bimodal sequence of meta-rhyolitic lavas
and tuffs and meta-basaltic lavas. The tectonic setting
appears to have been an arc-rift zone that formed during the
transition from arc volcanism forming the lower Gopher Ridge
Formation to younger basinal sedimentation forming the
Mariposa Formation.
The Besshi-type deposits are sediment-hosted in the Late
Jurassic Mariposa Formation. They appear to have formed in
the median part of a long linear basin between rifted arc
segments. The inferred tectonic setting of the sulfide
deposits was an early back-arc or interarc basin, which may
have been re'lated to transtensional tectonics.
-----------------------------.--"._.------,,_ ..... ,,- .... --. ... ._- ...... - .... -._ ..... _ ..
CHAPTER ORE
The Triangular Element-ratio Diagram: Applioation
to Disoriminating Voloanio Rooks from Differsnt Teotonio
settings and Different Differ~ntiation Trends
INTRODUCTION
16
Two applications of an original type of diagram are
presented. The diagram, a triangular plot of three ratios
of elements or oxides, was developed to help determine the
tectonic settings of massive sulfide deposits, the subject
of Chapter Three. The first application is a trace-element
plot of the ratios Ti/V, Tb/Ce, and Th/Ta for basic volcanic
rocks, which discriminates rocks erupted in various tectonic
settings. The second application is a major-element plot of
the ratios MgO/AI203, MgO/FeO, and Ti02/Feo of subalkaline
volcanic rocks of any Si02 content, which discriminates
tholeiitic from calc-alkaline rock suites. For major ele
ments, all samples were normalized to 100 percent volatile
free, with F'e203/FeO = .20, typical of tholeiitic basalts.
TRACE ELEMENT DISCRIMINATION OF BASIC ROCKS
FROM DIFFERENT TECTONIC SETTINGS
The TVRT Diagram
The TVRT (Ti-V-REE-ThTa) diagram (Figures lA and lC) is
a ternary trace element ratio plot of (Ti/V)/5, (Tb/Ce)x50,
and Th/Ta of unmetamor~hosed young basic rocks with Si02 S
• ... ·_··_··· __ ~~~··,IIIf··~···w... • .••. .-__ ."- .. ,.- _ ... _._ ... -_.
-.. -
l' .... A.
(Tb/C.)xSO
B.
Th
(n/V)/5
Hf/3
c. (n/V)/5
Th/Ta (Tb/Ce)xSO
Explanation
• OIB - oceanic Island basic rocks
.. lAB - Island/continent. arc bas./bas.andes. • MORB - mid-oceanic rfdge basalt
o BAS - back-arc/Interarc basin basalt & FAa - forearc basic rocks
o CRB· - continental rift zone basic rocks
IPB - Intraplat. basic rocks. OIB+CRB
To
I'igure 1. (A) P1otte4 basic rocks (~56 wt.% 8i02) froll cU.ff8%ellt tectonic settings in the ~ cU.agraa. (B) fta Baas sup1es p1otta4 in the U/3-ft-~a ScJUDI8 of W004, at al.. (1979), with their fie14 l:Io1m4arias. (0) fta ~ cU.agraa with fie14 ho1Ul4aries. Data sources are liste4 in ~ A.
Th/Ta
~ ..
18
56 wt. percent. These elements are relatively impervious to
low-temperature metasomatism, and the diagram may be used
with discretion for low-grade metamorphic rocks. The data
sources for the diagram are listed in Appendix A.
For comparative purposes, the same data sets of Figure
1A are plotted on the Hf/3-Th-Ta diagram of Wood, et ale
(1979), Figure lB. It should be noted that in any type of
diagram such as these, the future addition of new data
points undoubtedly will expand fields. Classification
should be based on a group of analyses and their overall
plot-area rather than on the particular field of individual
samples.
Figure 2 is another representation of the data sets used
to construct the TVRT diagram. This figure consists of
simplified extended coryell-Matsuda plots of chondrite
normalized rare earth elements against their atomic number
(Ce - 58, Tb - 65), plus the elements Ta, Th, Ti, and V,
positioned according to their relative incompatibility with
respect to the rare-earth elements. The abscissa positions
of the non-REE elements were determined by intersections of
the normalized element abundances with the hyperbolic REE
plot of a suite of undersaturated continental-rift basic
rocks from the Hessian depression (Wedepohl, 1985). The
derived abscissa values were Ta - 56.1, Th - 56.5, Ti -
65.0, and V - 76.1. For easier visualization, Ta is plotted
A. 10' ... Id-oceanlc RIdge BalOlI.
10'
j to'
.... l to
II" Ta TIl C. Tbn v
8. 10' VolcanIc Arc Balle Rockll
to'
• i 10'
J ~ B to
to" Ta '" C. Tbn v
OceanIc IIIIand Balle Rockl
to .. ·~To~Th~C.----------~~--------------~-
D. to' Back-arc BalIn ealalll
ID'
f to'
! • l to ~Q5 & n_ZI
to" Ta TIl C. Tbn
E. 10' Forearc Balle Volcanic Rock.
to'
I to'
.... lto
neZ'
II" Ta '" C. Tbn
Wigure 2. 8iaplified eztende4 Coryell-Katsuda plots of young basin roolts fro. different teotonc settings, i.e. (A' .td-ooean ridge., (D' volcanic aros, (0, intraplate oceanic island., (D, back-arc basins, (B) forearc regiona. 8ae tezt for azplanation of eleaent positioning. Data souroes are listed in Appendiz A. Obondritic conoentrationa ara the average C1 values of ADders and Bbibara (U82).
v
v
--------,------~--.--~-.. ~--.. ---.. -- ..... " ...... _ ... .
19
20
at 56, Th at 57, and Ti at 65.1. In Figure 2, the slopes of
the Ce-Tb line segments represent LREE enrichment or deple
tion in the source area, Ti-V slopes represent Ti depletion
or enrichment with respect to V, and Ta-Th slopes represent
relative high-field strength element (Ta) and incompatible
lithophile element (Th) behavior.
The distinction in trace element chemistry with tectonic
setting is related to the degree of effect of Ta (and Ti)
depletion and Th and LREE enrichment in basic magmas from
settings influenced by subduction-related processes. In the
TVRT diagram, magmas lacking the characteristic Th enrich
ments and Ta depletions of arc magmas plot away from the
Th/Ta apex. Magmas from sources which have variable degrees
of the subduction-related signature, such as BAB and FAB,
plot in in the central area and overlap both lAB and MORB.
The Tb/Ce component serves to separate magmas with different
REE enrichment or depletion patterns. Variability in rela
tive Ti and V abundances may reflect that during genesis or
differentiation of basic magmas in some settings, V behaves
as a compatible element, and in other settings, it behaves
incompatibly as does Ti.
The depletion of high field-strength elements is proba
bly related to stabilization of a titaniferous phase such as
sphene in the source area of either (1) hydrothermal fluids,
..... , ..... __ .~._ ... _ • ., ..•• _ •••• ,.u .• _._~ •• ~ ..••.• __ ~_ ••• -~_.
21
which metasomatically enrich the upper mantle source of arc
magmas, or (2) the arc magmas themselves. Ti/V differences
appear to have multiple causes, including characteristic
degrees of partial melting for magmas from different set·
tings and either or both fractional crystallization or
melting producing variation similar to the Ti/Fe relation
ship discussed below.
The VTRT Diagram
In the TVRT diagram in Figure 3A, the fields of arc rift
zone basic rocks (ARB) and Guaymas basin basalts have been
added along with the field of back-arc basin basalts. The
ARB and Guaymas fields were not placed on the diagram of
Figure 1, because many of these analyses are lacking one or
more critical elements such as V. In these cases, the
abundances of such elements were estimated by means given in
the caption to Figure 3.
The ARB analyses from the literature are supplemented by
data from Tertiary arc rift zone basaltic andesites from
Mexico, analysed in this study (Table 1). Titanium and
vanadium were analysed by inductively coupled plasma (ICP)
methods by a commercial laboratory, with reported errors of
5-10 percent. The author analysed other trace elements by
instrumental neutron activation analysis (INAA), by tech
niques detailed in Appendix A. One standard deviation in
counting statistics is reported in Table 1.
---•.. _ .. _ .... _ •.. _ ........... - , ... .
~
A.
(Tb/Ce)x50
'.;
(TI/V)/5 Explanation
DARB - arc-rif't zone bas./bas.andes.
o BAB - back-arc/Interarc basin basalt
• Guaymas basin basalt (pull-apart basin unrelated to subduction)
B.
(Tb/Ce)xSO
Th/Ta
(V/TI)x100
, 0 Is:I 'tI, ARB "to
8'§ '& 1IIt;:;e,
8'0 ~D8 )-..~D~ , DO 1:0.. '- I!I
BAS ...... 0
:rigure 3. (A) The ~ tiagraa or basic rocks (:SSG wt.% 8i02) rrma p1ate IUlZ'giJl rirt basiDs. SUbduction-associated rirt basin baic rocks inc1u4e ABB and JmB. Data sources are 1iate4 in Appendiz A. :For a mnority of samp1es scme e1aaents not avai1ab1e rrma the 1iterature were derived as ro1101f8: 'ra=1lb/1&.& (chonGritic ratio, :e1ative1y invariant with anviroJment), 'V'=(Sc+Y)5 (re1ative1y invariant with anviroJm8Jlt): for acme llBB 1acJdDg V, 'riz Sc or Y data, ractors rrma silli!.ar ])asa1t anal.yses rraa the SIlllS area were useeJ, i.e. 'V'=Scz8.2, V=YX20, 'ri=SCZ325. (D) orbe ftll'r diagram or the subduction-associated rirt basin basic rocks de1inaateeJ in (A). Dashed 1ine is the transitioJla1 IKnmdary babean JmB aneJ ABB.
Th/Ta
23
TABlE 1. AllALYSEI OF TElTIAIY IlAlALTiC IllllEIITEI. NIt: II" lmIE. IIEIIA IWIIII! ClCCIDEllTAL. IlEllICO
...,Ie. .001A .as" • os" ... .,oA .,2" lleJor els.!ta (lit I)
1f0z 54.09 54.81 53.114 54.11 53.55 54.36 TIOz 1.13 1.14 1.13 1.16 O.VI 1.13 AlzO.J 17.76 17.93 17.67 17.65 18.00 18.74 f~ 7.94 8.02 7.88 7.93 7.86 8.ZS IIrO 0.11 0.10 0.10 0.10 0.11 0.11 IIgO 3.96 3.91 3.61 4.12 3.53 3.110 cao 7.51 7.54 7.33 7.57 7.47 7.67 Mazo 3.49 3.55 3.81 3.42 3.48 3.81 ICzO 1.92 2.13 1.88 2.18 1.58 2.04 Pz05 0.44 0.46 0.45 0.52 0.44 0.46 LOI 1.25 1.22 1.09 1.37 0.96 0.90 lotal W.6O l00.ii' VI.79 100.13 97.96 101.31
~tlled to 100.0 1 wlatlte-free. with fe2rIJ/flll • 0.211 SIOz 55.33 55.44 55.49 55.17 55.59 54.52 TIOz 1.16 1.15 1.16 1.18 1.02 1.13 AlzO.J 18.18 18.12 18.21 17.W 18.69 18.79 f~ 1.24 1.24 1.24 1.ZS 1.24 1.26 fill 6.211 6.18 6.19 6.17 6.22 6.29 tIrO 0.11 0.10 0.10 0.10 0.11 0.11 IIgO 4.05 3.95 3.n 4.211 3.66 3.81 cao 7.69 7.62 7.55 7.n 7.75 7.69 Mazo 3.57 3.59 3.93 3.49 3.61 3.88 ICzO 1.97 2.15 1.94 2.22 1.64 2.03 PzOs 0.45 0.46 0.46 0.53 0.46 0.(,6 Total 100.0 100.0 100.0 100.0 100.0 100.0
elN _tlw _Iple Q 3.49 3.211 2.63 2.90 4.64 0.110 C 0.00 0.00 0.00 0.00 0.00 11.00 or 11.61 12.71 11.45 13.13 9.69 12.09 Db 3O.ZS 30.34 33.22 29.50 30.56 32.83 an 27.76 26.97 26.34 26.88 29.92 27.81 lie 0.00 0.00 0.00 0.00 0.00 0.00 dl 6.12 6.41 6.69 6.52 4.60 6.05 hy 15.74 15.29 14.57 15.79 15.78 15.35 01 0.00 0.00 0.00 0.00 0.00 0.00 111: 1.110 1.79 1.110 1.79 1.110 1.112 It 2.20 2.19 2.21 2.25 1.93 2.15 lip 1.07 1.10 1.10 1.26 1.08 1.09
Tr_ .I.-ente. not _tiled (All» ,. I. ,. I. ,. I. V 170. 160. 170. 160. 1110. 170. Ba 565. 10. 555. 111. 583. 211. 583. 10. 470. 10.4n. ZS. Co 24.0 0.1 ZS.8 0.1 24.0 0.1 24.0 0.1 22.1 .1 22.7 0.1 Cr 42.0 5.9 42.3 5.3 41.9 6.9 46.2 6.5 10.2 .6 19.8 4.7 ca 0.757 0.048 0.856 0.112 0.450 0.046 0.741 0.055 0.745 .006 1.50 0.05 Hf 2.88 0.04 2.95 0.06 2.97 0.06 3.35 0.06 2.34 .04 2.29 0.03 Rb 37.2 0.8 311.7 0.9 ZS.3 0.8 31.3 0.8 25.2 .7 18.2 0.6 Ta 0.259 0.028 0.357 0.027 0.292 0.034 0.335 0.028 0.259 .028 0.267 0.027 Th 2.67 0.07 2.70 0.03 2.60 0.05 2.86 0.045 3.10 .05 2.311 0.05 U 0.918 0.041 D.WSZ 0.032 0.l1li4 0.053 0.1183 0.031 0.960 .033 0.754 0.031 In 111. I. 105. 1. 109. 1. 111. 2. VI.8 1.9 99.6 1.3 Zr 110. 7. 131. II. 1111. 7. 144. 12. 106. 6. 114.1 6.0 Sc 13.8 0.1 13.7 0.1 13.8 0.1 14.6 0.1 14.1 . , 12.9 0.0 Le 18.9 0.1 19.0 0.1 19.1 0.1 22.1 0.1 16.3 .1 15.4 0.1 Co 39.2 0.6 39.4 0.4 39.3 0.6 43.6 0.9 33.6 .8 31.6 0.9 Md ZS.l 0.8 21.8 0.8 ZS.O 0.8 25.4 0.9 211.2 1.3 19.5 1.0 s. 4.39 0.01 4.40 0.01 4.47 0.01 4.91 0.01 4.08 .01 3.79 0.01 Eu 1.311 0.04 1.36 0.03 1.41 0.04 1.44 0.03 1.29 .04 1.25 0.03 Tb 0.505 0.019 0.510 0.014 0.4114 0.014 0.532 0.014 0.481 .015 0.445 0.015 Te 0.178 0.039 D.1V1 0.046 0.221 0.045 0.171 0.049 0.2Z4 .048 0.197 0.04 Tb 1.15 0.02 1.15 0.02 1.21 0.02 1.26 0.02 1.41 .02 1.08 0.02 Lu 0.163 0.007 0.171 0.004 0.1112 0.004 0.186 0.005 0.213 .007 0.158 0.004
A Major .1 ... : ICP-AES. c:~I.t lob.; V: MS, _relet lab; other t~ .Is.!t.: IW, M.C. NeIlton, III, LWIU" .... PI_tary Lab., ~Iv. of Ariz., Tuc:aon.
----~ .. -.. _ ... _-_ ..•.............. -
24
The uppermost part of the ARB fields in Figure 3 com
prises analyses of basic rocks from the Tertiary "Green
Tuff" of Japan (Ebihara, et al., 1984), which hosts the
Kuroko deposits. The analysed values of these samples are
plotted in simplified extended Coryell-Matsuda space in
Figure 4A1 actual and derived elements for all ARB used in
this study are presented in Figure 4B. The Green Tuff arc
rift zone sequence exhibits the variable Ta-Th relationships
also typical of BAB, but generally has greater LREE enrich
ment. In the TVRT diagram (Figure 3A), this positions the
ARB field above the BAB field, while the variable Th/Ta
relations spread ARB across the diagram from the volcanic
arc field to the field of E-MORB.
Because there is a greater potential for overlap between
ARB and BAB in the TVRT diagram, due to the commonly lower
Ti/V of ARB, these fields are better portrayed by reversing
the order of V and Ti in the upper ratio. consequently, in
Figure 3A, the fields of arc rift zone and back-arc basin
basic rocks have been joined as a superset of subduction
associated rift basin basic rocks. In Figure 3B, ARB and
BAB are plotted in the VTRT (V-Ti-REE-ThTa) diagram with an
upper ratio of (V/Ti) 100. The Guaymas field totally over
laps the BAB field in the VTRT diagram and is not plotted.
It is recommended that once a suite's affinity has been
---.-_., --_._ .. _ .. _ ................ .
A.
8.
,. -5 10' c o .c u ~ u to &
Basalts of the Green Tuff, northeast Japan
10 -I To Th Ce Tbn
10 •
.,. -5 10' c . 0
.c u ~ u 10 o
0::
10 -I To Th Ce
Arc-rift Zone Basalts
Tbn
25
v
v
pigure 4. simplified extended coryell-Matsuda plots of basalts of the Green Tuff (A) and arc-rift zone basalts (D). Some values in (D) derived as indicated in pigure 3 caption.
---~------------------ --_._---_ .•...... _-.. _ .. -_ ............... - ...... - .... --_ .... - ..... _ ....... .
I .; !
26
narrowed down to the sUbduction-associated rift field with
the TVRT diagram and stratigraphic/petrologic characteris
tics, the VTRT diagram may be used to help discriminate BAB
and ARB. Again LREE-enriched, commonly alkaline, basic
rocks from early back-arc and intra-arc settings are not
distinguished and are here included with ARB.
Arc Rift Zones
Most of the tectonic environments mentioned above are
well understood, with the exception of arc rift zones. This
tectonic setting has been recognized in the rock record and
is an important setting for volcanic-associated massive
sulfide deposits, e.g. the Hokuroku District, Japan (Silli
toe, 1982). The following is a discussion of this tectonic
setting.
Karig (1971) and Karig and Kay (1981) described tectonic
settings they referred to as volcanotectonic rift zones.
These zones are marked by bimodal volcanic compositions with
explosive silicic volcanism and more effusive basic volca-
nism. Karig (1972) suggested that explosive silicic volca
nism may occur on the back-side of an arc or on top of a
remnant arc. Karig (1972) suggested volcanotectonic rift
zones represent the earliest stage of extension in a trench
arc system, and Karig and Kay (1981) suggested they repre-
---..---.. -..... - ...... -.. -........ ~ ....... " .... -_. ~-." ... ~. _._._ ... ---.
sent a transitional stage between arc and marginal basin
activity.
27
Analyses in the literature indicate the bimodal
compositions are generally basalt-basaltic andesite and
dacite-rhyolite with a dearth of.andesite, although some
such zones may have a gap higher in the silica range,
between andesite and rhyolite, e.g. Quaternary volcanics of
Guatemala (Rose, et aI, 1979). The bimodal assemblages
generally overlie andesitic volcanic rocks.
Spectacular examples of arc rift basins with explosive
silicic eruptions have developed on continental crust, e.g.
the Sumatran segment of the Sunda arc (the setting of the
great Toba eruption), the Nicaraguan segment of the Middle
America arc, and the Taupo volcanic zone of New Zealand. If
volcanism takes place in a more aggressively rifting
environment, felsic eruptions may be more effusive than
pyroclastic, and mafic volcanism proportionally increases,
e.g. the early Basin-and-Range bimodal suites in the south
western U.S. (Christiansen and Lipman, 1972).
Subsidence in arc rift zones may be great. Guber and
Green (1983) suggest that, on the basis of foraminifera in
the Sasahata Formation in the Hokuroku district, subsidence
to a depth of more than 3500 meters took place prior to
eruption of felsi(;: tuffs of the "Green Tuff". These authors
---~ .. -" ~ ,. "_"'- ........ .,. ,........... .. -- ......
28
also suggest that subsidence was produced by block-faulting
along high-angle faults cutting basement.
In a volcanic arc setting, large tensile stress compo
nents would be expected in the upper plate of an oblique
convergence zone or in the more advanced stages of arc
rifting of a direct convergence zone. The distinction
between these effusive arc-related rift zones and strictly
extensional continental rift zones, such as the East African
rift zone, lies in the arc geochemical affinities of the
magmas, most notably lower Ti and high-field strength ele
ments such as Ta and Nb.
Both caldera-forming zones and the more-effusive zones
are here included under the designation "arc rift zones".
They are intra-arc extensional zones that represent the
early stages of rifting of arcs, which mayor may not pro
gress into interarc or back-arc basins. Subalkaline basic
rocks in arc rift zones commonly have relatively high alumi
num contents p which also is typical of mature arc sequences.
Bimodal arc rift zones may form over either continental or
oceanic crust: bimodal basic and low-potassium (high
Na20/K20) acidic magmas form the younger portions of some
ensimatic arc suites, e.g. Deception Island, scotia Arc
(Weaver, et al., 1982). The classification "arc rift zone"
here also includes intra-arc rift basin suites with Mgo-rich
,,-_ •• _~ ..... ___ .'1lIf"~' ~ ... '. ' ••
29
alkaline basic rocks, such as the intra-ocean Aoba island
suite, vanuatu (Gorton, 1977). Extensional provinces with
alkaline basic rocks that form on the rear side of arcs,
such as Vulsini, Italy (Rogers, et al., 1985) may be consid
ered either as arc rift zones or early back-arc basins.
Such rocks have very enriched LREE patterns, high Th/Ta, and
lower Ti/v than intraplate alkaline basic rocks. They may
have moderately high MgO contents. As there appears to be
no fruitful distinction between arc rift zones and early
back-arc basins, basic rocks from all such settings are
included here under ARB. ARB therefore are transitional
into BAB.
No distinction is made between normal rifting and trans
tensional rifting, and pull-apart basins in the upper plate
of an obliquely convergent plate boundary are included as
arc rift zones.
An arc rift zone with a bimodal assemblage of silicic
volcanic rocks and subalkaline basic volcanic rocks is a
major tectonic setting in which volcanic-associated massive
sulfide deposits form. In subaqueous environments, ore
deposits are commonly exhalative Zn-Pb-Cu deposits, exempli
fied by the Kuroko deposits. In subaerial environments,
base metal vein systems may be the counterpart of the sub
aqueous massive sulfides (Si11itoe, 1982). In the North
.... _ ...... - •. _-.- ·W'·· , .•. ' •. ~.
30'
American Cordillera, some skarn and carbonate-hosted Pb-Zn
Ag replacement and karst-filling chimney and manto deposits
spatially and temporally associated with felsic shallow
intrusive bodies also may be equivalent.
CALC-ALKALINE VERSUS THOLEIITIC DISCRIMINATION
Igneous rocks may be broadly classed as either alkaline
or subalkaline, e.g. chayes (1966). Subalkaline rocks
generally may be grouped as either tholeiitic or calc
alkaline, e.g. Irvine and Baragar (1971). The term calc
alkaline was derived from the calc-alkali field of Peacock's
(1931) alkali-lime discrimination scheme. Calc-alkaline
rocks generally have total alkali contents between those of
alkaline rocks and tholeiitic rocks and plot in the calc
alkali field of Peacock's diagram. Members of tholeiitic
suites characteristically have low K20 contents and higher
Na20jK20 (at given Si02 content) than members of calc
alkaline suites. As differentiation sequences, the real
distinction between calc-alkaline and tholeiitic suites is
the presence or absence of Fe-enrichment in early to middle
stage liquids.
The archetypal Fe-enrichment series is the Skaergaard
intrusive suite (Wager, 1960). The Icelandic Thingmuli
suite (Carmichael, 1964) also is a classic example. Calc
alkaline suites are characterized by the absence of Fe-
---------------------~--,------~.- .. -.. ---.. --.......... '- ..•. ., ...... _ .... - .---- ... --
31
enrichment or even the presence of Fe-depletion relative to
Mg in early to middle stage members. Classic examples are
volcanic arc suites of the Cascade and Andean mountain
ranges (Hyndman, 1972). The prevalence of calc-alkaline
rocks in arc settings has led to· some overapplication of the
term to include all arc sequences and to necessarily indi
cate an arc origin for all such suites.
Arc settings actually produce at least five distinct
igneous suites - boninitic, high-Mg alkaline, alkaline,
tholeiitic, and calc-alkaline. These suites can be differ
entiated on the basis of major and trace element chemistry,
and their recognition is important in characterizing the
setting and relative maturity of arc assemblages. Only
tholeiitic and calc-alkaline sequences will be dicussed
extensively here. Tholeiitic arc sequences have been sug
gested to compose the early phases of arc activity, while
calc-alkaline suites dominate mature arcs, e.g. Jakes and
Gill (1970). Calc-alkaline suites predominate in arc assem
blages developed on continental crust, and intra-ocean calc
alkaline suites may have tholeiitio affinities, e.g., st.
Kitts, Lesser Antilles. Tholeiitic and calc-alkaline suites
may be contemporaneous but spatially separated in the same
arc, as documented in Japan where a Quaternary tholeiitic
belt (pigeonitic series) nearest ~he trench is respectively
-----------------------------~ .. -.. -.... - ... -.................. ..
.:
supplanted inboard by a calc-alkaline belt (hypersthenic
series) and an alkalic belt (Kuno, 1966).
Calc-alkaline arc suites commonly have abundant ande
sites, whereas arc tholeiitic suites have more abundant
basaltic andesites to basalts. Calc-alkaline lavas are
typified by plagioclase phenocrysts with groundmass hyper
sthene being more common than pigeonite (Jakes and Gill,
1970). Arc tholeiitic r.ocks commonly have pigeonite, and
they have flatter REE patterns (less LREE enrichment) than
calc-alkaline rocks (Jakes and Gill, 1970).
32
The tholeiitic Fe-enrichment trend generally is accepted
as the result of crystal-liquid fractionation. The origin
of the calc-alkaline neutral to negative Fe-enrichment trend
is more problematical as, in such rocks, there is abundant
evidence of crustal contamination and m~gma mixing in addi
tion to igneous fractionation processes. Regardless of the
origin of tholeiitic and calc-alkaline suites, the Fe
enrichment discrimination is real and of importance in
characterizing petrotectonic assemblages. This is demon
strated in diagrams which plot FeO vs. MgO against a
differentiation index such as Si02, or Na20+K20 (e.g. the
classic AFM diagram) or A1203 (e.g. Jensen, 1976). Si02 is
a poor choice as a differentiation index, as would be the
case for Wager's (1960) derived Skaergaard liquids, which
_____ ..... _ •• _ •. _._ •• __ .. tII/l .••• ~ ,.,. '''' •.. - ... ~ " -'--'-'-."
.~ ..
33
actually decrease in si02 content through the middle part of
the sequence.
Although Fe-enrichment relative to Mg has been well
documented in tholeiitic suites, it has been underemphasized
that T.t is enriched even more than Fe in early to middle
stage liquids. This Ti/Fe increase is characteristic of
intraplate and spreading center tholeiitic suites such as
Skaergaard and Thingmuli. Tholeiitic Ti/Fe and F~/Mg
increases are demonstrated by their respective convex and
concave patterns in Figures SA and SD.
Ti and high field-strength elements such as Ta and Nb
are depleted in volcanic arc magmas, and Ti/Fe is low in arc
magmas compared to magmas from other settings. Calc
alkaline magmatic series generally exhibit lower Ti/Fe, as
well as lower to neutral Fe/Mg, in earl¥ to middle stage
members, as illustrated in Figures SB and SE. Rare calc
alkaline series such as the st. Kitts suite (Figure SG) are
exceptions to the rule and show increases in both Fe/Mg and
Ti/Fe. Boninitic series rocks (Figure SC) exhibit marked
Fe/Mg and Ti/Fe decreases in early to middle stage members.
Arc tholeiitic magmas are distinguished from arc calc
alkaline and boninitic magmas by patterns of Fe/Mg increase,
and they may have patterns of either increasing or decreas
ing Ti/Fe, e.g. Figure SF indicates that Japanese Quaternary
----------------------~---.--.- --- - . - .-- .. _-
.". M
.;
A. CI.JA 1 Skaergaard tholeRtlc tnnd
M;O/AI2Ol III'. . s
CI.JA Thlngmul tboJelltlc am.
• • D. • R2=.54
F.
CI.2Q
~ 0.11'"1 •
~
~0.12 t • S • • o.ce
0.0.
M;O/AI2Ol CItfa s.
CI.JA Japan Quatwnary tholel!llc tr.nd
CI.2Q
~ 0.11
... ,001Z
~CLOI 0.0.
• R2 =.88 •
• R2=.85
eoacadea calc-alkaline trend [1.20 CI.JA
1.2l) B. CI.2Q
E 2 CIAOE
~ r"i:'::: ~ ~
'0.12· •
~ a · OAO .o.ce OAO
0.0.'1
1.20
E.
o.8OE CI
~
~ CI.4Q
1.20 CI.JA
G. CI.2Q
CIAO ~ ~ 0.11
l ~IZ CIAO ~o.ce
"IIO/AI~_~_
Crater Lab, U.s., calc-a\Jc. aulte 2.011
1.00 W;a/AI2Ol
CIffc ......
1.50 • E
~ 1.00 ~
Q.5O
st. kltta, W. Indlea calc-allc. aerlft 1.20
W;O/AI~~ .4 ____ _
o.8OE
CIAO
~ ~
c. CIAO
a.n
iG.2· ~ So.ll •
CI.CI
Guam bonlnltea
R2=.96
I
2.011
1.50
E a o
1.00 ~ ~
Q.5O
WIIO/AI~~
Figure 5. Plota of best·fit c:urwa for -., rod; .... ites in Ti~F.o- II1d J/tfIJIF.o- ft. J/tfIJIAlZ13 sp>ee. (A,D,n are tholeIItic suites. (I.E.Ii) .... calc·.lbline suites. II1d (C) la • b:nlnltic suite • For (D·G) the arws ...... deriwd for a.ples with ~.15 IIgO/A12tl3. CCII'I'eSpCIdI~ to .... ltlc thraugb 8ftdesltic ~Itiora. TfJFe I..."..... II1d FeJIIg decreases are indicated by ccnwu pattema. TI/Fa decreases II1d FeJIIg I~ are Indicated by ccn:ave ~ttema. AU CUIWS are 2nd order pol".....' fln:ticns that are atatiatlcally silJtificant .t the 99+% confidonce 1,.".,1.
RZ .. 1 •• ZO'I - 'lJ%CTi . il. Generall --...t of correlation:
12 1.00 Perfect :>.110 Very good
.60· .110 Goad
.40· .60 lladerate <.40 Poor
eyotal Fe reported .. Fell.
Data -..ces: CA)~. 1960 (1.0) CIInIichael. 1964 (C) 1_ II1d lleljer.
19M; lloaeer II1d .... lna.19117
(E) IKat II1d Drultt. 1_
(F) fino. 1959; \Iood, et al •• 1'la1
(Il) later. 19168.
arc tholeiites have slightly decreased Ti/Fe in early to
middle stage members. Those with increased Ti/Fe exhibit
more subdued patterns than intraplate and spreading center
tholeiites.
3S
The antithetic Ti/Fe and Mg/Fe patterns illustrated in
Figure 5 may be combined, either being reinforced or subdued
in a triangular plot against a differentiation index, such
as MgO/A1203, as in the TFMA (Tio2-FeO*-Mgo-A1203) diagram,
Figures 6 and 7. In such a plot, the general positions of
suites are controlled by ratio differences among suites,
while inflections in patterns are controlled by changes in
ratios among members within suites. The derivation of the
dividing line in the TFMA diagram is given in Figure 6. In
Figure 6A, overlapping tholeiitic and calc-alkaline suites
are plotted in an AFM diagram with the tholeiitic/calc
alkaline dividing line of Irvine and Baragar (1971) and the
dividing line between the pigeonitic and hypersthenic series
of Kuno (1968). As noted by Irvine and Baragar (1971) there
appears to be a complete transition from tholeiitic to calc
alkaline suites. Therefore the dividing line between fields
in the TFMA diagram (Figure 6B) was derived by fitting
parabolic curves to the suites in Figure 6, which completely
overlap and span the transition zone between Fe-enrichment
and Fe-depletion. The derived dividing line in the TFMA
... ~- .. --.-.-.. - .. -........... .
\0 «')
A.
Na20+K20
B.
(n02/f"eO*)5
FeO*
MgO
(MgO/ A1203)2
MgO/f"eO*
c. (MgO/AI203)2
(n02/f"eO*)5 MgO/f"eO*
Fi~ 6. Derlvatien of field b:an!arles In tile TRIo\ dl __ by.-.engirv saoples f .... calc-alkaline ard tholeiitic: mites that averUp. (A) AfII plot of ..... suites that averUp. ~irv tile ~itfen zc:ne bet>Ieen Fearlc:bed ard Fe-depleted series. IIpon c:lrc:les .,.. tholeiitic: uop!es. solid .tara .... calc-alkaline ~Ies. DallIed line I. tile b:udory be-. calcalkaline ard tholeiitic: rccb of Irvine ard IIanpr (971). Solid line I. tile lIfIIX'CXi-.te Ix:In:Iary ~ tile bypenthenlc (calc-alkaline) ard plgec:nitfc (tholeiitic) ~temII"y series of ~ ..... after lin> (1!1611). (I) The __ suites Iq In (A) plotted In tile TfIIl diagna. Solid line I. tile beat·fit cg"'ft cr = .67) for aU polnta en tile diagna. (C) The TfIIl dl_ "ith general tholeiitic ard calc-alkaline fields ... Irv tile diridirv line sbqIn In ca). Elpltfen of tile diridirv line: T = '13.26i6 + 1.25OOX -D.0II53l"; X • 2D to 60. IObere X acale (bBe of trlqle) ard T acale (hel~t of trfqle) .0 to 100; X • [(1 • TF/Total • lIF/TotalU2 + lIF/Total)1oo; T ,. OIoVTotaU1oo; TF=CTiOzlFtO")5; M=(II;QIAIZl3)2; IIF=tWlIFrD"; Total.~.
Data scon:es: a-JI Gnq>. rer-Iec • EIIart. et al •• 1m ~ tholeiites - 1In>. 1959; 1Iood. et al. 1_ ~ calc-alt. suite - 1Iood. et at •• 1_ Ella. Torva - &art ard • ..,... 1m V. Epl. V ... tu - Garton. 1m: 1Iupiy. et at •• 1!i1112
*Total Fe reported _ FeD.
';'
r-rt')
(Ti02/FeO*)5
Na20+K20
(MgO/AI203)2
FeO·
MgO/FeO* AI203
* .lap.,... thalei1t.-a - Kuno. 1959
• Skaerga .... d - Waver, 1960
D ~U - Coraid>ul, 1964
TH" T~ - E-n, et aL, 1977
CA 1t c...~dn; - CarIaJ.c:ha.l, 1944
o st. Kitt. - Baker, 1968
FeO+Fe203+Ti02
MgO
fl~ 7. c.:.perlscn of TIM dI .. _ P). ARI dI_ (I). In! rwlad ~ catlen plot (I:) .. Irv repreAntatlw tholeiitic. calc-albllrw:. In! bcnlnltlc suites. Plotted FOints .. ~ltI_1 ___ CiA ....... dlrv.
• s.ntor.in1 AicraUri-ThJr.a s.rw. - NicheU..., 1971
except for ~. to basalt (<52 IItS 111Iz). basaltic onIeslte (52-56 IItS Sio,). onIesfte (56-63 IItS Sillz). dlcfte C~ IItS 110,). In! rtlyallh: ~ 711 IItS SIIIz). TIIJtA airidirv lI .... In ARI In! J..-. plata .. Ipprad-uly tile TIM dl_ dlridlft;l lI .. b • .,......s-.. en centrol points trw fi~ 6 suites. DcdIle lI .... In tile reriad ~ cat.
t erater LaQ - Bacon ancI Dru1tt, 1988
o 8onJnJ.tes - R •• van and ... u.r. 19SC1 Bloo.er ancI --. 1987
MgO
fen plot .... arllIl .. 1 II .... fna J..-. (1915). All SIIIIpl ___ lIzed ""Ietn ... t ... with ~ • 0.20.
-total Fe reporzed _ feO.
38
diagram (Figure 6C) roughly separates dominantly tholeiitic
and dominantly calc-alkaline fields, but a better determi
nant is the combination of field position and shape of trend
as demonstrated in Figure 7.
Figure 7 compares plots of averaged compositional
classes in the TFMA diagram, AFM diagram, and Jensen cation
plot. In the TFMA diagram, Figure 7A, decreasing Ti/Fe
accentuates the convex downward patterns (toward decreasing
Fe/Mg) of calc-alkaline suites, producing noticeable kinks
in the early stages of some series such as the Cascades and
Japan Quaternary calc-alkaline trends. The same dividing
line developed for the TFMA diagram in Figure 6 is drawn in
AFM and Jensen cation space. In Figure 7C, because of the
small calc-alkaline population used by Jensen, all calc
alkaline suites examined in this study plot outside his
delineated field.
Of particular interest in Figure 7 are suites with
definite Fe-enrichment trends, such as the Recent suite of
the Tonga Islands, which plots mainly in the tholeiitic
field, but, toward felsic compositions, sharply turns into
the calc-alkaline field. This same pattern is exhibited by
rare suites with definite Fe-enrichment patterns which plot
entirely in the calc-alkaline field, such as st. Kitts,
Lesser Antilles. Kinks in the middle parts of such series
are accentuated in the TFMA diagram.
39
Any diagram like the AFM plot, that employs alkali
elements or alkaline earth elements such as Ca and Ba, is
unusable with low-grade metamorphic rocks, as these elements
are too mobile to remain reliable indicators. Diagrams such
as the TFMA diagram and the Jensen cation plot may be
employed for such rocks, if there is no indication of Fe or
Mg depletion or enrichment.
Felsic members of tholeiitic and calc-alkaline suites
overlap in all three plots. The field of Guam boninitic
rocks intersects the calc-alkaline field in all three dia
grams. However, the TFMA diagram appears to be a better
discriminator in the Mg-rich range than the other two. For
example, the calc-alkaline Crater Lake suite (Bacon, et al.,
1988) has high-Mg primitive members. In the AFM and Jensen
plots (Figure 7B and 7C), the high-Mg end of the suite
totally overlaps the boninite fields. However, in the TFMA
diagram (Figure 7A), the high-Mg end does not intersect the
boninite field, largely owing to the higher Ti/Fe of the
crater Lake suite. These high-Mg basalts differ from
boninitic basalts in several ways, including being less si
saturated (lacking normative quartz), having more enriched
LREE patterns, and plotting in the arc rather than forearc
------------------------ ---- -- - .-- - .
40
fields in the TVRT diagram. This distinction on major
element diagrams, commonly used for differentiation trends,
is important as a genetic association has been suggested
between boninite-type rocks and either tholeiitic or calc
alkaline suites, e.g. Bloomer and Hawkins (1987).
----------------------_ .. __ ._-_. - - -.. - -
CHAPTER '!'WO
Nevadan Sinistral Transpression in the Southern Part of
the Foothills Terrane, westorn Sierra Nevada, California
INTRODUCTION
41
The Foothills terrane (nomenclature after Si1ber1ing, et
a1., 1984) is in the western Sierra Nevada foothills, cen
tral California (Figure 8). Approximately 1000 square
kilometers of the southern part of the terrane, south of the
Tuolumne River, were mapped by the author in detail and
reconnaissance. stratigraphy is summarized here and pre
sented in detail elsewhere (Chapter Three).
This paper is an account of the structural geology of
the terrane, with interpretation of the style of Late Juras
sic to Early Cretaceous tectonism, the Nevadan orogeny. In
particular, it emphasizes the effect of synmetamorphic 1eft
oblique shearing along an intra-terrane shear zone system,
the southern part of the Bear Mountains fault zone. Men
tioned but discussed in less detail is the Melones fault
zone, the eastern terrane boundary. It exhibits the same
synmetamorphic structural patterns and elements as the Bear
Mountains fault zone, consistent with dominantly 1eft
oblique shear, but these were overprinted by Cretaceous
brittle normal and right-lateral movements associated with
hydrothermal alteration and mineralization of the Mother
....... _ .. -•.... _ .... _ .... " ...... -. . ..
+'
• Sacramento
Bear Mountains Fault
Study Guadalupe Intrusive Complex
Foothills Terrane
"Sierra Nevada batholith
/
• Fresno
Foothills terrone - Metavolcanic rocks, metasedimentary rocks, minor uHramaflc rocks
Figure 8. Location map of the Foothills terrane, central California; after silberling, et ale (1984).
42
---~-- .. ~-.--.. - ............. --........... - " ...... _. __ .. _" ....... .
43
Lode region and intrusion of some peripheral facies of the
Sierra Nevada batholith. The detailed structure of the
Melones fault zone and the genesis of Mother Lode gold
deposits are the subject of a forthcoming paper (Newton, in
prep.).
Previous workers addressing the synmetamorphic structure
of this area have emphasized the reverse component of slip
on shear zones, suggesting only a small left-lateral compo
nent was present (Schweickert, et al., 1984; Paterson, et
al., 1987). This work presents evidence that there were
subequal components of reverse dip slip and left-lateral
strike slip along shear zones, and that deformation and
translation were related to sinistral transpression.
The age of synmetamorphic tectonism is constrained by
the age of deformed and sheared intrusive rocks in the area.
Early shearing was synchronous with intrusion of the north
ern facies of the Guadalupe intrusive complex, which has
been dated at 151 ± 1 Ma by U-Pb zircon methods (Saleeby, et
a1., 1989).
Figure 9 illustrates the gl:mlogy of the southern part of
the Foothills terrane, with major shear zones emphasized.
Shear zones composing the southern part of the Bear Moun
tains fault zone generally dip steeply eastward, at a small
angle to flattening foliation, are discontinuous, and are
... ~_., ___ •• __ "_.,,,,,. ~. '... •• _. • ••••• 0"' ~ •• ' • ' ____ 4' .... _ ••
II ~.
Geology of the southern Foothills terrane
Jm
r Jm
Scale
o 6km
--------------- --_._._ .. __ ." ... __ ... _--._ ........... ..
o 'w en c ~
•
I
I
I
BXPLANATION
Guadalupe Intrusive complex - northern raoies" , ga))bro/diorite and leuootonalite, looally mylonitio; southern raoies '\J, quartz monzonite to granite.
High-Kg basalts.
Mariposa Pormation - dominantly phyllite and slate (metamudstone), looal metaarenaoeous and metaoonglomeratio intervals.
Gopher Ridge Pormation (undirrerentiated) - lower part dominantly meta-basaltio andesite ooarse breooias with looal meta-andesite to meta-rhyolite lavasl upper part dominated by meta-felsio tuff and lava with looal metabasalt lavas.
Penon Blanoo Pormation - , lower Penon
44
Blanco metabasalts, locally pillowedl _ , lower to upper Penon Blanco Pormation (undifferentiated) , basal meta-chert and tuff overlain bymeta-mafio to intermediate brecoias, orystal tuffs, and lavasl includes intermediate meta-intrusive rocksl uppermost part dominated by metasedimentary rocks.
ultramafic/mafio rocks, locally amphibolitic •
Anticline I 9'. ' overturned anticline.
syncline I ~, overturned syncline.
Left-oblique shear zone.
Thrust zone.
contaots mapped by the author.
contaots extrapolated by the author or transposed from Rogers (1966), Best (1963), or Evans and Bowen (1977).
Pigure 9. Geology of the southern part of the Poothills terrane. Bear Mountains fault zone shaded.
45
right-stepping. Major right-stepping offsets took place in
t:he area occupied by Lake McClure and in the area north of
the Guadalupe intrusive complex (Figure 9). Displacement
transferred from eastern strands, along the western border
of the eastern metavolcanic belt, to a southwestern strand
of the Bear Mountains fault zone. This strand continues to
the south between the western metavolcanic belt and the
Guadalupe intrusive complex. To the north, it terminates
within the field area. The cross-over regions are north
east-trending thrust zones. A similar right-stepping offset
on the Melones fault zone can be seen in Figure 9, just
north of the Merced River.
Metamorphic grade ranges from lowest greenschist facies
with relict pumpellyite to low-pressure lower amphibolite
grade with amphibole, garnet, staurolite, biotite, and
andalusite. High-pressure epidote-amphibolite grade rocks
with blue amphiboles are present in melange in the northwest
part of the area and may be allochthonous blocks. Rocks in
most of the field area are chlorite-grade, lower greenschist
facies. As protoliths are generally quite recognizable, the
prefix "meta-" will be omitted from stratigraphic descrip
tions for brevity.
"9" __ '_'_"~_"_".'-'--"" ~ ••• - ...... ,- .- •• ----.-- •• ~.~-.-.--- .... - ••••
I ~'
46
STRATIGRAPHY
As shown in Figure 9, two metavolcanic belts, comprising
the Jurassic Penon Blanco and Gopher Ridge Formations, are
flanked by belts of pelite comprising the Late Jurassic
Mariposa Formation.
Penon Blanco Formation
That part of the eastern metavolcanic belt east of the
Bear Mountains fault zone mainly comprises the Penon Blanco
Formation. This Triassic(?)-Jurassic formation is greater
than three kilometers thick in an anti formal structure south
of the Tuolumne River, and the base is not exposed. The
unit ranges from basaltic pillow lavas at the base through a
thick middle portion of dominantly basaltic andesitic
pyroclastic rocks to an upper volcaniclastic/epiclastic
sedimentary sequence. In parts of the western edge of the
eastern metavolcanic belt, the Penon Blanco Formation is in
apparent unconformable contact with overlying mafic to
intermediate volcanic rocks correlated with the Gopher Ridge
Formation (see Chapter Three). On the east side of the
eastern metavolcanic belt, the upper Penon Blanco Formation
is in sheared unconformable contact with the Mariposa Forma
tion.
-----------------------------~ .. -...• - .. -.. - .. ---........ ........ . ................. _._ ....... .
47
Gopher Ridge Pormation
The Middle to Late Jurassic Gopher Ridge Formation makes
up the western metavolcanic belt and the western part of the
eastern metavolcanic belt (Figure 9). In the southern
Foothills terrane, the Gopher Ridge Formation is dominantly
a bimodal basalt/basaltic andesite - rhyolite sequence with
a large percentage of hydroclastic deposits. It is probably
in unconformable contact with both the underlying Penon
Blanco Formation and the overlying Mariposa Formation.
Mariposa Formation
The Late Jurassic Mariposa Formation in the field area
comprises greater than one kilometer mainly of metamorphosed
mudstone, with sandstone and conglomerate facies and rare
mafic volcanic and felsic tuff intervals. The unit is in
sheared unconformable contact with all lower metavolcanic
units. In the central pelitic belt, mafic to ultramafic
bodies were intrusively or tectonically emplaced into facies
of the Mariposa Formation. In the southern part of the
central pelitic belt, mafic to felsic plutonic bodies of the
Guadalupe intrusive complex intruded the Mariposa Formation.
STRUCTURAL GEOLOGY
Flattening Foliation
Greenschist-grade rocks have a penetrative flattening
foliation, either slaty cleavage or schistosity. In syntec-
-------------__________ ~ _____ ~··-··_··"·_··_ .. w··.···· .. ,,·,·.. . ........... ---- ..... - .
I"~
48
tonic intrusive rocks, the flattening foliation ~ncludes
compositional layering. The dominant flattening foliation
for the entire field area is plotted in Figure 10, giving a
modal orientation of 340/82. The foliation is axial planar
to folds, which generally plunge.moderately to steeply to
the south and rarely to the north. Later generation folia
tion, ranging from crenulation cleavage to penetrative
cleavage, is locally developed. For example, in the area of
the Blue Moon deposit, western metavolcanic belt, a second
spaced to penetrative flattening foliation trending north
west, best developed around shear zones, cuts and partially
transposes an earlier penetrative northeast-trending flat
tening foliation that is axial planar to northeast-trending
folds. In general, however, where a second flattening
foliation is discernable it is at a small angle to or copla
nar with the earlier foliation.
Penetrative flattening foliation is well-developed in
the field area, particularly in the western metavolcanic
belt, and it suggests finite bulk shortening across the area
in an east-northeast to west-southwest direction. The
eastern metavolcanic belt is less deformed, and a foliation
earlier than the penetrative Nevadan foliation was not
observed, despite the fact that these rocks may have been
I ~ ..
49 N
--·nodal flattening foliation
Poles to flattening foliation Stretching lineation
n=815 n=320
Figure 10. stereogram of foliation and lineation for the entire field area. Numbered oontours represent % points per 1% area. The modal flattening foliation (340/82) intersects the 153/42 mode of the stretching lineation.
Nt!
accreted prior to deposition of the overlying units (see
Chapter Three).
Lineation
so
Linear structural elements include pencil fJt:ruotures in
low-grade pelites, stretched lithic fragments in schists and
phyllites, and mineral growth lineation in greenschist to
amphibolite-grade rocks. The shape of stretched lithic
fragments ranges from prolate to oblate. The most common
shape is prolate and plotted axis ratios cluster in the
field of constriction or rotation in Flinn-type diagrams,
with k-values (k=(X/Y-1)/(Y/Z-1» > 1. Linear structures
generally are oriented subparallel to mesoscopic fold
hinges.
Lineation and measured fold hinges are plotted as the
stretching lineation for the entire field area in Figure 10.
The stretching lineation has a 90 degree fluctuation in the
southeast quadrant, from subparallel dip to subparallel
strike of the foliation. The circular mean value is 139/45.
Figure 10 indicates two modes of orientation, 116/68 and
153/42; the trace of the modal flattening foliation (340/82)
passes through the 153/42 mode.
Folds
Several fold sets developed within the field area.
Folds of bedding are cut by an axial planar penetrative
---.. _ .. __ .. _ .. -..... _.... . .... . ..... _ ..... __ .......
51
flattening foliation. This foliation is locally folded with
the development of a spaced to penetrative second flattening
foliation. Fold and foliation generational designations are
not used as folding probably formed during heterogeneous but
progressive shearing across the area rather than in separate
tectonic episodes. The two metavolcanic belts are anti
formal and the central pelitic belt is probably synformal,
plunging moderately to the southeast. Locally, earlier
folds plunge moderately steeply to the north-northeast, e.g.
at the Merced River, western metavolcanic belt, Figure 9.
These folds are cut by shear zones which comprise a system
of oblique shears, strands of which are folded around the
southern terminations of the anti formal western and eastern
metavolcanic belts.
At the eastern edge of the western metavolcanic belt,
south of Lake McClure (Figure 9), a mappable horizon between
basaltic andesite and rhyolite within the Gopher Ridge
Formation shows the geometry of asymmetric folds of So in
the trough area of a syncline which has been truncated by
the main shear zone composing the Bear Mountains fault zone
in this area. Subvertical penetrative foliation is axial
planar to the asymmetric folds, which have counterclockwise
senses of rotation, shear zones forming the long limbs, and
hinges and stretching lineation plunging moderately steeply
'4--'
S2
to the south-southeast. These folds have the geometry of
shear folds and indicate apparent sinistral sense of shear.
Folds at all scales are asymmetric and almost all have
counterclockwise senses of rotation, regardless of the limb
of larger folds on which they occur. Clockwise asymmetric
folds are associated with rare northeast-trending dextral
shear zones. In mylonitic zones, sheath folds are devel
oped.
Shear Zones
The Bear Mountains fault zone, coming into the field
area from the north, cuts down the west side of the easte~';:~
metavolcanic belt (Figure 9). This part of the shear system
died out to the south as new strands picked up en echelon to
the southwest. The main shear zone continuing to the south
forms the eastern border of the western metavolcanic belt
(Figures 9 and 11). This strand subsequently will be refer
red to as the southwest strand of the Bear Mountains fault
zone. It terminated to the north of Lake McClure, and it
increased in magnitude of shearing to the south.
The region of right-stepping transfer was through the
central pelitic belt, from Lake McClure south to the Guada
lupe intrusive complex. Shearing transferred from northeast
to southwest via parallel en echelon northwest-trending
left-oblique shear zones and northeast-trending northwest-
A
Figure 11. vertical cross-section along A-A' (Figure 9). SW BHFZ = southwest strand of the Bear Mountains fault zone; BHFZ = Bear Mountains fault zone; HFZ = Melones fault zone. stratigraphic units designated as in explanation to Figure 9.
S3
54
vergent thrust zones. One en echelon strand forms the
western boundary of the strip of metavolcanic rock north of
Lake McClure and separated from the eastern metavolcanic
belt by pelite (see Figure 9). It is truncated to the south
in the area occupied by the northeast-trending region of the
lake. Partially exposed along the shores of Lake McClure
are folded lenses of metavolcanic rock in pelite (Figure 9).
This zone appears to be a folded northeast-trending shear
zone which connects to the southwest with the southwest
strand of the Bear Mountains fault zone.
The main Bear Mountains fault zone comes into the area
from the north as two strands that sliced along the western
border of the eastern metavolcanic belt. The western strand
cut through the trough of a syncline, juxtaposing Mariposa
Formation with Penon Blanco Formation (Figure 9). The east
ern strand cut the boundary between the lower and middle
Penon Blanco Formation (Figure 9). It died out to the
south, an en echelon shear zone picked up to the south from
the hinge region of a fold, and this shear zone merged into
a northwest-vergent thrust zone north of the Guadalupe
intrusive complex. The thrust zone has northeast-striking
foliation with lineation plunging to the southeast subparal
lel the dip of foliation, and southeast side to the north-
-----_.- --------------------~
I • .'
55
west sense of shear from rotated porphyroclasts in mylonitic
rocks.
The central pelitic belt, from Lake McClure south,
contains tectonically-emplaced lenses of Penon Blanco and
Gopher Ridge volcanic rocks, and lenses of sheared and
unsheared high-Mg basalts, which originally may have been
intrusive (see Chapter Three). One of these high-Mg basalt
lenses can be seen in Figure 9 to be offset by a series of
shears with left-lateral separation. The main shear of this
series continues to the north truncating folds in the west
ern metavolcanic belt and displacing Gopher Ridge and Mari
posa Formation rocks, again with left-lateral separation.
The southwest strand of the Bear Mountains fault zone
termi~~tes to the north (see Figure 9) with a zone of broken
formation and melange. The disrupted zone contains lenses
of ultramafic rock and metavolcanic rock in a matrix of
pelite. At the south end of this zone, quartzose pebbles in
metaconglomerate are highly elongated, prolate and steeply
plunging. Foliation in the zone is commonly anastomosing,
separating schistose rock into tapered kernels moderately to
steeply plunging southeast. True melange is found locally
as narrow zones of highly deformed pelite with lithic frag
ments from adjacent macrolithons and abundant dispersed
milky white quartz pods representing disrupted quartz veins.
S6
within the disrupted zone are blocks of mylonitic
amphibolite with blue amphiboles + muscovite + low An
plagioclase + epidote + sphene + rutile, suggesting epidote
amphibolite facies metamorphism at pressures greater than 6
kb (see Chapter Three). These high-pressure rocks are
interpreted to have formed at depth and to have been tecton
ically emplaced to a higher structural level wi'i:hin the
shear zone during progressive shear.
Relationship of struotural Elements
Bedding is almost everywhere parallel to foliation.
Where transposition was incomplete, offset of bedding by
shearing parallel to foliation may be obse~ved, with appar
ent left-oblique sense of offset. Along brittle-ductile
shears, folds of foliation on either side of a shear may be
asymmetric, have counterclockwise sense of rotation, and be
steeply plunging. The direction of metamorphic mineral
elongation may be steeply plunging, subparallel to the
asymmetric fold hinges. Flattening foliation may bend into
a shear zone through a strike angle of up to 40 degrees.
Flattening foliation/shear angular relationships, sense of
rotation of asymmetric folds, and sense of drag from more
irregular drag folds, indicate a shear sense of east side to
the northwest. In areas of higher shear strain, fold hinges
and stretching lineation generally are moderately to gently
------------------------------~ .. -.. -..... - ... -........ ".. ... . ........... -- ..... .
57
plunging to the southeast. In mylonitic zones, particularly
associated with the borders of facies of the Guadalupe
intrusive complex, sheath folds have hinges parallel to
stretching lineation (see Figure 12C). Fold hinges and
lineation in the outcrops illustrated by Figure 12 plunge
20-40 degrees to the southeast. Sheath folds (Figure 12A)
have small asymmetric folds along shears deforming their
limbs (Figure 12B). In Figure 12B, asymmetric folds on both
limbs of the larger fold have counterclockwise senses of
rotation. Porphyroclasts in mylonitic rocks in the field
area are rotated along the stretching lineation. Shear
sense indicators such as fine-mineral tails streaming away
from rotated porphyroclasts, s-c angular relationships, and
sense of rotation of asymmetric shear folds consistently
indicate east side to the northwest sense of shear.
Domain 1. Domain 1 includes all parts of the field area
outside of Domain 2. In these areas, the dominant foliation
is northwest striking. These rocks vary from very low total
shear strains away from shear zones to high strains in shear
zones. strain intensity also varied across the region.
Most of the eastern metavolcanic belt experienced low
strain, and the dominant structure is a faulted antiform,
first recognized by Taliaferro (as reported by Cox and
Wyant, 1948), that has been partially dismembered and thrust
..... _ ...... -.,.-.,,-_ .. '-..... . . .... . .. -. ,-- ..
A.
B.
-----__________ 0 _____ ._ •••• 0_0
c.
Figure 12. Sheath folds, moderately gently plunging to the southeast, within the southwest strand of the Bear Mountains fault zone, In country rock near the southwest border of the Guadalupe Intrusive complex. (A) Sheath folds; plunge approximated by knife. (B) Opposing limbs of a larger sheath fold shown In (A); note the counterclockwise asymmetric folds of the quartzo-feldspathlc layers along sinistral shears on both limbs. (e) Quartzo-feldspathlc ellipsoids defining the gently plunging stretching lineation parallel to fold hinges.
------------_._-- --- -'_." .. _ .... _ .. _ ... _ ...... .
58
59
westward. Boge~ (1985) also recognized the anti formal
nature of the belt. strain intensity increased to the west
and rocks of the western metavolcanic belt are strongly
deformed, with minimum estimated shortening of 50-60 percent
(Paterson, et al., 1987).
Domain lA. Domain 1A (see Figure 9), was a high shear
strain region, along a northwest-trending brittle-ductile
shear zone segment of the southwest strand of the Bear
Mountains fault zone. In this domain, lithologies are domi
nantly greenschist-grade pelite with lesser amounts of
metavolcanic and meta-intrusive rock.
tion has a mean orientation of 148/45.
The stretching linea
Modal foliation in
this portion of the shear zone is 345/81 and intersects the
mode of the lineation cluster at 156/44, yielding a rake of
45 degrees to the southeast (Figure 13). The exact orienta
tion of the general shear zone is uncertain, but the zone
has an average map orientation of 340 degrees, and the dip
would be expected to be slightly less than the dip of the
foliation. The dip of the shear zone was independently
determined to be 78 degrees by a method discussed in Appen
dix B. At a shear plane dip of 78 degrees, the rake of the
modal lineation on a 340 degrees shear plane would be 45
degrees to the southeast.
N
Poles to flattening foliation stretching lineation
n=49 n=50
Figure 13. stereogram of foliation and lineation in Domain 1A. Numbered contours represent % points per 1% area. Both the modal flattening foliation (345/81) and the general shear plane (340/78) intersect the modal regions of the stretching lineation oluster at a rake of 45- SEe
60
,---~.--~-".--,-.-" .... ,." .' ... . -.... " .... __ .......
61
Plots of axial ratios of lithic fragments from shear
zones (Newton, unpublished data) on a strain diagram pro
duced by factorizing pure shear and simple shear components
(Sanderson and Marchini, 1984) indicate that simple shear
was dominant over pure shear. Assuming simple shear and
plane strain, for first-approximation modeling, the shear
strain, eTa) may be estimated by the relation:
Ta = 2/tan 29 (1)
(after Coward and Potts, 1983; Jaeger, 1956), where e is the
angle between the poles to flattening foliation and shear
plane. For a dip on the shear zone of 78 degrees in Domain
lA, e is 6 degrees, which yields a shear strain estimate of
9. With this method, any dip on the shear zone greater than
71 degrees (considered to be lower than the actual value)
yields a shear strain greater than 5, a relatively high
value.
If, a priori, the stretching lineation reasonably
approximates the displacement direction at high total shear
strains, (e.g., Hobbs, Means, and Williams, 1976), the 45
degree rake suggests subequal components of strike slip and
dip slip. Figure 14, after Ridley (1986), indicates that at
a total shear strain (rtot8') > 6.5 and subequal components of
strike slip and dip slip (Tw/r~l), fold hinges, stretching
lineation, and displacement direction would all be within 10
~-------------------------\ t,_o , 80 \ \
1\ \ o I" ,
\\ \
70 \'\ " I, \ \ , 1\' \ \ "
00 I I \ \ \ " '\ ' \ \ , \\' \' ,
I I, ' ,\ " DO I \ \. \ \' "
II II \ \', ........ , \ \ \ ..... II I \ , \ , ......... p\ I , , , ...
40 \1' \ \ " .... \ \ \ \ , , ... -:- 0.1 \ \', , ,
\ \ \ '\ " ' .... \ \' , , ..... ,
\ " " " .......... , '." " ......... ..... ........... ,', '......... ............. - .... 0.2 1L
"" .. : .... .... ....... 0.25 1, 20
10
o 1 :I 4 o o 10 )',0/0'
Figure 14. Diagram illustrating relationships between fold hinges, finite stretching direction, and line of slip, after Ridley (1986) and Coward and Potts (1983).
e = angle between fold hinge and shear slip direction. T(total) = total shear strain. TW ITt = ratio of wrench shear to thrust shear.
62
In the cross-hatched area, fold hinges will be within 10· of the finite stretching direction. In the shaded region, for a total shear strain >6.5 and approximately equal wrench and thrust shear components in Domain lA, fold hinges, stretching lineation, and the line of slip would all be within 10· of each other.
---~ .. -.. -•.... -.-.......... , ..... -. . .. - , ......... _-_ .... _ ..
I ~;
63
degrees of each other. Therefore, in Domain lA, the
stretching lineation may indeed be assumed to be subparallel
to the displacement direction, with a rake of 45 degrees to
the southeast with respect to the general shear plane. This
indicates subequal components of stri.ke slip and dip slip
along this portion of the Bear Mountains fault zone.
Independent evidence locally supports a more strike
parallel slip line than the 45 degree rake estimated above.
A large syn-kinematic gabbroic dike developed extension
fractures and pegmatitic veinlets late in its crystalliza
tion history. The veins have subhorizontal coarse straight
elongate pyroxene and plagioclase feldspar crystals that
grew normal to the vein walls. The pegmatitic veins strike
35-40 degrees, dip vertically to steeply to the southeast,
and are perpendicular to northwest-striking, vertical shears
that truncate the veins. The slip line (kinematic x direc
tion) would be 90 degrees along the shear plane from the
intersection of the veins and the shear plane (Ramsay and
Huber, 1987). This intersection (kinematic y direction) is
subvertical, and the kinematic x direction is subhorizontal,
northwest-southeast. The subhorizontal slip line supports a
dominant wrench shear component during late-intrusive shear
ing along this part of the Bear Mountains fault zone •
...... _ .. -•.... - ... -.-. -.... ~.. . . -. ... .. _ .................... --_ •..... - ..
64
Domain 2. Domain 2 (see Figure 9) is an anomalous zone
dominantly comprising meta-intrusive rock, lenses of upper
greenschist to lower amphibolite-grade mica schist (pelite)
and greenstone to amphibolite (metavolcanic rock). The
domain contains a zone 2500 meters wide (northwest to south
east) dominantly of mixed metaplutonic feldspar-quartz
biotite schist and metasedimentary quartz-muscovite schist.
The dominant foliation within the zone strikes northeast and
dips gently southeast, while foliation north and south of
the zone generally strikes northwest. The modal orientation
of foliation in the domain is 46/30 (Figure 15). The folia
tion includes compositional layering as well as schistosity.
In most places, flattening foliation and shear surfaces can
not be distinguished, but where they are, S-surfaces are
more steeply dipping than C-surfaces. Locally, the rocks
are protomylonitic to mylonitic with sheath folds which have
hinges parallel to a well-developed metamorphic mineral
lineation. stretching lineation in this high shear-strain
domain is subparallel to the dip of the foliation. This
lineation is parallel to lineation in the high shear-strain
portions of Domain 1, where lineation is oblique to
northwest-~triking foliation. Mineral elongation lineation
and parallel fold hinges are gently plunging southeast, with
a mode of 170/30 (Figure 15), subparallel to dip of the
N 6S
Poles to flattening foliation
n=151
n=66
Figure 15. stereogram of foliation and lineation in Domain 2. Numbered contours represent % points per 1% area. Modal flattening foliation is 46/30; modal stretching lineation is 170/30.
---~ .. -.. -.... -.. - .. -...... ".... ... . ................ __ ... _ ... .
.. '
p.
foliation. This lineation is essentially colinear with
lineation in adjacent portions of the bounding northwest
striking shear zones.
66
The high metamorphic grade, with andalusite and its
sericitic pseudomorphs common in metamorphic assemblages,
undoubtedly is related to these intrusive rocks. Paterson
and others (1987) suggested the possible importance of near
surface intrusive rocks in this domain in deflecting
foliation into anomalous patterns. However, the presence of
mylonitic and protomylonitic facies, and the persistence of
a northeast-trending foliation in mappable lithologic hori
zons across the domain, as illustrated in Figure 9, suggests
that the anomalous orientations are related to shearing.
Simple deflection would not account for southeast-dipping
foliation around both southern and northern ends of the
bodies. The elevated temperatures associated with these
intrusive bodies as well as in the northern, western, and
southwestern aureoles of the Guadalupe intrusive complex
facilitated the development of mylonitic fabrics and ductile
folding. Sheath fold hinges rotated into alignment with the
stretching lineation indicate that the lineation may be
taken as the line of slip (Cobbold and Quinquis, 1980),
suggesting a north-northwest to south-southeast line of
slip. Kinematic indicators mentioned earlier consistently
, __ ,,,, .• _ •• - ," '_"'_ .JIJI1 . • ".~ .., •••.• ". •
67
indicate southeast side to the northwest sense of shear
along the northeast striking shears in Domain 2, which here
are interpreted as northwest-vergent thrusts.
COMPARISON OF FOOTHILLS' STRUCTURE WITH TECTONIC MODELS
Figure 16A is an outline of the major features of the
shear zone system from Figure 9. Figures 16B and 16C are
examples' of the orientation of structures predicted in
sinistral transpressive regions, after Sanderson and
Marchini (1984).
The southwest strand of the Bear Mountains fault zone is
an oblique brittle-ductile shear zone system which displays
mylonitization to the south and terminates to the north.
Figure 16B illustrates expected zones of extension and
shortening, plus fold orientations, at the end of a left
lateral wrench fault (Sanderson and Marchini, 1984). In the
model, extensional zones occur on the west side of a north
ern terminus and northeast-striking compressional structures
occur on the east side.
The zone of broken formation and melange at the northern
termination of the f~outhwest strand formed on the west side
of the shear system. This zone probably formed by exten
sional flow within or along the shear zone during periods of
tensional stress. By analogy with the sinistral wrench
fault model (Figure 16B), extension would be expected on the
---~ .. -., ~ .... - ... -" ............... .
I ~:
A.
,
s
c.
\
"-"" ~ , ,
Figure 16. (A) outline of the major struatures along a portion of the Dear Kountains fault zone. (D) ~2peate4 zones of extension (B) and shortening (8) in transpressive regions at the terminations of a sinistral wrenah fault, after Sanderson and Karahini (1984). (0) orientation of thrusts in transpressive regions at an offset of a sinistral shear zone, after Sanderson and Karahini (1984).
68
---------------------------------~ .. --~-.... - .. -.... "' ..... -.. , .. -.... - .. -... , _ .. _--_ ... _ ..
69
west side of the northern terminus of a sinistral shear
system, which in this case probably was a sinistral-oblique
shear zone.
Folds adjacent to and truncated by the southwest strand
of the Bear Mountains fault zone around Lake McClure (Fig
ures 9 and 16A) are northeast-trending as would be expected
of folds along a sinistral wrench fault (cf. Figure 16B).
Figure 9 shows, cutting through Domain 2, the trace of a
gently southeast-dipping thrust zone. This zone roughly
strikes 46 degrees, and has the same orientation as the
modal foliation in the entire domain, 46/30. The thrust
zone trend intersects the northwest-trending shear zones
that bound Domain 2 at an angle of approximately 75 degrees.
Figure 16 juxtaposes these generalized field area trends
(Figure 16A) with model orientations of transpressional
regions in a 'compressive' bend in a sinistral wrench fault
system undergoing simple shear (Figure 16C). The trace of
the thrust zone is similar to the predicted orientation of
thrust faults. This geometry is analogous to that of a
restraining bend in a sinistral wrench fault system.
Figure 17 shows the suggested orientations of the North
American and Farallon plates during Late Jurassic/Early
Cretaceous time, from paleomagnetic studies by Engebretson,
et al (1985). Note the sinistral oblique approach of the
I ~;
70
140 Ma (135-145)
ElJ • f II f ~ bf '\ # ~ .t;>( VI t""\ '"" I" ...
V' ~~ f5l'r1J "- ~ -~ f\ ~
~ ~ " ... II it
Va ~~ " , tv - -- (
~ ~ 'i/ FAF ALL DN I. - - ~
20· ~ .., -. -- ~ NA --0 ~~ - f7l.H
-./ l~ • ~~ ~\ c::.:.", ... ~ ~ - -. ..... _ ... r ~
~AGI ...
.... "7/ " ~l, I?~ .... -. IZA ... ... ... .9~
~~ ~~ JL ~ ~ -. - -- ..... 1-
~~ ;;0-
" ,
I;,a ~ ~ ~'::<I> I) '\'
.A II' PA
140· 220· 260· 300·
Figure 17. Suggested plate motion between 135 and 140 Ma ago, from Engebretson, et ale (1985). Lengths of arrows indicate relative distance of movement in that time interval. Note th~t the absolute motion of the North American and Farallon plates with respect to each other was sinistral oblique convergence.
-----------------------~----~~----.---.. -....... "" ................ - .-- ...... - ... --_ ... -_ ..
71
pl~tes. Figure 18A shows the suggested orientations of
convergence velocity vectors at 150 Ma and 125 Ma ago (Enge
bretson, et al., 1985). The two vectors that bracket the
area of central California range in azimuth from 85 to 92
degrees, yielding an average of 88-89 degrees, during the
period 125-150 Ma ago. The line in Figure laB is oriented
88 degrees, through the area of the southern Foothills
terrane.
The average convergence velocity vector trend does not
parallel the minimum principal strain axis (perpendicular to
the regional flattening foliation), nor does it parallel
either the steeply or moderately plunging modes of the
stretching lineation (maximum principal strain axis). How
ever, it does parallel the expected maximum principal stress
direction to produce a sinistral transpressive shear zone
system parallel to the Bear Mountains fault zone.
In Figure 19A, the maximum principal stress direction
(a1) is drawn parallel to the convergence velocity vector
trend from Figure 18B. Figure 19A shows the orientations of
structures expected in a sinistral transpressive shear zone
undergoing bulk shortening across the zone, after Sanderson
and Marchini (1984). Figure 19B juxtaposes the outline of
the major structural elements of part of the Bear Mountains
fault zone. The major shear zones of the Bear Mountains
___ ~' __ .' __ .. ___ .. _h .. ·._~ .... ··_ ............ _ ... _.~ .. -.. ___ .. _ .... .
72
A.
125 Ha
190 HI
Figure 18. (A) Convergence velocity vectors of the oceanic Farallon plate relative to the North American plate, after Engebretson, et a1. (1985). The mildd1e two vectors on each diagram bracket the area of central Californ~.a containing the southern Foothills terrane. These four vectors range from 85 G
to 92°, yielding an average of 88-89·. (B) The line of the mean vector (88·) through the area of the southern Foothills terrane.
---~.- ... -.. ~---.... , .......... '.' .- .. ' . _ ........ -""--'''-~ ,
A.
Figure 19. (A) For a maximum principal stress direction of 8S-, the expected orientation of a sinistral transpressive shear zone with associated structures, undergoing shortening across the zone, after Sanderson and Marchini (1984). (B) Outline of the major structures along a portion of the Bear Mountains fault zone.
73
---~ .. -.. -..... ---........... " ...... _ ........ -. -- ........ __ .... -•..
74
fault zone are subparallel to the model boundary shears of
Figure 19A. The folds trending north-northeast in the
western metavolcanic belt parallel the model orientation of
folds in Figure 19A. The shear zone with left-lateral
separation just south of Lake McClure parallels the model
orientation of a synthetic Riedell shear (R) in Figure 19A.
DJ:SCUSSJ:ON
The structural data presented above are consistent with
either subequal components of strike slip and dip slip, or a
more dominant component of strike slip, along northwest
trending segments of the Bear Mountains fault zone in the
southern part of the Foothills terrane. Sense of shear from
various kinematic indicators is consistently east side to
the northwest. The evidence indicates that deformation and
shearing were related to sinistral transpression. The
common counterclockwise asymmetry and moderate to steep
plunges of folds suggests that most folds in the area formed
in response to left-oblique shearing, rather than merely
forming in response to orthogonal shortening. However,
concomitant with shearing, bulk shortening took place across
the terrane in an east-northeast to west-southwest direc
tion. Translation was systematically transferred from
northeast to southwest via right-stepping en echelon left
oblique shear zones. The main region of transfer was from
---~--.. -.... - .•. -.. -....... -.............. - .............. -_ ..... -"
7S
Lake McClure south to the Guadalupe intrusive complex. The
region occupied by the northeast-trending part of Lake
McClure and the area north of the Guadalupe intrusive com
plex appear to have behaved as restraining or compressive
bends in the sinistral oblique shear zone system.
The right'·stepping offset through the area of Lake
McClure is hi.ghly folded and sheared with east-west bulk
shortening, which took place after the northeast-trending
shear zone formed. This deformation, related to progressive
shear, obscured earlier structural relationships by result
ing in a highly folded contact roughly trending northeast,
with outcrops dominated by the well-developed northwest
trending flattening foliation. This relationship has been
observed elsewhere along the Melones fault zone (Newton, in
prep.).
The southwest strand of the Bear Mountains fault zone,
separating the western metavolcanic belt and the central
pelitic belt, propagated a short distance northward where it
terminated with a zone of extension with melange on the west
side of the shear zone. This zone contains blocks of amphi
bolite with assemblages indicative of higher pressures of
formation than indicated for the surrounding rocks, suggest
ing the amphibolitic blocks were brought up from depth.
This extensional zone probably was a region of dilatant
76
back-flow during oblique underthrusting in the basin repre
sented by the central pelitic belt.
This sinistral transpressive model with a suggested
large wrench shear component is at odds with previous work
in the southern part of the Foothills terrane, by
Schweickert, et ale (1984) and Paterson, et ale (1987) who
suggested that the main sense of shear along Foothills shear
zones was reverse with a small left-lateral component. At
low shear strains, there is no reason to assume that
stretching lineation at a small angle to foliation dip
indicates a predominant dip-slip component. In the Foot
hills terrane, even adjacent to shears, stretching lineation
commonly is steeply plunging and subparallel to the hinges
of steeply plunging asymmetric folds. As demonstrated above
in Figure 14, in systems with a large wrench shear to thrust
shear ratio, at low shear strains, fold hinges and stretch
ing lineation may be subparallel while still being oblique
to the displacement direction at angles up to 40 degrees.
Fold plunges steeper than slip lines have been reported from
other transpressional shear zones. For example, in sinis
tral transpressional shear zones in the Southern Uplands of
Scotland, fold hinges and stretching lineation are steeply
plunging in some shear zones while folds are gently plunging
outside the shear zones (Kemp, 1987). In the southern
-----------------------------~--.. -~---.-.---.-.. -.---...... - . --- --- --" -- ---_ ..... -. -
.:
77
Foothills terrane, as demonstrated above, in zones of higher
shear strain, lineation and fold hinges cluster around a
rake of 45 degrees to foliation and shear surfaces, consis
tent with transpression. The evidence suggests that the
dominant mechanism of Late Jurassic-Early cretaceous strain
and displacement in the southern Foothills terrane was
sinistral transpression. Sinistral oblique underthrusting
took place during closure of the basin represented by the
central pelitic belt.
Nevadan Orogeny
Jurassic deformation and ancient mountainous areas in
California were recognized as early as 1880 by Whitney.
Blackwelder, in 1914, was the first author to suggest that
Late Jurassic deformation was of more limited extent and
less intense than earlier Jurassic deformation, and he
coined the name "Nevadian Orogeny" to indicate its restric
tion to the Sierra Nevada area.
crickmay (1931) produced a series of paleogeographic
reconstructions of western North America at different times
in the Jurassic period. On the basis of unconformities,
volcanism, and changes in coarseness and thickness of sedi
ments, he demarcated mountainous areas which he related to
orogeny. He suggested there had been a m~jor mountainous
area in Early Late Jurassic time in the region of the Sierra
---~-.. -... ---... -.. -........ --.... ... . .. _-- ....... _ .... _ ...... -_ ..
78
Nevada, the Coast Range, the Klamaths, and what is now
called greater stikinia. Crickmay termed this tectonic
episode the Agassiz orogeny, for the Agassiz Prairie area of
British Columbia in the Coast Plutonic Complex, where clas
tic sedimentary rocks bearing Oxfordian-Kimmeridgian fossils
unconformably overlie Callovian volcaniclastic rocks (crick
may, 1931). He suggested that folding in this orogenic
episode began in the Middle Jurassic. Crickmay (1931) used
the term "Jurasside" for the Late Jurassic "Nevadian" oro
geny of Blackwelder. crickmay believed the Agassiz orogeny
was the major Jurassic deformational event, and that the
Jurasside was merely a "disturbance".
Adopting the contracted term "Nevadan" after Hinds
(1935), Taliaferro (1943a) suggested there was no evidence
for Crickmay's Agassiz orogeny, and that there was no
evidence for intense folding in the Sierra Nevada in the
Jurassic earlier than the Late Jurassic event that intensely
deformed the Mariposa Formation.
Bateman (1963) and Bateman and Clark (1974) promulgated
the idea that the major Sierra Nevada structure is a giant
synclinorium formed during the Nevadan orogeny. The synfor
mal interpretation was based on inward facing limbs and
stratigraphic units younging toward the center of the batho
lith. Schweickert, and others (1984) again invoked the
.. __ ......... _ •• ___ ,. ••• _ •• , ••• '. 0-.
I ~.
79
synclinorium as the major Nevadan structure. They proposed
that the Nevadan orogeny was an intense but short-lived
event, specifically dated around 155 Ma ago, that affected
the entire width of the Sierra Nevada, possibly reaching to
deep crustal levels during east-west shortening.
Schweickert and others (1984) indicated that Nevadan defor
mation in the Sierra Nevadan region was most intense in the
north and decreased to the south. Tobisch and others (1986)
suggested that Nevadan deformation in the southern part of
that region was most intense in the western foothills and
decreased significantly east of the Melones fault zone.
They suggested that Nevadan effects may be entirely absent
in the eastern Sierra Nevada.
controversy continues about the timing, duration,
extent, style, and cause of the Nevadan orogeny. There is
great disparity in dates of suggested Nevadan deformation in
different areas. For example, a 154 Ma younger age limit on
Nevadan deformation in the Northern Sierra terrane was
suggested by Schweickert, et a1. (1984), based on an unde
formed pluton which cuts across foliation. This is in
contrast to the 147-151 Ma age range of plutons bracketing
deformation attributed to the Nevadan orogeny in the western
Klamath Mountains (Wright and Miller, 1986).
---~.---.------,--.-.-- ... --.. .... - -- ......... _---"--"
! ~:
80
The Guadalupe intrusive complex is part of a suite of
intrusive rocks in the Foothills terrane dated at -145-151
Ma (Saleeby, et al., 1989) which are deformed and sheared.
These dates agree with the dates of Klamath Nevadan tecto
nism. Deformed intrusive rocks as young as 123 Ma in the
southern Foothills terrane have been dated by Saleeby and
others (1989), indicating that Foothills tectonism continued
into Early cretaceous time as suggested by Paterson, et al.
(1987), Tobisch, et al., (1989) and saleeby, et al. (1989).
There also are brittle-ductile overprinting manifestations
of Late Early cretaceous tectonism, particularly apparent
along the Melones fault zone (Newton, in prep.). However,
major southern Foothills synmetamorphic tectonism overlaps
Nevadan ages, and it is here considered Nevadan rather than
post-Nevadan as suggested by Paterson, et al. (1987),
Tobisch, et al. (1989) and Saleeby, et al. (1989).
The end of penetrative deformation in one area cannot be
taken as the end of related deformation elsewhere in an
orogen. This may be particularly true in an obliquely
convergent orogen where shortening and shearing may alter
nate with extension. Along the length of such an orogen,
transpressive tectonism would be diachronous and regionally
heterogeneous.
. .. -_ .. _._. __ ... _ ......... ~ ...... ,
81
As summarized by Moores and Day (1984), the models for
the Nevadan orogeny fall into two categories: (1) oceanic
island arc collision with the North American continental
margin, and (2) "in-situ" development of a volcanic arc and
arc-rift basins with transpressional deformation. Edelman
and Sharp (1989) suggest that in the Sierra Nevada foot
hills, the Tuolumne River terrane, containing the Penon
Blanco Formation and ~~Jurassic melange, had been accreted
prior to the deposition of Callovian and younger units,
which formed an overlap assemblage. Previous accretion of
rocks forming the basement upon which Callovian and later
arc sequences developed eliminates the possibility of a
Nevadan arc-continent collision.
In the Sierra Nevada foothills, the evidence for Juras
sic deformation earlier than Late Jurassic is indeed
cryptic. Edelman and Sharp (1989) present evidence, such as
remnant fault strands distinct from and cut by Late Jurassic
faults, that Sierran terrane boundary faults were estab
lished prior to 165 Ma, with some faults cut by plutons as
old as 177 Ma. In the Merced River and Northern Sierra ter
ranes, east of the Foothills terrane, Schweickert and others
(1984) placed a minimum age constraint on pre-Late Jurassic
deformation by cross-cutting undeformed intrusive rocks
ranging from 164 to 170 Ma. In the Foothills terrane,
82
melange units which contain probable Early Jurassic volcanic
fragments have pre-Callovian mylonitic fabrics (Behrman,
1978; Saleeby, 1982). Ave Lallement and Oldow (1988) cite
evidence indicating left-oblique tectonism in the western
Cordillera from Middle Jurassic into Early cret~ceous time.
Edelman and Sharp's (1989) pre-Callovian accretion model
and Moore and Da~·'s (1984) second category, the "in situ"
Nevadan model, are supported by this study. The preferred
interpretation from this study is that from Early Jurassic
to Early Cretaceous time, continued alternating and coeval
development and deformation of arc sequences and rift-basin
sequences took place on accreted oceanic basement at the
edge of the continental plate over an east-dipping subduc
tion zone. From Late Middle Jurassic into Early Cretaceous
time, this upper-plate segment was undergoing sinistral
transpression due to sinistral oblique relative motion
between the Farallon and North American plates. It is
likely that outer margin terranes were sinistrally offset
during th~s time period, with displacement to the south
relative to the North American plate (Newton, 1986; Ave
Lallement and Oldow, 1988).
The Nevadan orogeny in the study area has a maximum age
limit indicated by Oxfordian-Kimmeridgian fossils in de
formed pelitic rocks. This work indicates that sinistral
---~ .. -.. -•... --.- ...... , .......... , ... ' ....... . .. __ .. -_.
83
transpressive tectonism was the dominant style of Nevadan
deformation and shearing in the southern Foothills terrane.
Nevadan tectonism was long-lived and continued into Creta
ceous time in the southern Foothills terrane and in the
western Klamath terrane.
Sinistral oblique convergence between the Farallon and
North American plates during Late Jurassic-Early Cretaceous
time is suggested by Engebretson and other's (1985) plate
reconstructions. Also solely on paleomagnetic grounds, May
and others (1989) have suggested that such sinistral oblique
relative plate motion between 150 Ma and 135 Ma ago would be
expected from the rapid northward migration of North America
indicated by the apparent polar wandering path reconstruc
tions of May and Butler, 1986. sinistral transpressive
tectonism on the western North American margin would be an
expected consequence of Late Jurassic-Early Cretaceous
sinistral oblique relative plate motions (Newton, 1986; May,
et al., 1989).
-----------------------~-----~ .. -.. ~ ...... ---............... . ... . ............... __ ... -.
CHAPTER THREB
Mesozoio Teotonio Environments of Massive Sulfide
Deposition in the Southern Part of the Foothills
Terrane, western Sierra Nevada, California
INTRODUCTION
This paper documents the evolution of stratigraphic
units and interpreted tectonic environments of deposition in
the western Sierra Nevada foothills, central california,
south of the Tuolumne River (Figure 8). This is the
southern part of what was named the Foothills terrane by
Silberling, et ale (1984).
The terrane comprises superimposed Mesozoic and Paleo
zoic intraocean assemblages ranging from mid-ocean ridge
ophiolitic rocks, through multiple island arc and arc rift
zone sequences, to interarc basin sequences. Within these
different tectonic settings, three types of syngenetic poly
metallic massive sulfide deposits formed: cyprus-type Cu
(Cu-Zn) deposits, Kuroko-type Zn-Cu-Pb deposits, and Besshi
type Cu-Zn deposits. These disparate petrotectonic
assemblages are now closely juxtaposed owing to a combina
tion of unconformable depositional overlap, deformation and
displacement.
This study focuses on characterizing the settings of the
syngenetic massive sulfide deposits, in the hope of aiding
8S
expluration for such deposits. Regional structural geology
and the style of Late Jurassic to Early cretaceous tectonism
(Nevadan orogeny) are treated elsewhere (Chapter Two).
Approximately 1000 square kilometers of the terrane were
mapped by the author in detail and reconnaissance, and
samples of all igneous units in the area and in correlative
strata to the north were analyzed for major and trace ele
ments. Analyses were performed by several methods and
laboratories, listed in the data tables. The methods of
instrumental neutron activation analysis (INAA) performed by
the author are discussed in Appendix C. For trace elements
analyzed by INAA techniques, one standard deviation in
counting statistics is reported in the tables. For elements
analyzed by inductively coupled plasma CICP) and lithium
metaborate fusion X-ray fluorescence (XRF) techniques, esti
mated errors supplied by the laboratories are 5-10 percent.
In plots of rocks from the field area, for trace element
values reported as being less than a lower detection limit,
a value of 80 percent of that limit arbitrarily has been
used for plotting, e.g. Th reported as <0.05 ppm is plotted
as 0.04 ppm.
As a tool in discriminating tectonic settings, extensive
use has been made of the trace element characteristics of
basic igneous rocks. A type of diagram employing three two-
86
element ratios was developed by the writer and was presented
in Chapter One. The trace element plot employed here is
capable of distinguishing basalts formed in mid-ocean ridge,
intraplate, marginal basin, forearc, and arc settings. This
discrimination scheme is used to help infer the tectonic
environments of formation of basic rocks, and by implica
tion, associated mineral deposits in various stratigraphic
units in the Foothills terrane.
Topics in this paper are addressed in the following
order: element immobility, stratigraphic units, massive
sulfide deposits, rock assemblages based on interpeted
tectonic environments of formation of stratigraphic units,
and the tectonic settings of massive sulfide deposits.
ELEMENT IMMOBILITY
Rocks from the field area, analyzed for this study, have
been metamorphosed to lower greenschist grade. Metamorphic
assemblages commonly contain chlorite, epidote, and musco
vite, with no biotite, garnet, or amphibole. Samples
analyzed were lavas or homogeneous tuffs; care was taken to
get the freshest, least metamorphosed and least altered
samples possible.
Plagioclase phenocrysts are weakly to pervasively
replaced by epidote, white mica, and rarely calcite. Amphi
bole and pyroxene phenocrysts are less altered than the
.----------------------------~ .. -.. -.... --- .............. '" . -- ... - ..
87
feldspars, and commonly have large areas of relict igneous
mineral, with rims and patches of chlorite, epidote, rare
prehnite, quartz, and iron oxides. Matrices of the volcanic
rocks are commonly fine grained quartzo-feldspathic material
± chlorite, epidote, and white mica. petrographic descrip
tions of representative analyzed samples are given in
Appendix D.
Elements may have been mobile during low grade metamor
phism, during hydrothermal activity on the sea floor, or
during weathering either while on the sea floor or while
subaerially exposed. Of the major elements, sea floor
alteration may result in net enrichments in K, Fe, Ti, and
Mn, and net depletions in Ca, Si, and Al (Gelinas, et al.,
1982). Na and Mg may be either enriched or depleted. Of
the trace elements, generally the rare earth elements and
high field strength elements such as Zr, Ta, Nb, Th, and V
are considered to be relatively immobile (e.g. Floyd and
Winchester, 1978). However, Finlow-Bates and Stumpfl (1981)
suggested that V, Th, and Nb (presumably therefore also Ta)
can be mobile during alteration. In their study, only Ti
and Zr appeared to be consistently immobile.
To evaluate the immobility of elements used in the plots
in this study, a series of variation diagrams is presented.
In suites of consanguineous igneous rocks, if elements
1/<'
I
88
remained immobile, they should produce coherent variation
trends related to processes such as fractionation, partial
melting, mixing, contamination, and assimilation. Deviation
from such trends would be an indication either of element
mobility during alteration or of analytical imprecision
(e.g. Arndt and Jenner, 1986). A caveat is that two ele
ments may covary systematically during alteration, and would
still yield coherent trends when plotted against each other
(Arndt and Jenner, 1986). In this case, both elements
should show scatter when plotted with other elements with
which they did not covary.
For two immobile incompatible elements, a simple plot of
one against the other should produce a linear trend which
passes through the origin (Finlow-Bates and stumpfl, 1981).
However, for elements that are not strictly incompatible,
trends in this type of plot may be curvilinear and will not
necessarily pass through the origin.
A better plot for this purpose is one of the form cJ/c'
vs. cJ, in which the variation of the two elements through-
out the range of magma compositions is evaluated in a linear
array. For example, during either equilibrium melting or
fractional crystallization, for H elements (for which the
partition coefficient ~ 0), a plot of cHIC' vs. cH will
always be a straight line which does not intersect the
--_.---_ .... _ ... _., ...... ,...... . ... , .. _,.- .. "--'''--'
89
origin (Minster and Allegre, 1978). If the element chosen
for cl has an invariant partition coefficient, the line will
always be straight for any element with which it is plotted
(Minster and Allegre, 1978). Therefore, linear regression
statistics yield a measure of the degree of scatter which
may be attributable to element mobility.
Heavy rare earth elements such as Yb and Er are commonly
considered to have essentially invariant partition coeffi
cients in a related compositional suite, commonly have
nearly constant abundances (partition coefficient ~ 1), and
yield straight lines when used for cl (Minster and Allegre,
1978).
To evaluate immobility of trace and major elements in
the field area, the suite of related volcanic rocks with the
most analyses was chosen for statistics. Ten analyses of
basalts, basaltic andesites, andesites, and rhyolites of the
lower Gopher Ridge Formation plus rhyolites of the upper
Gopher Ridge Formation from the study area (Table 3) and
from north of the study area (Newton, unpublished data)
constitute a related volcanic arc suite. Basalts overlying
the upper Gopher Ridge rhyolites are not included as they
obviously are not consanguineous. Major and trace elements
were adjusted to a volatile-free basis and normalized to 100
percent. The linear regression coefficient (R2) and the
---~ .. -.. ~ .... --.- .................................... __ ......•..
standard error of the calculated ordinate values (Sy) are
given for each plot in Figures 20 and 21.
90
Figure 20 demonstrates the variation of the trace ele
ments Th, Ta, La, Ce, Tb, and Yb for the Gopher Ridge suite.
The coherence of these trends suggests that these elements
were immobile, and therefore Yb is valid to use as an immo
bile element in subsequent plots.
Figure 21 displays cl/e' vs. e1 plots of Ti, V, Th, Ta,
Ce, FeO., MgO, and A1203 against Yb for the Gopher Ridge and
Penon Blanco suites. Tb exhibits too little variation with
Yb to be used in this plot, but its immobility was demon
strated in Figure 20. The linearity of these plots suggests
that the trace elements Ti, V, Th, Ta, Ce, and Tb used in
this study can be considered to have remained immobile in
the Gopher Ridge rocks.
In the major element plots of Figure 21, except for a
couple of samples, there is generally good linearity. The
linearity of the plots suggests that Ti and Mg, and for the
most part Al and Fe, can be considered to have remained
immobile in the Gopher Ridge rocks, and are valid to be used
in the TFMA diagrams in this study.
other units analyzed in the field area either were not
represented by enough samples or were not genetically
,.-_., _.- .. _ .. - .. .., ..... ~-., ..... , .. -~ .... ,',' -' .~-.-, ... - - .
....
Ce VI. Ta, Gopher Ridge F'm.
48.00 R2 :.92
E Ihz.oo
~
le.oo
E ft:,.IO
o.eo
0.40
Sy:~.19
Th VI. La, Gopher Ridge F'm.
10.00 La ppm
15.00
Ta VI. La, Gopher Ridge F'm.
R2=.91
0.32 Sy :.O~2
E ft:O.24 ~
0.111
o.os
o
0.00 '*TT1..,....TT1..,..,..,..,.....,...,....,..,.,..,.., ...... T'nI'"M',..,.....,...,..,., 0. 10.00
La ppm 15.00
Ce VI. Yb, Goph.r RIdge F'm.
48.00
E Ihz.oo ~
le.oo
Yb
La VI. Yb, Gopher Ridge F'm.
111.00
R2=.83
IZ.OO Sy:I.72 E D. D.
..9 e.oo
4.00
Yb
Tb VI. Yb, Gopher RldgeF'm. 1.10
R2=.9~ I.ZO
Sy=.076
0.40
o
o
0.00 '*TT1..,..,.. ....... r'TT.,..,..,.,..,..,..,..,...,.,...,...,....,...,...,...,.,.,.,..,.....,,., 0.
Fiqure 20. Of vs. oj plots of basalts to rhyolites of the Gopher Ridge Formation.
----------------------_ .. _-_. - .
91
~ c./Yb n. C.. Gopher Ridge Fm. 7.1O~
R2 =.74
e~1 s." .... ....... 0
~
.c..IO 0
0
2Il.Oo ~ 4Uj-SQ.bo c.ppm
n/Yb n. TJ, Gopher Ridge Fm.
32DO.aO ~ 0
R2=.78
/ e~lsY'''''' ~ICICI.IIO 0
0
n ppm
A1203/Yb n. A1203, Gopher Ridge Fm.
I.DO~ 0 0
R2 .. S6 ~ 7..101 . . Suo Sy=L6I 0 !:! 000(4.20 'I ./.. 0
14.11C1 I A1203 wi. percent
.;-
Ta/Yb VL Ta, Gopher RIdge rm.
CI.OI
~ ,IIJU ~
CI.IIS 0
0
-0:00""" 0.10""" iI3ii 0" 00 iI3O" 0 000 illiJ" " Ta ppm
y/Yb YS. Yo Gopher RIdge Flit. ~ 1_ ~ R2=.92
,_ ISY'= / ~,~ 0
, 0
> -1 DOJIo 2OCLoO ~oo 4OOJIo -=co
Y ppm
FeO-/Yb YS. FeO-, Gopher RIdge Fm.
~-I R2=.8S
$.uo 3 Sy =.89
~ lAO
o
CI.OO lei " i Ii:&i " ",O&i i " ",1bo 1 Feo- wi. percent
Th/Yb YS. Th, Gopher RIdge Frn. 0.51
~ R2
=.74 0
D.Q Sy=.071 ~ 0 >-....... ~ ....
~'1 /0
00 0
Pigure 21. c1 IC' vs. cl plots or basalts to rhyolites or the Gopher Ridge Pormation.
WgO/Vb YS. MgO, Gopher Rldg. Fm.
l.8O -I R2=.95
e ~ Sy".312
~2.20 Q
:IE
o
4.' WgO wi. percent
.:
93
related, so they were not amenable to rigorous statistics.
However, their degree of alteration is no greater than that
of the Gopher Ridge rocks, and the same elements evaluated
above are considered to have been immobile in those rocks.
STRATIGRAPHY
The field area conta'ins NW-trending belts of dominantly
two lithologies - metavolcanic rocks and pelitic rocks (see
Figure 22). Two metavolcanic belts are present: the eastern
one comprises mafic, intermediate, and minor felsic metavol
canic rocks of the Gopher Ridge Formation and the Penon
Blanco Formation. The western belt comprises felsic, mafic,
and intermediate metavolcanic rocks of the Gopher Ridge
Formation. All formations below the Mariposa Formation are
part of the Amador Group as defined by Taliaferro (1943a).
The two metavolcanic belts are surrounded by belts of
dominantly pelitic rocks. These pelitic units historically
were given different names on different sides of the meta
volcanic belts, i.e., Merced Falls Slate west of the western
metavolcanic belt, Salt Springs Slate in the central pelitic
belt, and Mariposa Formation east of the eastern metavol
canic belt (Clark, 1964). Following the usage of Bogen
(1984), they are grouped in this study as the Mariposa
Formation.
---~-- .. -.... ,----., ...... , .... ,'... ."- .. '" ..... __ .... - '
.:
Geology of the southern Foothills terrane
r Jm
dum
Scole - - - I
0 6km
o 'c;; en c ~ ..
, I
I
BXPLANATION
Guadalupe Intrusive Complex - northern facies. , gabbro/diorite and leucotonalite, locally mylonitic; southern facies \:J, quartz monzonite to granite.
High-Kg basalts.
Mariposa Pormation - dominantly phyllite and slate (metamudstone), local metaarenaceous and metaconglomeratic intervals.
Gopher Ridge pormation (undifferentiated) - lower part dominantly meta-basaltic andesite coarse breccias with local meta-andesite to meta-rhyolite lavas; upper part dominated by meta-felsic tuff and lava with local metabasalt lavas.
Penon Blanco FormatioD - , lower Penon
94
Blanco metabasalts, locally pillowed; _ , lower to upper Penon Blanco Formation (undifferentiated), basal meta-chert and tuff overlain by meta-mafic to intermediate breccias, crystal tuffs, and lavas; includes intermediate meta-intrusive rocks; uppermost part dominated by metasedimentary rocks.
Ultramafic/mafic rocks, locally amphibolitic •
Anticline; 9'., , overturned anticline.
syncline; ~,overturned syncline.
Left-oblique shear zone.
Thrust zone.
contacts mapped by the author.
contacts extrapolated by the author or transposed from Rogers (1966), Best (1963), or Evans and Bowen (1977).
Figure 22. Geology of the southern part of the Foothills terrane.
___________________ ._._ .• 0 __ '. "0'.' _,._._ •• ' ' •• " . ' "
.:
9S
Most contacts between the underlying volcanic rocks and
the Mariposa Formation are shear zones. However, deposi
tional contacts in outcrop have been observed in five
places, and evidence is presented below that the primary
contact is an unconformity surface.
Metamorphic grade in the study area ranges from
prehnite-pumpellyite facies in parts of the eastern metavol
canic belt to amphibolite grade in the aureole of the
Guadalupe intrusive complex. The general grade is lower
greenschist facies, and as protoliths are quite recogniz
able, the prefix "meta-" subsequently will be omitted from
rock names for brevity.
The stratigraphy of the southern part of the Foothills
terrane is summarized in Figure 23. Tables 2, 3, and 4
present data from geochemical analyses, performed for this
study, of representative volcanic and hypabyssal rocks from
different units in the Foothills terrane. The development
of the ternary major-element ratio plot employed here, the
TFMA diagram, was given in Chapter One.
Penon Blanco Formation
This formation composes the eastern metavolcanic belt
east of the Bear Mountains fault zone, Figure 22. The
formation is more than 3 km thick in an antiformal structure
south of the Tuolumne River, and the base is not exposed.
'?.-... - .. , .. - .. ~-.-.. -... ,'>.... .... . ..... _ ...... -~- .... --.
..
<0 en
.. ~
o u; '" < a: :::I
.. 1-:> ;; "t:J
~ I
o ILl
o iii
'" < ~
Western Metavolcanic
Belt
PElR1lTEtTOIIC ASSEIIIlAG£!j
Central Pelitic
Belt
x: Forurt/pruta-trnth, Asulblilge 6
=: _BiItt-ilrt/iatuut bilsin, Asulblilge ~ ///Att-rift loaf, ASSfiblilgf 4
IiiIiiI Eltl!llsioad ,a!t.nic Irc, ASUlblilgz 3
'\. ~Prilitin YDlcanie orc, AS5Ilbhge 2
IIII Deua fhnr, Assflbhge 1
West Side of Eastern
Metavolcanic Belt
East Side of Eastern
Metavolcanic Belt
Figure 23. stratigraphy and petrotectonic assemblages in the southern part of the Foothills terrane. SW BHFZ = southwest strand of the Bear Mountains faUlt zone; BHFZ = Bear Mountains faUlt zone; HPZ = Melones fault zone •
97
The unit ranges from basaltic pillow lavas at the base
through a thick middle portion of dominantly basaltic
andesitic pyroclastic rocks to an upper volcani
clastic/epiclastic sedimentary sequence. In parts of the
western edge of the eastern metavolcanic belt, the Penon
Blanco Formation is in apparent depositional contact with
overlying mafic to intermediate volcanic rocks here corre
lated with the Gopher Ridge Formation. On the east side of
the eastern metavolcanic belt, the upper Penon Blanco is in
sheared unconformable contact with the Mariposa Formation.
Basaltic Lavas, Basalt Member, Lower Penon Blanco Forma
tion. The lowest exposed unit in the area consists of a
thick sequence of tholeiitic basalts with thin interspersed
chert intervals. The base of this sequence is not exposed
in the field area, but a minimum thickness from exposure is
1 km. The basalts are black to dark green, massive, aphy
ric, and locally pillowed. Thin discontinuous iron and
manganese-rich cherts are intercalated at the top of the
unit, and the unit contains small cu-rich massive sulfide
occurrences such as La Victoria (Cox and Wyant, 1948).
Analyses of two representative lower Penon Blanco
basalts are presented in Table 2. The bas.alts are tholei
ites, high in normative hypersthene and mildly either quartz
or olivine normative. They have higher Fe/Mg ratios (Figure
-------------------------~ .. '~----.-'.--.. -'.'''.'.- ._"
TABU! 2. IETAVIIlCAIIIC IDCC MALnEI. PEb IIMCO FIIIMTlIII. SUllIED 'Air OF TIE FOIJJlIILlI rEIIIIAIE
... It .......... Lower .. ic-int=-ai.te .oaa, AI&I( • ,eRan 11 __ F •• to I%Jper ........ 11 __ r ..
SeIple' N5.~ N5-1~ .,-1oA .,·zuA .,-2141 "-34~ .,.m! 115.6468
llejor el_ (lit ") SIOz 50.40 5c1.50 52.20 53.50 49.45 51.17 52.44 56.16 TIOz 0.87 M3 0.31 0.36 0.57 0.411 0.57 0.60 AlzOl 14.10 12.60 lZ.50 13.50 16.39 15.D\I 14.111 16.39 FezG3 11.30 15.11D 9.46 9.31 10.37 9.115 10.27 7.611 lInD 0.19 0.25 0.16 0.16 0.10 0.13 0.16 0.16 IIgO 11.09 6.05 10.110 11.56 7.19 7.29 6.93 4.n ClIO 11.00 10.10 9.66 11.20 9.54 7.71 4.97 6.10 118z0 2.46 2.D\I 1.37 1.11Z 2.37 3.311 5.57 4.70 ICzO 0.13 0.05 0.90 0.06 0.34 1.i!2 1.311 0.42 Pz05 0.06 0.10 0.03 0.03 0.21 0.22 0.15 0.23 LOI 1.911 O.IID 3.38 1.99 1.26 2.D\I 1.65 2.119 Totel 100.58 100.57 100.57 100.09 97.79 100.23 911.90 99.66
II_I fled to 100.0 " wletlle-free, ultll Fe2rJJ/FfIJ • 0.20 SIOz 51.62 51.31 54.16 54.77 51.70 53.02 54.41 511.0 "Oz 0.119 1.45 0.32 0.37 0.60 0.50 0.59 0.62 AlzOl 14.44 12.110 12.76 13.67 17.14 16.47 15.37 17.05 FezG3 1.77 2.45 1.50 t.46 1.65 1.56 1.61 1.22 Feo 8.113 12.24 7.411 7.30 8.27 7.78 11.13 6.09 lInD 0.19 0.25 0.17 0.16 0.10 0.13 0.17 0.17 "gO 8.29 6.15 n.20 8.110 7.52 7.55 7.19 4.50 ClIO 11.27 10.26 10.02 11.51 9.97 7.99 5.16 6.35 118z0 2.52 2.94 1.42 1.87 2.411 3.50 5.78 4.D\I ICzO 0.13 0.05 0.93 0.06 0.36 1.26 1.0 0.44 Pz05 0.06 0.10 0.03 0.03 0.22 0.23 0.16 0.24 Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0
CIPU no ... tfw _Iyal. D 0.00 0.92 3.78 6.64 0.94 0.00 0.00 5.11D C 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 or 0.79 0.30 5.52 0.36 2.10 7.47 8.46 2.511 ab 21.32 24.84 12.03 15.113 20.97 29.M 44.08 41.311 an 27.70 21.60 25.611 28.72 34.511 25.47 11.76 23.29 ne 0.00 0.00 0.00 0.00 0.00 0.00 2.61 0.00 dl 22.611 23.115 19.23 22.87 11.01 10.35 10.411 5.46 hV 22.92 21.94 30.91 22.69 26.36 15.67 0.00 17.99 01 0.20 0.00 0.00 0.00 0.00 7.67 18.77 0.00 R 2.56 3.55 2.17 2.12 2.40 2.26 2.36 1.77 It 1.69 2.76 0.61 0.70 1.13 0.94 1.12 1.19 lip 0.15 0.24 0.07 0.07 0.52 0.54 0.37 0.57
Trece eleaentl, not _tlzed (!'PI' Y 23. !! 311. !! 10. !! 9. !! !! !! !! !! V 314. 451. 264. 274. 400. 310. 330. 250. BI ~1D. 61. 10. 27. 7. 41.7 21.2 1IZ.6 9.9 l!II.l 12.4 75.3 11.5 Co 47. 0.5 52.5 0.5 37.2 0.4 39.1 0.4 36.9 0.2 n.6 0.2 39.115 0.18 19.1 0.1 Cr 276. 11. 50.6 1.5 359. 11. 290. 6. 61.5 2.3 57.6 2.6 59.3 2.22 7.27 4.37 C. 0.12 0.03 0.731 0.007 1.4 0.3 0.344 0.1611 0.192 0.066 1.1145 0.077 0.350 0.072 Hf 1.23 0.02 1.94 0.04 0.541 0.032 0.705 0.014 0.1151 0.048 1.n 0.09 1.34 0.056 2.02 0.05 Rb 4.7 1.2 13.3 0.9 4.37 0.59 5.23 0.56 14.3 0.75 6.27 0.53 Sb 0.196 0.01 0.14 0.04 0.23 0.05 0.480 0.00 .-Ta ~.14 0.059 0.006 0.0203 0.0016 0.0319 0.0019 O.D4I!II 0.D3P4 0.276 0.0311 0.0912 .- 0.161 0.031 Th ~.05 ~.05 0.101 0.008 0.18 0.04 0.327 0.027 1.03 0.04 0.407 0.019 0.9117 0.035 U ~.05 ~.05 0.0:J6 0.015 ~.05 0.166 0.027 0.446 0.029 0.159 0.034 0.407 0.037 Zn 120. 23. 73.1 10.2 84.7 4.4 19.5 1.9 611.45 2.14 1ID.55 2.13 Zr ~o. IIZ.O 8.2 n.7 7.4 54.9 10.4 45.7 7.67 74.6 6.4 Se 40.2 0.5 411.7 0.5 52.1 0.5 47.7 0.5 46.6 0.2 39.5 o. I 42.9 0.2 23.8 0.1 La 0.956 0.057 1.78 0.04 1.03 0.05 1.32 0.04 3.00 0.05 7.28 0.05 3.13 0.062 8.925 0.096 Ca 3.57 0.36 6.30 0.76 2.72 0.35 3.14 0.13 6.99 0.66 13.95 0.24 7.60 0.18 19.9 0.3 Nd 3.87 0.27 5.78 0.35 2.3 0.5 2.49 0.27 5.11Z 0.51 8.53 1.01 6.80 1.71 14.5 0.7 SII 1.87 0.17 2.611 0.32 0.950 0.057 1.04 0.04 1.61 0.01 1.935 0.007 1.95 0.01 3.19 0.01 Eu 0.7311 0.022 1.16 0.01 0.374 0.011 0.405 0.004 0.584 0.008 0.647 0.011 0.653 0.017 0.973 0.015 Cd 2.7 0.5 4.25 0.34 1.40 0.21 Tb 0.561 0.017 0.8110 0.026 0.24 0.04 0.257 0.010 0.310 0.018 0.301 0.020 0.404 0.035 0.527 0.015 T. 0.3119 0.019 0.625 0.019 0.187 0.011 0.211 0.055 0.219 0.059 0.224 0.065 0.223 0.049 Yb 2.55 0.15 4.08 0.16 1.10 0.01 1.17 0.06 1.34 0.03 1.325 0.020 1.74 0.02 2.05 0.02 Lu 0.379 0.007 0.612 0.012 0.180 0.011 0.111P .- 0.213 0.008 0.214 0.005 0.2611 0.006 0.318 0.006
A Major el ... : IIRF (gl ... ). U.S.G.S., Denver; Y.V: ICP, U.S.G.S., Denver; other t..- .1 ... : IIWI, U.S.G.S., Denver. 8 "jor al ... : Icp·AES, c_rcf.1 lab.; V: MS, ~I.I lab; other trece .Ieaenta: IIWI, II.C. lIewton. III, LWlllr and PI_tary Leb., Unlv. of ArIz •• Tucaon.
... __ ........ _ ... _' •..... - ,-
98
I~;
(U90/ A1203)2
(TI02/F eO·)5
99
• MIddle Penon Blanco basIc-Intermediate volcanic rocks
o lower Penon Blanco basalts
MgO/FeO·
Figure 24. TFMA diagram with lower Penon Blanco basalts and middle Penon Blanco basalts to andesites plotted.
*Total Fe reported as FeO.
... __ .... _. __ .. _ .. .,r ... _ .. ,_.~ .. ... " .. _. _._.~_". _. _._._ .... ~_.,
100
24) than the overlying middle Penon Blanco volcanic rocks.
The basalts are low in potassium, titanium, and incompatible
trace elements, and they have depleted REE patterns (Figure
25A). Along with overlying radiolarian cherts, these pil
lowed basalts are probably the top of an ophiolitic
sequence, as suggested by Schweickert (1978). A younger
age limit on the basalt sequence is 200 Ma, the U/Pb zircon
date (Saleeby, 1982) of the Don Pedro intrusive suite, which
cuts the lower and middle parts of the Penon Blanco Forma
tion. The lower Penon Blanco basalts unconformably overlie
deformed serpentinitic melange just north of the study area
on the Tuolumne River (Saleeby, 1982). Saleeby suggested
the lower Penon Blanco basalts could be correlated with
ophiolitic rocks probably unconformably overlying deformed
serpentinitic melange in the Kaweah terrane directly south
of the Foothills terrane. Shaw and others (1987) have dated
the southern ophiolitic complex at 485 ± 21 Ma by Nd/Sm
whole rock methods.
As will be discussed below, the overlying radiolarian
chert sequence is continuous and gradational into intermed
iate arc-related tuffs, so the duration of marine pelagic
sedimentation between pillow basalt extrusion and initial
arc activity is constrained by the length of time necessary
to deposit approximately 100 meters, the typical thickness
I ,,:
A.
CD :0-.fi to' c o .c u ~ u to o 0:
10·'
10 •
B.
to •
CD :0-.fi to' c 0 .c u ...... ~ to u 0
0:
La Ce
;:
Tholeiitic basalts, Lower Penon Blanco Formation
Nd Sm Eu Tb Atomic Number
: : :
Tm Yb Lu
Tholerrtrc basalt - basaltrc andesrte - andesrte, Mrddle Penon Blanco Formation
!S:J t:D I en
10 .,.J...._-,-~_---,~_~~~--::::--__ -:::---:-:.:--:-__ La Ce Nd Sm Eu Tb Tm Yb Lu
Atomic Number
Figure 25. Coryell-Matsuda plots of basio to intermediate voloanio rooks of the Penon Blanoo Formation.
101
(A) Basalts of the lower Penon Blanoo Formation. (B) Basalts to andesites of the middle Penon Blanoo Formation.
---~ .. -.. _ ..... _-_ ................. -.. ... - _ .... , ..•.... __ .'.'.' .
102
of the chert (Bogen, 1984). At a slow deep-marine pelagic
sedimentation rate of 1 mm /1000 yr (Press and Siever,
1982), the depositional time required would be approximately
100 million years. The probability that some of the cherty
sequence has exhalative and volcanic rather than pelagic
origins, as subsequently discussed, suggests an even shorter
depositional span. The 485 Ma date is inconsistent with the
thickness of the overlying marine pelagic sequence. The age
would appear to be slightly earlier than the 200 Ma age on
the main part of the arc sequence (discussed below), proba
bly Triassic to possibly Permian. This is consistent with
poorly preserved radiolaria in the overlying chert being no
older than Triassic (Jones, D.L., pers. communication
reported by Schweickert, 1978).
Thin-bedded Pelites, Lower Penon Blanoo Formation.
Directly overlying the Basalt Member are approximately 300
meters of metasedimentary and metavolcanic rocks. The basal
hundred meters of the unit comprises thin-bedded to thick
laminated green-red-white-black chert with thin mUdstone
interlayers. Many of the cherts are Fe-rich (red jasper)
and Mn-rich, with local historically-mined Mn concentrations
(Taliaferro, 1943b). Radiolarians have been identified in
the chert (Taliaferro, 1943b; Schweickert, 1978). The
remainder of this thin-bedded sequence largely consists of
green tuff of probable mafic/intermediate composition,
impure ashy and cherty mudstones, and interbedded chert,
mudstone, and clinopyroxene/amphibole-plagioclase crystal
tuff. The tuffaceous component increases upward, and the
unit is gradational into overlying massive intermediate
crystal tuffs and breccias (see also Footnote 1, Appendix
E) •
103
Intermediate crystal Tuffs and Breccias, Middle Penon
Blanco Formation. Overlying the thin-bedded sequence are
1700 meters dominantly of poorly-bedded massive tuff brec
cias and thick to thin-bedded massive crystal tuffs, with
minor pillow lava intervals. Intercalated with the crystal
tuffs and breccias are thin-bedded fine tuffs and ashy mud
stones. Phenocryst assemblages in crystal tuffs are 5-10 %
clinopyroxene + 10-20 % plagioclase, 5-20 % amphibole + 2-15
% plagioclase, or 2-5 % amphibole + 3-5 % clinopyroxene + 7-
10 % plagioclase. Common lithic fragments are intermediate
to mafic aphanitic to holocrystalline hypabyssal rocks with
clinopyroxene or amphibole + plagioclase phenocrysts.
Analyzed homogeneous tuffs, based on silica content, are
basaltic andesites to andesites, which are Ti-poor and Mg
rich (see Table 2). pillow lavas are pyroxene phyric
basaltic andesites to basalts. In SUbstantial areas of the
eastern metavolcanic belt east of the Bear Mountains fault
-----------------------------~ .. -.. -.... - .. -.. .., ...
104
zone, shallow intrusive augite-plagioclase and hornblende
plagioclase basaltic andesite to basalt porphyries and
intrusive breccias crop out. Pyroxene-plagioclase porphy
ritic rocks are common north of the Merced River and
hornblende-plagioclase porphyritic rocks are common south of
the river in the southwestern part of the belt. These
hypabyssal rocks are quite similar to crystal-rich middle
Penon Blanco volcanic rocks. They may represent subvolcanic
feeder systems, as suggested by Morgan (1976) and Saleeby
(1982), and they also may represent magma bodies intruding
their own volcanic piles.
These intrusive bodies north of the Merced River are
part of the Don Pedro intrusive suite, which has U/Pb zircon
dates of ca. 198-200 Ma (Saleeby, 1982). This places a
younger age limit of 200 Ma on the volcanic rocks.
The basic to intermediate middle Penon Blanco volcanic
rocks constitute a relatively high-Mg tholeiitic sequence
(see Figure 24). They range from mildly LREE depleted to
mildly LREE enriched and exhibit slight negative Ce anoma
lies (Figure 25B).
Voloanio1astio/epio1astio Turbidites, Upper Penon Blanoo
Formation. The upper 500 meters of the Penon Blanco Forma-
tion is a heterogeneous package of well-bedded thin to
medium-bedded volcaniclastic arenites with plagioclase and
.... -.. -.. ~ .. - .. - ... , ...... -
lOS
clinopyroxene and/or amphibole crystals (reworked crystal
tuffs), minor volcanic breccias and lavas, thin-bedded to
thick-laminated tuff, mUdstones and ashy mudstones, epi
clastic feldspathic arenites, and minor cherts. Normal
grading and cross stratification are present in the are
nites, and these clastic rocks have been interpreted as
turbidites (Bogen, 1985). In the uppermost 60 meters on the
east side of the eastern metavolcanic belt, above the
highest volcaniclastic breccia exposure, crystal-bearing
volcaniclastics are not present and the sequence consists of
thin-bedded to thick-laminated impure ashy and cherty mud
stone, fine-grained felsic tUff, chert, and argillaceous
mUdstone intercalations.
On the west side of the eastern metavolcanic belt, the
Penon Blanco Formation was dismembered by activity along
various strands of the Bear Mountains fault system with
stratigraphic throw increasing to the north where all of the
middle to upper Penon Blanco Formation is missing. To the
south, where shearing transferred to more-western strands of
the shear zone system, upper Penon Blanco volcaniclastic/
epiclastic rocks and minor basaltic andesitic lavas are in
contact with mafic to intermediate plagioclase crystal tuffs
and breccias of the lower Gopher Ridge Formation. The
physical contact has not been observed, but no shearing is
106
evident in adjacent outcrops of either the upper Penon
Blanco Formation or the lower Gopher Ridge Formation or
along strike. The contact is probably depositional, and
based on the age difference of approximately 30 million
years between the sequences, it probably is unconformable.
This is consistent with the interpretation of Edelman and
Sharp (1989) that the Penon Blanco Formation is part of the
oceanic basement (their Tuolumne River terrane) upon which
Callovian to Kimmeridgian deposits formed an overlap assem
blage.
Gopher Ridge Formation
In the southern Foothills terrane, the Gopher Ridge
Formation is dominantly a bimodal sequence of basalt/bas
altic andesite and rhyolite with a large percentage of
hydroclastic deposits. Equivalent rocks to the north have
been dated at 160-170 Ma (Saleeby, 1982). The unit crops
out in the western metavolcanic belt and depositionally
overlies the upper Penon Blanco Formation in the western
edge of the eastern metavolcanic belt (Figure 23). As
mentioned above, this contact is probably an unconformity.
This eastern exposure of the Gopher Ridge Formation is
in shear zone contact with the Mariposa Formation to the
west. Felsic volcanic intervals at the top of this sequence
were placed in the Gopher Ridge Formation by Bogen (1985).
---~ .. -.. _ .... _ .. - ....... .
, .;.
107
Clark (1964) assigned the eastern exposure to the Copper
Hill Formation, which is a mafic volcanic dominated sequence
directly north of the eastern strip of the unit in the study
area. Because of the similarities between western and
eastern exposures of these rocks, discussed below, they are
here included in the Gopher Ridge Formation, which may also
be partially equivalent to the Copper Hill Formation.
Intermediate/mafia Breooias and Tuffs, Lower Gopher
Ridge Formation. This sequence comprises 350 meters of
intermediate to felsic breccias and tuffs. The dominant
lithology is very thick-bedded massive matrix-supported
breccia and lapilli tuff. Fragments are rounded, 3-6 cm in
the maximum stretched direction, and generally are homogen
eous in each deposit. There are two most common fragment
compositions. One is medium green aphanitic porphyritic
basaltic andesite with 5-10 % 5 mm white plagioclase pheno
crysts and white to green 2-4 mm quartz ± chlorite amygdules
or white calcite amygdules. The other type is a light green
aphanitic lava with fewer and smaller plagioclase pheno
crysts and either no amygdules or 1-2 mm green amygdules
filled with chlorite, sericite, and quartz. Compositions
range from basaltic andesite to dacite. Locally, purple
tinted magnetic fragments bearing magnetite and hematite are
abundant in a green fine-grained non-magnetic matrix.
108
Vesicle-content in fragments ranges up to 50 percent. The
breccias lack scoria and pumice and many have very fine
grained non-vesicular matrixes, possibly meta-hyaloclastite.
These deposits are dominantly hydroclastic breccias and less
abundant flow breccias. Some, particularly at the base of
the sequence, have crystal tuff as matrix, and may be true
tuff breccias and lapilli tuffs. At the base of the
sequence, minor pyroxene crystals are present with the more
abundant plagioclase crystals in fragments and matrix. Some
lapilli tuffs higher in the sequence have heterogeneous dark
green, dark blue, and gray aphyric lithic fragments in a
fine-grained light green tuff matrix. Intercalated within
the sequence are well-bedded, normally-graded volcaniclastic
conglomerates and minor epiclastic lithic arenites and mud
stones. Minor felsic amygdaloidal lava lenses and fragments
in intermediate breccias have quartz phenocrysts. Locally
at the top of the intermediate breccia sequence are light
green dacitic to rhyolitic plagioclase and quartz
plagioclase crystal tuffs and lavas. The quartz-feldspar
rhyolites are distinctive rocks with 2-3 % up to 5 mm subhe
dral quartz phenocrysts.
Actual andesite locally is common, but in general it
appears to be subordinate to basaltic andesite, analyses of
which are listed in Table 3. These basaltic andesites are
___ ~._ •. _ •. o. ___ ....... , 0" ••• ,
109
TAIl!! 3. IETA1IOI.CAIIIC lOCI: AlAI. nu, CIII'IEII .IDeI! fCIIIIATiCII. SOUJIIEII 'MT Of TE FOIITIIILLI TEaAII!
SaIVII •• 15·14D" IIN''- 117-0t1
IIoIlor .1-.0 (lit I' IIIIz 52.90 51.70 61.40 72.60 74.40 "'.40 48.70 46.32 46.12 TlIIz 1.26 1.17 0.13 0.41 0.41 o.n 0.90 0.87 1.09 AlzOl 15.70 15.10 16.110 12.70 12.70 12.ZO 16.DO 15.65 16.77 fezO] 12.40 13.10 7.22 3.76 2.sa 2.42 10.DO 10.711 11.111 lInD 0.22 0.22 0.14 0.16 0.04 0.04 0.111 O.ZO 0.16 IIgO 4.26 '.87 2.06 1.16 O.DO 0.76 7.95 9.10 11.94 c:ao 6.60 3.99 3.97 2.90 0.'" 1.03 9.61 9.ZS 10.sa IIezO 3.76 '.03 3.15 4.115 5.29 '.35 2.52 2.48 2.47 rza 0.51 2.32 1.35 0.03 1.95 2.04 0.15 0.62 0.611 PzOs 0.17 0.13 0.15 0.10 0.07 0.03 0.17 0.23 0.19 LOI 2.30 3.00 2.76 0.911 0.54 0.69 2.10 2.49 2.sa Tot.1 100.OS 99.63 99.71 99.65 99.53 99.29 99.l1li 911.02 911.39
IIorallzed to 100.0 "volltlle-'ree. IIlth fe2t1J/f1O • O.ZO
"liz 54.69 54.13 63.13 13.82 "'.n 711.63 SO.ZS 48.96 48.52 TlIIz 1.30 1.22 0.76 0.42 0.42 0.34 0.91 0.92 1.15 AlzOl 16.23 IS.III 17.44 12.91 12.116 12.40 17.34 16.54 17.64 fezO] 1.96 2.09 1.14 O.sa 0.40 O.sa 1.70 1.74 1.41 flO 9.711 10.46 5.71 2.92 1.99 1.l1li II.SO 11.69 7.07 lInD 0.23 0.23 0.15 0.16 0.04 0.04 0.19 0.21 0.17 IIgO 4.40 5.10 2.14 1.111 0.111 0.77 11.21 9.62 9.40 c:ao 6.82 4.111 4.12 2.95 0.76 1.05 9.92 9.111 11.13 lIezO 3.119 4.22 3.ZS 4.91 5.36 4.42 2.60 2.62 2.60 rza 0.53 2.43 1.40 0.03 1.97 2.07 0.15 0.66 0.72 PzOs 0.111 0.14 0.16 0.10 0.07 0.03 0.111 0.24 O.ZO Tot.1 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0
CIPU _tlw _Ipl. Q 4.32 0.00 23.99 35.39 32.~ sa.46 0.00 0.00 0.00 C 0.00 0.00 3.46 0.00 0.70 1.05 0.00 0.00 0.00 or 3.12 14.35 II.ZS 0.111 11.61 12.ZS 0.92 3.87 4.23 ... 32.119 35.70 27.49 41.13 45.32 37.41 22.01 22.111 21.99 III ZS.ZS 17.02 19.43 13.01 3.30 '.99 35.19 31.43 34.36 IW 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 dl 6.20 2.34 0.00 0.110 0.00 0.00 10.51 12.70 15.77 fly 22.48 17.77 13.119 7.02 '.74 4.sa 22.47 11.56 0.55 01 0.00 7.14 0.00 0.00 0.00 0.00 '.ZS 16.43 111.42 lit 2.11J 3.03 1.66 0.115 O.sa 0.54 2.47 2.52 2.OS II 2.47 2.n 1.44 0.79 0.79 0.64 1.76 I.'" 2.111 II' 0.42 0.32 0.37 0.24 0.17 0.07 0.42 0.511 0.47
Trace .I.....,t •• not _lIzed (All' , zs. I! 21. I! 37. I! 47. I! 42. I! 51. !! 16. I! I! I! V 290. 404. .. II. III. ZS. !S. 2l1li. . . 2110. .. 230. Ba 176. III. 194. 12. 219. 4. 22. 4. 291. 12. m. 15. c10. .. 115. 9. 611.3 11.7 Co 26.6 0.3 34.' 0.3 5.sa O.OS 2.sa 0.02 2.11J 0.03 1.53 0.02 40.9 0.4 46.2 0.2 43.15 0.19 Cr 1.7 0.4 11.21 0.16 3.10 0.62 1.70 0.37 2.35 0.14 1.30 0.23 90.9 2.7 247. 3. 3311. 4. Co cO.5I 0.15 0.03 1.53 0.03 0.065 0.010 0.090 0.017 cO.099 0.090 0.077 0.073 O.a.'. Hf 1.91 0.06 1.55 O.OS 3.55 0.04 3.53 0.04 5.34 0.11 7.44 0.07 1.13 O.OS 1.26 O.OS 1.69 O.OS Rb cl.O .. 34.7 1.4 30.3 0.6 cl.O 111.7 0.4 ZO.5 0.6 6.97 0.61 9.OS 0.60 Sb cO.09J 0.087 0.025 0.608 0.006 0.1110 0.004 0.074 0.001 0.095 0.017 cO.12 T. 0.101 D.OOJ D.D69 0.005 0.0764 0.004 0.151 0.005 0.2311 0.007 0.39J 0.004 0.194 0.004 0.1OS 0.0311 0.194 D.1B7 Th 0.576 D.OS2 0.371 0.015 1.06 0.02 1.19 0.01 2.00 0.04 2.67 O.OS 0.48 D.OS 0.222 0.017 0.124 0.C117 u CO.OSO 0.147 0.013 0.641 0.006 0.412 .. 0.677 0.041 0.1130 O.DOS 0.51 0.10 O.on 0.027 0.031 0.1B7 Zn 99.7 15.0 101. 12. 157. 2. 63.1 1.9 47.1 0.9 62.90 0.6 87. 14. 141. 2. 67.115 1.54 Zr 65.0 7.11 115. 7. 1ZO. I. 170. 3. 2511. 3. 62.7 7.5 51.3 7.2 64.7 6.9 Sc 39.2 0.4 43.3 0.4 21.11 0.2 14.4 0.1 9.27 0.09 9.56 0.10 36.1 0.4 34.7 0.1 34.6 0.1 La 5.51 0.06 3.111 O.OS 5.54 0.11 9.31 0.09 10.9 0.1 16.11 0.2 7.611 0.15 3.21 0.04 2.94 0.04 Co 13.4 0.4 9.27 0.56 15.0 0.11 22.2 0.2 ZS.4 1.1 45.9 1.11 17.11 0.5 11.39 0.27 11.34 0.16 lid 9.97 0.110 11.32 0.67 12.5 0.4 14.0 3.2 19.2 0.11 27.0 0.11 10.5 0.2 7.5OS 0.1195 11.03 0.51 51! 3.12 0.09 2.70 0.03 4.42 0.22 4.611 0.09 5.03 O.OS 7.29 0.07 2.92 0.09 2.07 0.01 2.SO 0.01 Eu 1.19 0.02 1.00 0.04 1.47 0.01 1.53 0.02 1.12 0.01 1.46 0.1ll 0.9114 0.020 0.794 O.DOS 0.7l1li O.lXl! Gd 3.99 0.24 3.51 0.42 5.69 0.23 5.70 0.11 7.110 O.OS 3.27 0.23 Tb 0.699 0.021 0.580 0.012 0.967 0.019 1.10 0.01 0.967 0.010 1.sa 0.01 0.453 0.0111 0.401 0.0111 0.526 0.017 r. 0.4311 0.0111 0.347 0.017 0.592 O.OIIJ 0.769 0.031 0.677 0.034 0.975 0.039 0.263 0.013 0.2111 0.OS7 0.2IIJ 0.161 Yb 2.75 0.06 2.19 0.044 3.117 0.04 '.l1li 0.10 4.49 0.04 6.111 0.06 I.sa 0.03 1.51 0.02 2.02 0.012 Lu 0.401 0.008 0.340 O.OOJ 0.620 0.006 0.730 0.007 0.671 0.007 0.917 0.009 0.247 0.002 O.ZZS 0.005 0.308 DJIlj
A Malar el ... : XlF (glu.,. U.S.G.S •• Denver; '.V: Ia>. U.S.G.S •• OerMlf'; other trece .1 ... , !NAII, U.S.G.S •• Denver. I IIoIlor .1 ... : lep·AES. <,-rel.I IDb.; V: MS. <,-rel.I IDb; .... ton, III. L\I'1er MId PI_tery L ..... 1A11v. of Ariz •• Tucaon.
110
neither as Ti-poor nor as Mg-rich as basaltic andesites in
the middle Penon Blanco Formation. In the TFMA diagram,
Figure 26, field area samples are plotted along with basic
to intermediate samples of the Gopher Ridge Formation to the
north of the field area, which were analyzed in this study.
They have a pattern that is neither definitively tholeiitic
nor calc-alkaline in any plotting scheme, but they have a
definite convex upward (Fe-enrichment) pattern in the TFMA
diagram (Figure 26). Basic members plot in the tholeiitic
field and the trend bends sharply into the calc-alkaline
field. Because of the Fe-enrichment and the similarity of
the trend to the trend of the Recent tholeiitic suite of the
Tonga Islands (Ewart and Bryan, 1973), also plotted in
Figure 26, the Gopher Ridge rocks are considered tholeiitic.
They exhibit flat to mildly enriched REE patterns with
slight negative Ce anomalies in the more basic members
(Figure 27A and 27B).
The hydroclastic affinities of the basaltic andesitic
breccias (fine-grained non-amygdaloidal matrix with rounded
very amygdaloidal fragments), great extent of the breccias,
and intercalated well-bedded normally-graded volcani
clastic/epiclastic deposits and mUdstones suggest the unit
was subaqueously deposited. The abundance of vesicles
suggests eruption at relatively shallow water depths (Moore,
. ___ .... _ •. ___ .. ___ .. _ .......... _.. . a. . .. _ .~. .. .. , ___ •. ,_ ..
! '+"'
{MgO/AI203)2
{Tt02/FeO*)5
* Lower Gopher Ridge basic to felsic volcanic rocks
o Tonga Islands tholeiitic suite, Ewart and Bryan, 1973
MgO/FeO*
Figure 26. TFMA diagram with lower Gopher Ridge basic to silicic volcanic rocks plotted.
*Total Fe reported as FeO.
---------------------~.-~.~--.. -~-.............. '" ...... -.
111
A. to •
to •
II
~ to' c o .c
~ u 10 o 0:
Basaltic andesite to rhyolite, Lower Gopher Ridge F'ormatlon In study area
i : i : :
10 -I.J.... __ ~_~-:--_~-=-_--=: ___ -=-~::--: __ _ La Ce Nd Sm Eu Tb Tm Vb Lu
B. to •
to •
# -5 to· c o .c u ~ u to o
0:
AtomIc Number
Basalt to rhyolite, Gopher Ridge F'ormatlon (undlfferentrated), north of study area
: :
10 -I.J...._~~_-:"!"-:--_~-=-_-= ___ ~-.,...,--, __ _ La Ce Nd Sm Eu Tb Tm Vb Lu
AtomIc Number
Figure 27. Coryell-Matsuda plots of basic to silioio vOloanio rooks of the Gopher Ridge Formation. (A) Lower Gopher Ridge Formation in the study area. (D) Undifferentiated Gopher Ridge Formation north of the study area •
112
. ---~ .. -... --... --.. ""' ...... -...... ... ..-- .. - .' .... -.- ... _-
113
1965). Marker lithologies like the distinctive quartz
feldspar rhyolites as well as distinctive breccias of
plagioclase-phyric basaltic andesite, are found in the same
stratigraphic arrangement in the western metavolcanic belt
and on the west side of the eastern metavolcanic belt. This
provides compelling evidence that these separated sequences
are correlative. Either the eastern and western metavolca
nic belts at one time were contiguous and subsequently were
separated, or they may still be contiguous in the subsurface
in a synformal structure beneath the central pelitic belt.
Rhyo1itio Voloanio Rooks and Basalts, Upper Gopher Ridge
Formation. There are 450 meters of this unit in the western
metavolcanic belt, Figures 22 and 23. Rhyolitic lavas and
hydroclastic breccias of the unit host the Blue Moon Kuroko
type massive sulfide deposits. Throughout the field area,
the unit consists dominantly of poorly-bedded rhyolitic
aphyric, plagioclase phyric, or quartz-plagioclase phyric
fine-grained matrixed pyroclastic and hydroclastic phases,
local rhyolitic lava flows,' and minor intercalated volcani
clastic/epiclastic lithic arenites, ashy mUdstone and
mudstone, basaltic to basaltic andesitic breccias and lavas.
Many fragmental rocks with dense formerly glassy fragments
and no pumice in a meta-vitric matrix are probably hydro
clastic deposits. Aphanitic lavas with relict flow-banding
--------------- ---- --.- ----- .--.. __ ... _- -'''-- .- ...... - .
I ~_.
114
locally are common. Some formerly glassy deposits have
abundant tapered recrystallized collapsed pumice fragments
and were unwelded tuffs. Some volcaniclastic deposits are
well-bedded, normally graded, and exhibit cross
stratification and scour marks. These features, plus the
association of intercalated fine-grained epiclastic depos
its, are indicative of subaqueous deposition for the bulk of
the sequence.
Analyzed rhyolitic lavas, Table 3, are relatively high
in silica (ca. 75 wt. %), are peraluminous, have Na20/K20
ratios> 2, and are very low in Rb with K/Rb > 800. Norma
tive compositions plot in the dacite field (granodiorite
equivalent) of the lUGS Q-A-P classification scheme
(Streckeisen, 1979). There is no evidence of hydrothermal
alteration in these samples, and alkali ratios are probably
primary. The high Na/K ratios and high Si02 content are
consistent with eruption in an intra-ocean setting, e.g.
South Shetland Islands, Antarctica (Weaver, et al., 1982)
and Thingmuli, Iceland (Carmichael, 1964). REE patterns of
the upper Gopher Ridge rhyolites manifest only mild LREE
enrichment and moderate negative Eu anomalies (Figure 28).
Felsic tuffs are thin and rare in the Gopher Ridge
Formation adjacent to the Bear Mountains fault zone in the
eastern metavolcanic belt. At the southern tip of the
----------------------~ .. -... --.----.. -..... -...... '.
I~:
10 •
GI -=5 10· c o
.c (,)
~ CJ 10 o
0:
Basalts and rhyolites, Upper Gopher Formation In study area
115
10 .'-'--~--:::---~----:=---::----:::------:=--~~'---La Ce Nd Sm Eu Tb Tm Yb Lu
Atomrc Number
Figure 28. Coryell-Matsuda plot of basic and silicic volcanic rocks of the upper Gopher Ridge Formation in the study area. Lower three lines are high-Al basalts; upper two lines are rhyolites.
. .... ~-~.--~--..... -.. .,.-.... ~ ... - '. .... . ............... ' -.-.- •.. ~- ..
: ~.'
116
western metavolcanic belt, intercalated with and overlying
felsic tuff and lapilli tuff, is a thick unit of dark green
aphanitic basalt. The basalt is either massive or pillowed,
and the lavas range from aphyric to porphyritic with 1-2 mm
clinopyroxene phenocrysts. Rare porphyries have 1 cm clino
pyroxenes. Analyzed representative basalts are high-alumina
basalts with Al203 ~ 16.5 wt. % and Tio2 ~ 1 wt. %, Table 3.
The basalts have variable LREE depleted and enriched pat
terns (Figure 28).
contaot with the Mariposa Formation. Locally either
rhyolite or basalt/basaltic andesite of either the upper or
lower Gopher Ridge Formation is in depositional contact with
overlying pelite of the Mariposa Formation.
On the west side of the eastern metavolcanic belt, the
base of the overlying slate sequence of the Mariposa Forma
tion, contains channel scours and lenses of coarse detrital
quartz and feldspar crystals and crystal-rich lithic frag
ments similar to the underlying crystal-rich rhyolites.
strata on either side of the contact are subparallel, and
the contact is probably a diastemic or paraconformable
surface. Along strike felsic volcanic rocks are absent and
the slate rests directly on intermediate/mafic volcanic
rocks of the lower Gopher Ridge Formation. Locally, the top
of the volcanic unit contains pyrite and the rocks are
---~ .. -.,.- ..... _-_., ...... ,.
! ~:
117
bleached, whereas the overlying slate sequence is unaltered.
The pyrite was probably deposited during submarine hydro
thermal activity; the bleaching also may have been
hydrothermal alteration, or it may be supergene acidic
alteration owing to sulfur release from the pyrite. The
lack of pyrite in the overlying mudstone supports a diastem
or paraconformity.
In the western metavolcanic belt, where upper Gopher
Ridge fine-grained rhyolitic quartz-feldspar crystal tuff is
in depositional contact with the Mariposa Formation, the
Mariposa Formation is a great thickness of black shale with
no ashy interlayers or conglomeratic lenses at the base.
stratification in tuff and shale is subparallel. The tuff
is reversely graded in both crystals and pumice fragments.
The top 0.3 meters of the tuff is an altered pumiceous con
centration which contains pyrite cubes and is silicified.
There is no pyrite or silicification in the overlying black
shale. The alteration at the top of the tuff was probably
submarine hydrothermal alteration, prior to deposition of
mud without alteration, and is consistent with a paracon
formable or diastemic contact.
Mariposa Formation
The Mariposa Formation in the field area comprises more
than one kilometer of predominant meta-mudstone in s~eared
--------------------~----~ .. _ ... ~ ..... _ ... _._-...... '" .. , .-.
118
unconformable contact with all lowe~ metavolcanic units.
Pelitic units on different sides of the metavolcanic belts
traditionally have been differentiated as separate units.
west of the western metavolcanic belt (Figure 22), the
epiclastic rocks have been called the Merced Falls Slate
(Clark, 1964). In the central pelitic belt, the epiclastic
unit has been called the Salt Springs Slate, and east of the
eastern metavolcanic belt, it has been called the Mariposa
Formation (e.g. Clark, 1964). These units have been mapped
in this study continuously around the noses of south
plunging anti forms that mark the terminations of the
metavolcanic belts (Figure 22). Because of equivalent
stratigraphic positions and Oxfordian-Kimmeridgian fossil
contents across the width of the Foothills terrane (Clark,
1964), these units are probably correlative, and they are
here grouped as the Mariposa Formation, as suggested by
Bogen (1985).
Mudstone and Tuff, Lower Mariposa Formation. Up to 350
meters of pelite is exposed on the west side of the eastern
metavolcanic belt, in a faulted synclinal inlier of Mariposa
slate between volcanic units, Figure 22 (see also Footnote
2, Appendix E). The sequence comprises very thin-bedded to
thick-laminated mudstone, indurated mUdstone and ashy mud
stone, felsic tuff, minor chert, and minor immature volcani-
___ ...... _ •• _ • •. ___ .. 'IIIII.~.
I .:
119
clastic/epiclastic feldspathic arenite. The indurated
mUdstones are much harder than similar mUdstones elsewhere,
and detrital plagioclase grains have jagged metamorphic
plagioclase overgrowths. The induration is due to metamor
phic recrystallization, possibly associated with subsurface
intrusive bodies along the Bear Mountains fault zone.
Locally at the base of this epiclastic unit, east of the
Gopher Ridge volcanic rocks, normally-graded fine-grained
beds with channel scours contain lenses of coarse detrital
quartz and feldspar crystals and porphyritic lithic frag
ments, up to cobble size, indistinguishable from the
underlying felsic volcanic rocks to the west. The volcanic
debris may have been deposited by turbidity flows off of
volcanic edifices.
The unit's proper stratigraphic assignment is uncertain.
On the basis of stratigraphic position directly above felsic
and intermediate/mafic volcanic rocks of the lower Gopher
Ridge Formation, this dominantly epiclastic sedimentary unit
with minor felsic tuff may be partially equivalent to the
felsic volcanic-dominated top of the upper Gopher Ridge
Formation in the western metavolcanic belt. The upper
Gopher Ridge felsic tuffs may just have been thin in the
eastern belt or they may have been eroded away prior to
slate deposition. The unit is similar and may be equivalent
--------------------------'
1-'"
120
to fine-grained epiclastic, tuffaceous, and pelagic metased
imentary rocks placed as the topmost part of the upper Penon
Blanco Formation on the east side of the eastern metavol
canic belt, which may be coeval with the Gopher Ridge
Formation in that area.
Where this epiclastic unit is juxtaposed across shear
zones with Mariposa slate, it commonly weathers to a red
tinted brown soil compared to the dark gray to blackish
brown soils of Mariposa slate. Although the unit is slate
dominated, it has felsic tuffs and thin cherts that have not
been observed in the rest of the Mariposa Formation. Clark
(1964) designated the unit as distinct from the Salt Springs
Slate, which is here included with the Mariposa Formation.
It is possible that rather than being correlative with the
Oxfordian-Kimmeridgian Mariposa Formation, the nludstone-tuff
member could be Callovian-Oxfordian in age, as are Callovian
ammonite-bearing argillaceous sequences to the north in the
central Foothills terrane (Duffield and Sharp, 1975).
Mainly because of the common unconformable contact of all
slate sequences with underlying metavolcanic units, this
unit is here assigned as a basal member of the Mariposa
Formation (see Figure 23).
Mudstone, Mariposa Formation. The top of the Mariposa
Formation is everywhere truncated by shear zones or covered
.,.- .. _ .. -.-~ '-"'- '. -~. ',."
I .:
121
by cretaceous overlap sediments to the west. In most cases
the base of the sequence has been sheared at the contact
with underlying units. Where the contact has been observed
unsheared, strata on eit.her side are subparallel, there may
be alteration in the underlying units that is not present in
the overlying Mariposa, and volcanic contributions are
absent in the Mariposa directly above the contact with vol
canic rocks. As previously discussed, this basal contact is
probably a paraconformable or diastemic surface.
The age of the Mariposa Formation is constrained by its
fossils, specifically the Late Oxfordian-Early Kimmeridgian
bivalve Buchia concentrica and the Early Kimmeridgian ammo
nite Amoebites, found in northern and eastern parts of the
eastern pelitic belt (Imlay, written communication reported
by Clark, 1964). Imlay also identified Buchia concentrica
and other Late Oxfordian-Early Kimmeridgian fossils in the
Salt springs Slate - the Mariposa Formation in the central
pelitic belt (Clark, 1964).
In the field area the Mariposa Formation is more than
1000 meters thick. The dominant lithology is thin-bedded to
laminated dark gray to black fine grained meta-argillaceous
mUdstone variably metamorphosed to shale, slate, or phyl
lite. Thin-bedded poorly-graded epiclastic gray micaceous
feldspar quartz graywackes and arenites are intercalated.
122
Rare lenses of black limestone are present in the central
pelitic belt (Clark, 1964). Intervals of medium to thiclc
bedded mudstone-pebble conglomerate are not uncommon,
particularly in the central pelitic belt. Clasts in these
conglomerates are either intraclasts similar to the mUdstone
matrix or they are clasts of slightly more siliceous
mUdstone. In the central pelitic belt, rare lenses of
conglomerate with pebbles to cobbles of quartzite are inter
calated with feldspar quartz arenites and mUdstones. Bogen
(1984) suggested that quartzite clasts in Mariposa conglom
erates in the northern extension of the eastern pelitic belt
are metasandstones and meta cherts of probable exotic prove
nance. In the central pelitic belt, conglomerates with
quartzite clasts sporadically crop out for at least 40 km
strike-length south from the WGst side of Lake McClure.
In the eastern pelitic belt, fine graywacke sandstone is
common, intercalated with abundant slaty mudstone and con
glomeratic mudstone-pebble lithic wackes. Sandstones are
gra.ded, exhibit scour marks and rare cross-stratification,
and are probably turbidites.
Intrusive Rocks
There are two major suites of pre-cretaceous intrusive
rocks in the field area. The older suite is the Don Pedro
intrusive suite in the north central part of the eastern
123
metavolcanic belt. Plutons.dominantly are pyroxene diorite
and hornblende diorite and have zircon U/Pb dates of ca.
198-200 Ma (Saleeby, 1982).
The younger suite is the Guadalupe intrusive complex
(Best, 1963) in the south central part of the central
pelitic belt, Figure 22. It consists of an older gabbroic
to dioritic northern portion and a younger quartz monzonite
to granite southern facies. Analyzed representative samples
of the northern facies are presented in Table 4. The basic
rocks predominantly are olivine-normative tholeiites with
modal clinopyroxene + plagioclase ± amphibole. They exhibit
relatively flat REE patterns with slight negative Eu anoma
lies (Figure 29).
The northern facies includes other small gabbroic and
tonalitic intrusive bodies north of the main part of the
complex (e.g., the Hornitos pluton of saleeby, et al., 1989)
which are not demarcated in Figure 22 and are here consid
ered as part of the Guadalupe complex. In the northern
inner aureole of the main complex, gneissic-appearing rocks
containing red-brown biotite + quartz and plagioclase (some
grains cataclastic) are highly deformed and sheared intru
sive rocks and are associated with upper greenschist to
lower amphibolite-grade, locally mylonitic, country rocks.
At the contact between the northern gabbroic facies and
the southern felsic facies, the latter contains abundant
---------------------,----~ .. -.. _., .. _ .. - ,_ ........ " ' .. '-' ., .-...... _ ........... -. __ .... - .
I ' I ~.
124
TAIlE 4. WIC IETA·IGlBUlIIOIX AllALYIEI, CElTIIAL mlliC IELT, IIIIT ... PAIl Of TE RXITIIIW TEIIAIE
&Will.,. intl'Ulhw SllPI ... GrWI kCUit.in c.nt,..( p.li tic .. It northem f..,l_ • ltend .... tt h'~-III .... It •
...,1 •• 15·m' ... J't 15·'1f2' ~.R'l ,".n' ~·54· I .. OlA I 116-m' ~·5t'
IIIIjor .1_. (ut '" 50.40 47.31 46.69 ·'Oz 50.50 50.60 50.05 54.111 51.79 50.99
TlOz 1.32 I.D 1.37 1.81 1.21 2.OS 0.51 O.D 0.35 AlzOl 14.50 16.00 15.17 15.07 15.91 14.90 I!!.IO 9.27 9.43 Fez03 9.M 9.SS 10.15 9.97 11.79 12.91 6.43 9.611 9.111 lInD O.I!! 0.16 0.17 0.17 0.1$ 0.21 0.32 0.17 0.16 lItO 6.811 11.23 7.00 4.64 6.64 5.14 9.91 19.36 111.70 c:.o 10.40 11.90 11.111 11.10 10.11! 11.96 17.20 9.OJ 11.56
·aza 2.98 2.69 3.10 4.07 3.SS 3.97 0.51 O.II! 1.32 ICza 0.57 0.16 0.01 0.98 0.55 0.81 0.04 0.02 0.31 PzDs 0.12 0.11 0.111 0.27 0.24 0.29 0.41 0.12 0.14 LOI 0.74 0.20 0.47 0.20 0.13 0.01 I.I!! 2.9Z 2.75 Totel 98.00 101.23 98.SS 99.30 99.111 100.13 99.96 99.04 98.22
..... tlled to 100.0 "wl.tlle-free, with FeZ!JJlFdJ • 0.20 ·'Oz 52.37 50.50 51.32 SS.14 52.35 '1.49 51.211 49.64 49.34 TlOz 1.37 I.D 1.40 1.611 1.22 2.10 0.'2 0.35 0.37 AlzOl 15.04 15.97 15.56 15.34 16.10 15.05 13.35 9.73 9.96 Fez03 1.51. 1.50 1.59 1.55 1.36 1.99 1.00 1.55 I.SS Feo 7.80 7.50 7.94 7.74 6.711 9.96 4.99 7.75 7.90 lInD 0.13 0.16 0.17 0.17 0.15 0.21 O.D 0.111 0.11 lItO 7.14 11.21 7.111 4.72 6.71 5.19 10.OS 20.32 19.76 c:.o 10.79 11.811 11.46 11.24 10.95 9.05 17.50 9.48 9.05
·aza 3.09 2.611 3.111 4.14 3.59 4.01 0.52 0.111 1.39 ICza 0.59 0.16 0.01 1.00 0.56 0.66 0.04 0.02 0.33 PzDs 0.12 0.~1 0.111 0.27 0.24 0.29 0.42 0.13 0.15 Totel 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0
CIPU _tlWI _Ipl. G 0.00 0.00 0.00 2.59 0.00 0.00 3.91 0.00 0.00 C 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 or 3.49 0.94 0.06 5.119 3.29 3.811 0.24 0.12 1.94 ... 26.15 22.72 26.90 35.05 30.36 D.9Z 4.39 7.37 11.80 .... 25.41 31.05 211.15 20.31 26.19 21.12 D.9Z 22.51 19.96 ne 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 dl 22.23 22.01 22.44 15.49 21." 111.11 40.111 111.111 19.09 hy 16.'2 12.112 14.01 14.60 11.26 9.52 13.97 31.112 20.113 01 1.04 5.'1 3.05 0.00 5.48 5.119 0.00 16.11 23.04 lit 2.26 2.17 2.30 2.24 1.96 2.119 1.45 2.25 2.29 Il 2.60 2.'2 2.61 3.19 2.32 3.99 0.99 0.66 0.70 lIP 0.29 0.26 0.44 0.81 0.57 0.69 0.99 0.30 0.35
Trac. al ...... t •• not _tiled (pp.)
Y 35. !! 25. !! !! !! !! !! 23. !! !! !! V 248. 249. 2110. 250. 250. 360. 59. 190. 190. Be 66.5 5.3 57. 12. SS.2 15.9 tn. 34. 151. 14. 216. 14. 29.9 4.5 32.5 10.3 119.1 111.6 Co 37.0 0.4 41.7 0.4 40.2 0.2 46.25 0.19 30.4 0.2 22.7 0.1 11.31 O.OS 611.1 0.3 69.5 0.2 Cr 216. 4. 311. 6. 99.1 3.4 12.9 0.9 164. 4. 211.3 4.11 73.9 2.21561. 16. 144'. 15. ee 0.662 0.020 0.191 0.136 0.071 0.0111 0.146 0.6111 0.056 .. 0.111 0.121 0.801 0.1(6 Hf 2.9Z O.OJ 1.70 0.05 2.35 0.06 4.02 0.06 2.69 0.01 2.111 0.05 5.57 0.05 0.4113 0.045 0.456 om Rb 111.11 1.1 3.110 0.611 3.04 O.SS 4.315 0.724 0.617 0.557 111.5 0.7 d.O o.w 1.04 3.29 0.65 Sb 0.445 0.013 0.092 0.014 Te 0.213 0.004 0.133 0.004 0.0115 0.OJ9 0.429 0.045 0.204 0.036 0.457 O.DD 0.1161 0.009 0.102 0.0411 0.D19 Th 1.24 0.01 0.229 0.014 0.220 O.OJI 0.611Z 0.022 0.294 0.0211 2.595 0.026 II.II! 0.09 0.130 0.021 0.161 O.IPZ U O.!IIII O.O!III 0.073 0.031 0.297 0.049 0.201 0.031 0.696 0.114 2.69 0.05 0.0111 0.026 0.101 O.ml Zn 73. 15. 711.1 1.4 125. 4. 75.6 2.4 19.7 1.3 m. 66.6 4.15 63.0 1.9 Zr 115. 10. e75. 75.6 11.0 144. 10. IOJ. 7. 107. 9. 1111. 2. 15.1 7.6 24.1 6.6 sc 36.7 0.3 37.4 0.4 39.0 0.2 44.4 0.2 37.' 0.1 25.3 0.1 9.39 0.09 34.35 0.10 34.9 0.1 L. 4.57 0.05 3.24 O.OJ 3.57 0.04 11.71 0.06 6.97 0.05 10.' 0.1 24.6 0.2 0.967 0.024 1.035 om Ce 12.1 0.5 11.52 0.09 10.25 0.29 23.2 0.5 17.9 0.3 24.3 0.4 48.7 1.5 2.110 0.41 2.91 0.26 Id 11.0 0.3 7.46 0.37 9.95 e.~ 111.81 O.II! 13.4 0.6 15.7 0.7 21.6 0.4 2.21 0.44 2.70 0.61 SII 3.90 0.12 2.110 0.22 !.<o 0.01 '.80 0.01 3.70 0.01 4.21S 0.010 4.611 0.05 0.770 0.005 0.1162 o.m; Eu 1.32 0.01 1.111 O.O~ 1.24 O.OJ 2.07 0.01 1.15 0.01 1.20 0.01 1.14 0.01 0.365 0.00II 0.331 O.OP Gel 4.90 0.25 0.416 0.'.142 3.811 0.04 Tb 0.1173 0.017 0.725 0.001 o.rn O.O!III 1.21 O.OJ 0.759 0.021 0.1116 0.0111 0.632 0.006 0.1113 0.0111 0.221 0.121 T. 0.536 0.011 0.401 O.IIU! 0.452 0.057 0.665 0.016 0.467 0.061 0.4111 0.053 0.419 0.017 0.0119 0.052 0.159 OJra Yb 3.19 O.OJ 2.49 0.02 2.96 0.04 4.45 0.01 2.114 0.06 3.24 0.04 2.61 0.05 0.946 0.01S 1.01 0.02 Lu 0.466 0.005 0.362 0.004 1:'.448 0.007 0.671 0.009 0.435 0.007 0.471 0.007 0.3111 0.004 0.151 0.005 0.161 o.m;
A Major el ... : XRF (gleu). U.S.G.S., D<fMIr: r.v: I.::', U.S.G.S •• D<fMIr: other trace al ... : 111M. U.S.G.S •• D<fMIr. II IIIIjor el ... : ICP-AES. cOIIII!rc'al lab.: V: MS. c.-rel.1 , ... ; other trllC.!l al ... : N.C ..... t .... III. L&a>ar end PI_tary L ..... IkIlv. of Ariz •• Juea"".
-------------------'---'"'._"'- -
Basfc to sflfcfc hypabyssal rocks, northern part of the Guadalupe Intrusfve Complex
i I I ;
10 .I-'--_-:--~--:--:---~-~-~----=----:-~-:----La Ce Nd Sm Eu 1b 1m Vb Lu
Atomic Number
Figure 29. coryell-Matsuda plot of basio to silicio hypabyssal rooks of the northern part of the Guadalupe Intrusive Complex.
125
. ..... __ .- -~------.--. . ........ -, ........ _p ..... ~-.~-... --.-.. --.- .
126
xenoliths of gabbro and diabase identical to the northern
facies of the complex. Different facies of this complex
have been dated at ca. 146-151 Ma (Saleeby, et al., 1989).
Both the northern and southern facies were sheared and
deformed, and rolled garnets are present in staurolite
garnet-andalusite-biotite schist adjacent to intrusive rocks
on the west side of the complex. Intrusion of the complex
was synkinematic.
Isolated Mafic/ultramafic Lenses in the central Pelitic Belt
Much of the central pelitic belt in Figure 22 may be
described as broken formation as it contains meter to
kilometer-scale lenses of mafic metavolcanic rock and amphi
bolite, conglomeratic arenite, and serpentinite intercalated
with pelite. In the northwest part of the central belt in
the study area, broken formation to true melange contains
large serpentinite exposures and amphibolitic metavolcanic
phacoids containing blue-green amphiboles. The amphiboles,
which contain approximately 4 wt. % Na20 (Figure 30), are
part of an assemblage including white mica (muscovite),
plagioclase (albite), epidote, sphene, rutile, ± chlorite in
mylonitic amphibolite at the periphery of mafic to ultra
mafic lenses in pelite. The NaM4 versus AIW contents of the
amphiboles suggest 6-7 kb pressure during formation (Figure
30). The assemblage is similar to metabasite
.. ,._ ......... _ ... _., •••••• w ••• '... • .,'w • _ ••
I .:
No M4 1
.0
3kbl--__
o :~:~!i:!~Nftt;::·:),:,:·;\:,::··:.;i:,:·;,;;;::::;:~;:.:!;;;:t;::.::::;;;1?:.:.:/::.':;·,·.·.: :···:'j'··'·','··'.':':··E.::;;.:::;·:));Sierr.· '.:,,·:.:::<.:·':',:;\i,;::
·?'.O 0.5 1.0 1.5 2.0
A,rl:
Figure 30. Plot of N0H4 VB. AllY of blue·green -Pt.iboles from mylonitic oqilibolftes within ultrllllllffc/.fic lenses along the northern terllinus of the western strand of the Bear Homtoins fault zone, southern Foothills terrane. The diogra. (after Brown, 1977) suggests tentative esti.tes of pressure of fOlWltion. Thl following are overage high-No and low-No end-lIeIItler caqJOSitions of sllq)les which plot as the two bors in the figure:
Nal'J 1Cl'J COO Sill;! Alz03 FeO MgO Till;! MIlO TotolB
wt.X wt.X 4.20 3.63
.95 .95 8.21 8.34
43.50 43.95 12.95 12.80 11.61 11.26 9.39 10.46
.82 .16 ~ -:.1l 91.94 98.32
Erlds of bars plotJri represent onolyses adjusted to total Fe = Fe2+ and total Fe = Fe •
Aelectron microprobe onolyses by M.S. Gustin and M.C. Newton, III, B LUlOr and Planetary Laboratory, The ~Iverslty of Arizona. All other truce eleaent ions individually <.10 wt.X.
----------------------------------~-- .. -_ ... _ .. -............... '"
127
128
mineralogy in the Bessi-Ino area of the Sanbagawa terrane,
Japan (Miyashiro, 1973), and it is assigned to the epidote
amphibolite facies.
To the south, the main inclusions in pelite are mafic
metavolcanic rocks, which appear to be members of two dif
ferent suites. Aphyric black to dark green metabasalts are
highly enriched in magnesium (see Table 4) and are compo
sitionally similar to some serpentinitic pods to the north.
The second suite consists of basaltic andesitic metavolcanic
rocks with crystals of plagioclase and either pyroxene or
amphibole, similar to Penon Blanco crystal tuffs. These
lenses crop out in the eastern part of the central belt
along with lenses of aphyric and pyroxene-phyric metaba
salts. Some of these lenses are mylonitic amphibolite
intercalated with metasedimentary and meta-intrusive rock;
all of the Penon Blanco lenses in the central pelitic belt
probably were tectonically emplaced along shear zones.
The fine-grained high-Mg basalt lenses are not serpenti
nized south of Lake McClure and yield reliable geochemical
results. with MgO contents of ca. 20 %, these subalkaline
basalts are chemically similar to komatiitic basalts. Two
representative samples are plotted with other high-Mg arc
and komatiitic basalt suites in the TFMA diagram (Figure
31A) and in normative ol-di-hy space (Figure 31B). In
-------------------~-------~ .. -... ~---.. -.-...... -........... -.. _ .......... _. __ ...•..
~ - A. (M90/ A1203)2
{Ti02/FeO·)5
B. di
* High-totg basalts from lenses In the Central Pentlc Belt
• Hlgh-totg basic rocks of the suHes plotted In (8)
MgO/FeO·
Figure 31. (A) TFHA cti.agram with field of various types of high-Kg volcanic rocks plus high-Kg rocks from the Central Pelitic Belt. Plotted crosses are uncti.fferentiated members of the suites shown in Figure 31B. (B) Various high-Kg volcanic suites plotted in CZPW normative ol-cti.-hy space, after chayes (1966). All samples normalized volatilefree with Fe1}.yFeQ = 0.20.
*Total Fe reported as FeO.
Explanation
a Aoba lsi., Vcnuatu - Warden, 1970; Gorion, 1977
• New Georglc Gp., Solomon lsi. - Stanton and 8ell, 1969
• Gorgona lsi., Colombia - Echeverria, 1980, 1982
• Barberion greenstone belt - Smith and Erlank, 1982
• Betts Cove, Canada - Upadhyay, 1982
+ Mariana forearc bonrnltes - Reagan and o Merler, 1984; Bloomer and Hawkrns, 1987
hy
..
I .:
130
Figure 31B, the Foothills high-Mg basalts overlap the
Archean Barberton, South Africa komatiitic basalts and plot
between two Phanerozoic komatiitic basalt suites, the Creta
ceous Gorgona Island, Colombia basalts and the Ordovician
Betts Cove, Canada basalts. They are not Si-oversaturated
like Guam boninites or alkaline like high-Mg basalts of the
New Georgia and Aoba arc suites. REE contents are low and
REE patterns are flat (Figure 32).
The contact relations of these basalts are ambiguous.
In a southern exposure, pelite on one side of a basalt lens
has been strongly deformed and metamorphosed to biotite
grade: psammite on the other side of the lens is extremely
indurated with negligible foliation and appears hornfelsed.
The basalt is chlorite-grade and shows no apparent thermal
metamorphic overprint. No Guadalupe series intrusive rocks
crop out nearby, so the thermal metamorphism appears to be
due to the high-Mg basalt lenses and pods in the area. The
lack of shearing structure in the basalts or adjacent coun
try rocks suggests the basalts were not tectonically
emplaced. They may have intruded the sedimentary basin as
dikes or sills.
..... ... _ ... _.e .. _. __ .. w ... , .....
CD ;t'" ~ 10 J
C o .c (J
~. u 10 o
Q: .,-
Hfgh-Mg basalts, lenses In the central pelitic belt
I .. :
131
I •
10 .'..l-_~ ___ ~ _____ ~_~ ___ ~_-,--,-__ La Ce Nd Sm Eu Tb Tm Yb Lu
Atomic Number
Figure 32. coryell-Matsuda plot of high-Mg basic rocks which crop out as lenses in the Central Pelitic Belt.
I ~.'
132
MASSIVE SULFIDE DEPOSITS
The southern Foothills terrane affords an excellent
assortment of ore-deposit types in a small area. Copper ore
was first discovered in the terrane in 1860, and active
massive sulfide mining in the late 1800's and early 1900's
led to Heyl's (1948) appellation "Foothill copper-zinc
belt". This belt comprises interarc basin, mid-ocean ridge,
and arc rift zone sequences, which bear syngenetic massive
sulfide deposits, and are now juxtaposed by translation and
deformation. Later fracture-channeled hydrothermal activity
produced epigenetic auriferous quartz-mariposite-carbonate
and talc-sericite lodes of the Mother Lode system, which in
turn gave rise to important placer gold deposits. The
structural control and genesis of Mother Lode gold deposits
is treated in another paper (Newton, in prep.). This paper
deals exclusively with the genesis of the syngenetic massive
sulfide deposits, which include three types: Kuroko-type Zn
cu-Pb deposits, Besshi-type Cu-Zn deposits, and cyprus-type
Cu (Cu-Zn) deposits. Host stratigraphy, ore, alteration,
and tectonic environments of deposition are discussed below.
cyprus-type CU (Cu-Zn) Deposits
Small copper-rich massive sulfide deposits, such as La
victoria and Barrett, are present in the top of the lower
Penon Blanco basalts. La Victoria (Figure 22), first mined
I .,. I
133
in 1864, was the largest producer and is representative of
these deposits. No modern work has been done on La
victoria, and early mining mainly removed supergene enriched
material. Ore at lower levels mainly comprised fine-grained
pyrite, chalcopyrite and quartz, with minor supergene azu
rite, malachite, and chalcocite (cox and Wyant, 1948). Ore
occurs as pods one to five feet thick in sheared metabasalt
and between relict pillows in less deformed metabasalt (Cox
and Wyant, 1948). Only a few tens of thousands tons of ore
were produced grading around 4 to 5 % CU and trace to 2 % Zn
(Cox and wyant, 1948). Mn-rich cherts with small Mn depos
its, associated Fe-rich cherts (red jasper), and radiolarian
cherts overlie the basalts. This is common in ophiolite
suites with Cyprus-type massive sulfide deposits, e.g.
manganiferous iron-rich sediments associated with red jasper
and radiolarian chert overlie pillow basalts of the Troodos
complex (Franklin, et al., 1981). Perhaps a large percent
age of the chert overlying the lower Penon Blanco basalts
had an exhalative origin, with or without biogenic precipi
tation.
Kuroko-type Zn-Cu-Pb Deposits
Deposits of this type are present in the upper part of
the Gopher Ridge formation in the southern Foothills ter
rane. This stratigraphy may be correlative with similar
-----------------------------~-- .. -.-.-.. -.- .......
I •.
134
strata hosting massive sulfide deposits in the Gopher Ridge
Formation and in the Copper Ridge Formation to the north.
The Blue Moon deposits in the field area are located just
southwest of Lake McClure (Figure 22). The altered rhyo
litic host stratigraphy, which attained chlorite-grade
greenschist facies, is part of a bimodal assemblage of rhyo
litic breccia, lava, and tuff and basaltic andesitic to
basaltic breccia and lava. Two massive sulfide/sulfate
horizons are hosted by rhyolitic meta-vitric hydroclastic
deposits and lava that are either aphyric or have plagio
clase feldspar ± quartz phenocrysts. Sulfide/sulfate lenses
along the lower horizon commonly directly overlie massive
altered aphyric rhyolitic lava with relict flow banding.
Ore lenses of the upper horizon commonly are intercalated
with fragmental aphyric rhyolitic deposits that have dense
meta-vitric subequant fragments in a meta-vitric ash matrix
of similar composition. Pumiceous fragments and accidental
lithic fragments are rare, and the deposits are probably
hydroclastic breccias. Alteration around the ore bodies
dominantly consists of intense sericitization and strong
silicification manifested as weakly pyritic quartz-sericite
phyllite with local massive and stringer gypsum, and nodular
anhydrite and barite.
""--" -.... _- - .,., .....
1<-l
135
Ore has ,sulfide and sulfate facies. Sulfide facies
mineralization generally is sphalerite-dominated, consisting
of pyrite + sphalerite + chalcopyrite ± galena (MacVeigh and
Meade, 1987). Locally on the lower ore zone, a
chalcopyrite-dominated facies of pyrite + chalcopyrite ±
sphalerite is present. Sulfate facies is generally
anhydrite-dominated with less abundant gypsum and sulfides.
Thin intervals of massive barite are locally present.
Sulfate-facies ore has higher precious metal values and
higher cu/Cu+Zn ratio than sulfide facies ore (MacVeigh and
Meade, 1987). MacVeigh and Meade (1987) reported reserves
of 2 million tons of 8.77 % Zn, 1.13 % Cu, .43 % Pb (mini-
mum), 2.46 opt Ag, and .06 opt Au.
Massive sulfide ore commonly displays a mixed pattern of
sulfide mineral aggregates in a ribbony interleaved texture,
probably a tectonic shearing phenomenon. Rare continuous
compositional banding is probably relict primary layering.
Overlying some ore zones are horizons containing massive
sulfide or massive sulfate fragments similar to the underly
ing ore bodies. The massive sulfide/sulfate bodies probably
were syngenetic exhalative deposits, with local slumping,
disruption, and downslope resedimentation of massive sul
fide/sulfate fragments.
136
Like the type Kuroko deposits, the Blue Moon deposits
have abundant sulfate, including barite. Additionally, the
deposits are zinc-domin~ted with pyrite as the dominant Fe
sulfide. However, Kuroko deposits have higher overall Fe
and Pb contents than the Blue Moon deposits. Kuroko depos
its are Pb-rich; some deposits have more Pb than Zn. The
Blue Moon deposits have very low Pb contents for Kuroko-type
deposits. The lowest levels of the Foothills terrane
exposed are ultramafic complexes, and the basement upon
which Foothills arc sequences developed was oceanic litho
sphere, as suggested by several authors, e.g. Saleeby
(1982). The low ore-Pb contents may owe to hydrothermal
and/or magmatic scavenging in oceanic lithosphere rather
than continental lithosphere.
In the western metavolcanic belt, felsic pyroclastic
rocks thicken to the northwest and thin to the southeast.
The Blue Moon deposits did not form in a region of major
felsic pyroclastic eruption or deposition. Rather, the
deposits are associated with locally thick hydroclastic and
effusive rhyolitic lava facies which thin dramatically both
north and south, suggesting a possible lava dome structure.
These lava facies rest directly on intermediate to mafic
volcanic rocks with no intervening felsic tuff. No caldera
structure or caldera igneous/hydrothermal control is indi-
cated. The proximity of the paleo-hydrothermal system to
the contact between the western metavolcanic belt (an arc
complex) and the central pelitic belt (a back-arc/interarc
basin), suggests the possibility that the hydrothermal
activity was controlled by a high-angle fault system that
was to form the edge of a growing marginal rift basin.
Besshi-type eu-an Deposits
137
Green Mountain and similar eu-Zn massive sulfide depos
its are found in the southern part of the central pelitic
belt. The Green Mountain deposit itself is located south of
the area included in Figure 22. Mattinen and Bennett (1986)
recognized these deposits as being similar to the Besshi
massive sulfide deposits of the Sanbagawa terrane, Japan.
The Green Mountain deposit is hosted in meta-epiclastic
pelites and arenites containing minor basic sills (Mattinen
and Bennett, 1986). Early production was from supergene
enriched ore grading 6 % eu; current reserves are 4 million
tons grading .78 % eU J 1.42 % Zn, negligible Pb, .50 opt Ag,
and .003 opt Au (Mattinen and Bennett, 1986). The ore is
dominantly pyrrhotite-sphalerite-chalcopyrite. Like the
type Besshi deposits of Japan, the Green Mountain deposit is
an extensive tabular 20 ft thick sheet of uniform thickness
and mineral facies over its extent (Mattinen and Bennett,
... -_ .. -.. -- .~.-. -, .. _.. . -.
1986). An altered basalt sill or dike intrudes the Green
Mountain deposit stratigraphy (Table 4).
138
The deposit is within two kilometers of the exposed
cretaceous Sierra Nevada batholith, and the host rocks have
an amphibolite-grade thermal metamorphic overprint. The
immediate footwall of the deposit is a coarse-grained
cordierite-anthophyllite schist with accessory spinel,
biotite, and garnet, and local retrograde chlorite and talc
(Mattinen and Bennett, 1986). The pre-contact metamorphic
assemblage was Mg-enriched.
Unlike the host rocks of type Besshi deposits in Japan,
which have subequal amounts of basic volcanic rocks and
sedimentary rocks (Fox, 1984), the Green Mountain and asso
ciated deposits are dominantly sedimentary rock-hosted with
only minor, dominantly shallow intrusive, basic igneous
rocks. Some Besshi-type ore environments however are mainly
sedimentary with little or no volcanic members, e.g. in the
Prieska area of South Africa (Franklin, et al., 1981).
Mattinen and Bennett (1986) noted similarities between
the Besshi and Green Mountain deposits and sulfide blankets
currently forming in the Guaymas basin, Gulf of California.
The Guaymas basin contains two spreading centers which have
3-5 km wide rift valleys (Lonsdale, et al, 1980). These
rift valleys contain thick mixed terrigeneous clastic and
139
organic-rich pelagic sequences that Lonsdale, et al (1980)
suggest were rapidly deposited (>1 m/1000 yrs). The thick
unconsolidated sediment blankets are suggested by those
authors to have prevented the extrusion of basalt flows,
which instead characteristically form thin sills and thicker
doming intrusive bodies. Lonsdale, et al (1980) note that
bottom waters are poorer in oxygen in these semi-restricted
rift basins with organic-rich mud than in open ocean. In
the Guaymas basin, eu-rich pyrrhotitic hydrothermal deposits
are associated with massive talc deposits that form ledges
and terraces commonly exposed along rift-floor faults.
Lonsdale, et al (1980) note that sulfide deposits on unsedi
mented segments of the East Pacific rise lack associated
talc deposits, and that similar sulfide deposits with asso
ciated talc are found in the semi-restricted, oxygen-poor
floor of the Hess deep, Galapagos rift. The similarities of
eu-Zn pyrrhotitic mineralization, Mg-enriched alteration
assemblages, thick fine-grained sedimentary rocks, and basic
shallow intrusive rocks are striking between the Green
Mountain and the Guaymas basin stratigraphies.
Green Mountain and associated Besshi ·~type deposits to
the north are caught up in the main shear system which cuts
the western border of the Guadalupe complex and is the
southern continuation of the Bear Mountains fault system.
I •.
140
It is likely that this shear zone system formed along ear
lier fracture systems that may have controlled hydrothermal
and igneous activity during active basin development.
PETROTECTONIC ASSEMBLAGES
Tectonostratigraphic (lithotectonic) terranes consist of
combinations of "tectonic assemblages". Tectonic assem
blages, in addition to distinctive lith~logies, may include
superimposed tectonic fabrics, metamorphic facies, distinc
tive intrusive bodies, distinctive discontinuities, e.g.
widespread dextral brittle-ductile shear zones, and other
such non-primary features that distinguish the assemblage
from other assemblages. Many of these distinctive features
may have been superimposed after a considerable separation
by time or distance from the original tectonic setting, and
therefore tectonic assemblages may differ from "petrotec
tonic assemblages".
The characteristics of certain "petrotectonic assem
blages" are diagnostic of specific plate tectonic settings
(Dickinson, 1972). Such assemblages comprise facies that
are characteristic of depositional environments within a
tectonic setting, and their limits may not coincide with the
boundaries of stratigraphic units or tectonic assemblages.
Petrotectonic assemblages may include distinctive metamor
phic or intrusive facies or tectonic structures, but only as
----------------------------~.- .. -.- ._ ... - . ..,... ........ . .... - -' ....• __ ...... .
I >I
141
such featur~s relate to the tectonic environment in which
the protoliths formed (for example, paired metamorphic belts
and a convergent plate boundary).
Volcanic eruption styles, distinctive igneous composi
tional suites, and characteristic sedimen'tary associates are
important in distinguishing petrotectonic assemblages.
Discrimination also may be facilitated by recognizing trace
and major element signatures in igneous rocks that are
distinctive because of the geotectonic processes influencing
magma genesis.
Petrotectonic assemblages, characteristic of particular
tectonic settings in which protoliths in the southern Foot
hills terrane formed, are given informal names: from oldest
to youngest respectively, Assemblages 1,2,3,4,5,6. The
relationship of these assemblages to stratigraphic units is
shown in Figure 23. The development of the ternary trace
element ratio plots (TVRT and VTRT diagrams) and the
extended Coryell-Matsuda plot employed here was discussed in
Chapter One.
Ocean Floor. Assemblage 1
This assemblage includes the pillowed and massive
basalts of the Basalt Member and the overlying chert
dominated basal portion of the thin-bedded sequence of the
lower Penon Blanco Formation (see Figure 23). In Figure 33,
..-.. _ ......... _ .. _ .. ~ . .,. " ... , ... ,~ .....
I ~.
4D = ~ 10· c o .c o ~ () 10 o a:::
1
Basalts. Lower Penon Blanco Formation
o N5-09 * N5-198
10-1~----------------------------------------------------------To Th Ce TbTl V
142
Figure 33. Simplified extended Coryell-Matsuda plot of basic vOlcanic rocks of the lower Penon Blanco Formation.
-----------------------------.----~-.----.--.... -- .... -...... __ ............ __ .... ..
143
the two lower Pefion Blanco basalts whose REE were plotted in
Figure 25A, cf. Table 2, are plotted in extended Coryell
Matsuda form with Ta, Th, Ti, and V. These basalts are
depleted in LREE and extremely depleted in Th, typical of
very depleted N-type MORB, cf. Figure 34A. They are more
depleted in LREE and Th than typical back-arc basin basalts.
The basalts plot in the MORB field in the TVRT diagram
(Figure 35) well-separated from all other rocks from the
field area. The relativ6;!ly low Th/Ta ratios indicate that
their genesis was not influenced by sUbduction zone pro
cesses (see Chapter One). Geochemically, the basalts are
indistinguishable from very depleted N-type mid-ocean ridge
thoeiites.
The overlying cherts contain radiolaria (Taliaferro,
1943b) and they are probably ocean-floor pelagic sedimentary
rocks with a strong exhalative component as previously
discussed. The chert sequence is manganese-rich, a charac
teristic of ophiolitic sediments in ancient deposits.
Manganese also is greatly enriched in modern oceanic sedi
ments overlying the Mid-Atlantic Ridge and East Pacific Rise
compared to other oceanic sediments (Elderfield, 1978).
Petrotectonic Assemblage 1 represents a section of upper
oceanic crust, i.e. the upper part of an ophiolite suite.
The basalt chemistry suggests the ophiolitic sequence formed
------------------- ---- -.-.. _- ........ _ .. _.-_ ........ .
A. 1O' .,.Id-oc.anlc Rldg. Bllalt.
1O'
lo,
..... J 10
I." T. '" c:. lbn v
B. 10' Volcanic Arc Basic Rocks
10'
i 10'
I J 10
I'" T. '" c:. lbn y
C. 10 Oc.anlc Island Ba.lc Rocks
10"~~=-=---------~~--------'----~~
144
0.1. Back-arc Ba.I" Ba.alft
II'
jlo,
.......
11o
1\1121
I." T. '" c:. lbn v
E. 10' rorearc Baalc Volcanic Rocks
10'
lo ...... I II
1\112'
I." T. '" c:. lbn y
~lgura 3.. a!apllfla4 aztan4a4 eozyell-Katau4a plot. of 70unq basia rook. froa 4lfferant taotoDlo .atting., i.e. (a) .t4-ooean ri4g •• , (D) voloanio eroa, (e) lntraplata ooaaDia ialan4., (D) hok-aro IlaalDa, (B) forearo req!oDII. aa'l Cbaptar 1 for uplllllation of el_ant poaitioDlng. Data souroe. ara II.ta4 iD Appan41z a. ChoD4rltio oonoantratloDII ara the average e1 valua. o~ AD4er. an4 Bblhara (1182).
(TI/V)/5
(Tb/Ce)xSO
Explanatfon
A Lower Penon' Blanco F'm. basalt It Middle Penon Blanco F'm.
bal./bas.andel.
D Lower Gopher Ridge F'm. bas./bas.andes.
Th/Ta
Figure 35. The TVRT diagram with basio rooks of the Penon Blanoo Pormation and lower Gopher Ridge Pormmtion plotted •
145
. ----------------,---~-".--.-.. --.... --....... - .......... _ ....... _ .. __ . __ .. .
146
at a mid-ocean ridge rather than a marginal basin spreading
center. This ocean floor eventually became part of the
overlying plate of a sUbduction system, through which the
Penon Blanco arc volcanics erupted.
Primitive Volcanic Arc. Assemblage 2
Assemblage 2 comprises the Penon Blanco Formation above
and including the upper tuffaceous portion of the thin
bedded sequence of the lower Penon Blanco Formation (see
Figure 23). This assemblage is dominated by basaltic ande
sitic volcaniclastic rocks, and it represents a submarine
volcanic arc. Analyzed homogeneous samples are tholeiitic
basaltic andesites, low in Tio2 and high in MgO, generally
with flat to depleted LREE patterns. They exhibit the Th
enrichment and Ta depletion and low Ti/V characteristic of
arc volcanic rocks (Figure 36), cf. Figure 34B. In addi
tion, they display slight negative Ce anomalies (Figure
25B), a common trait in intermediate to mafic arc volcanic
rocks (White and Patchett, 1984). The samples analyzed in
this study define a tholeiitic arc suite with relatively
high Mg/Fe (Figure 24). The flat to depleted LREE patterns
are consistent with the chondritic patterns suggested by
Jakes and Gill (1970) to be typical of the earliest, most
primitive members of arc tholeiitic suites. In the TVRT
diagram (Figure 35) these rocks cluster in the lowest part
I • , ~.
QJ -
10 ~
10 I
~ 10 J
C o .c (.)
~ u 10 o
a::
1
Basalts-basaltic andesites, Middle Penon Blanco Formation
o NS-10 * NS-243 A NS-214 ~ NS-347 + NS-358 Ie N7-58
10 -'..L __________ ~:------------_::_ To Th Ce TbTr v
Figure 36. Simplified extended Coryell-Matsuda plot of basio voloanio rooks of the middle Penon Blanoo Formation •
147
. _---------------- --_._-_. __ .. -- ... __ ... _---- ... _--- ........................... -............. __ .... .
1~
and even below the field of arc volcanic rocks outlined on
the basis of complete analyses available in the literature.
They are suggested to have formed in an arc environment more
primitive than the settings of most of the arc samples used
in deriving the figure. Such a setting is quite compatible
with the upward transition from ocean floor pelagic sedi
mentary rocks to arc-related intermediate tuff observed in
the thin-bedded sequence of the lower Penon Blanco Forma
tion. Assemblage 2 most likely represents the development
of an intra-ocean primitive volcanic arc around 200 Ma ago.
Bogen (1985) also suggested the Penon Blanco Formation
formed by submarine island arc volcanism.
Extensional Volcanic Arc. Assemblage 3
Assemblage 3 comprises the lower part of the ca. 160-170
Ma Gopher Ridge Formation (see Figure 23). In the southern
part of the Foothills terrane, the lower Gopher Ridge
Formation dominantly comprises basaltic andesitic and basal
tic hydroclastic breccias with less abundant andesite and
minor local dacite/rhyolite. Basaltic andesite appears to
be the dominant composition; analyzed samples are tholeiitic
basaltic andesites that plot in the field of volcanic arc
basic rocks in Figure 35. These basaltic andesites have
flat LREE patterns (Figure 28), but they are not depleted as
are some of the middle Penon Blanco basaltic andesite pat-
I . , <.
149
terns. Also in contrast to the Penon Blanco tholeiitic
rocks, the lower Gopher Ridge tholeiitic rocks are lower in
Mg and higher in Ti, and they plot in higher Fe/Mg-Ti/Fe
portions of the TFMA diagram (Figure 26), cf. Figure 24.
There is a marked negative Ta anomaly (Figure 37) typical of
arc volcanic rocks. Although these rocks represent a less
primitive arc than the Penon Blanco arc, the flat REE pat
terns, tholeiitic compositions, and dominant basaltic ande
sitic composition indicate the arc was not mature. The age
difference of ca. 30 Ma between the middle to upper Penon
Blanco and the lower Gopher Ridge formations indicates that
the two arc settings they represent probably were not
related by evolution.
The lower Gopher Ridge rocks represent an immature
tholeiitic volcanic arc sequence, but they do not represent
typical arc explosive pyroclastic eruptions. Rather they
are dominated by fragmental deposits with rounded amygdaloi
dal lava fragments in a non-amygdaloidal fine matrix. such
features are typical of hydroclastic mafic deposits (Fisher
and Schmincke, 1984). Such eruptions may obtain their
explosiveness by flashing steam discharges of water encoun
tering lava, rather than the magmatic devolatization
associated with pyroclastic eruptions. Minor dacites and
rhyolites in the sequence are lavas of limited areal extent
and may represent lava domes.
. ... -._.·.,..····_·.-.flIlII···-,·
i ~;
A.
B.
to •
Basalttc andesites, Lower Gopher Ridge Formation In study area
o N5-333 .. N5-140
10-' To Th Ce Tbn
10 •
1
Basalts, Gopher Ridge Formation (undifferentiated), north of study area
oGR-03 .. GR-06
10 -, To Th Ce
150
v
v
Figure 37. Simplified extended coryell-Matsuda plots of basic volcanic rocks of the lower Gopher Ridge Formation in the study area CA) and from north of the study area CD).
1Sl
There is a predominance of explosive and effusive lava
phases, as opposed to the generally high percentage of
pyroclastic phases in most arc systems (Garcia, 1978). The
dominance of lava and explosive lava eruptions rather than
pyroclastic eruptions would be more expected in a tensional
rather than a compressional stress regime. The Farallon and
North American plates probably were undergoing sinistral
oblique convergence during the period of 160-170 Ma ago (see
Chapter Two). Such oblique convergence may have engendered
a tensional stress regime, producing a magmatic arc undergo
ing synvolcanic extension. Beard and Day (1987) suggested
that the Smartville ophiolitic complex in the northern part
of the Foothills terrane, dated at 159-163 Ma, formed as the
extensional core of a rifting arc, and that rifting was
coeval with calc-alkaline arc volcanism. That northern
igneous activity is directly along strike from the Gopher
Ridge Formation, and it probably took place in a northern
extension of the same arc system. In the southern Foothills
terrane, the predominance of lava facies suggests that even
the oldest exposed facies in the Gopher Ridge arc system
formed in an extensional environment.
Arc Rift Zone. Assemblage.
The rhyolitic hydroclastic, pyroclastic, and lava phases
and basalts of the upper part of the Gopher Ridge Formation,
I ~;
152
and the oldest parts of the Mariposa Formation, compose
Petrotectonic Assemblage 4 (see Figure 23). The northwes
tern swath of the western meta'lfolcanic belt, Figure 22,
contains abundant felsic vitric tuffs. The amount of felsic
tuff decreases to the south in the western metavolcanic
belt, and felsic tuff is rare in the upper part of the
assemblage in the eastern metavolcanic belt. Major felsic
pyroclastic vents may have been located to the northwest.
In the western metavolcanic belt, rhyolitic lava and hydro
clastic phases host Kuroko-type massive sulfide deposits.
Massive and pillowed high-aluminum basalts are common at the
top of the assemblage in the southern part of the western
metavolcanic belt.
The bimodal character of this sequence and the strati
graphic position above an arc-related basaltic andesite unit
and below a basinal mudstone sequence support an origin in
an arc rift zone. Such zones were referred to as volcano
tectonic rift zones by Karig (1971) and Karig and Kay
(1981). These zones are marked by bimodal volcanic composi
tions with explosive silicic volcanism and more effusive
basic volcanism. Karig (1972) suggested that explosive
silicic volcanism may occur on the back-side of an arc or on
top of a remnant arc. Such tectonic environments have been
recognized in the rock record and are important settings for
I •.
153
volcanic-associated massive sulfide deposits, e.q. the
Hokuroku District, Japan (Sillitoe, 1982). Kariq (1972)
sugqested volcanotectonic rift zones represent the earliest
stage of extension in a trench-arc system, and Kariq and Kay
(1981) suggested they represent a transitional stage between
arc and marginal basin activity. This tectonic environment
was discussed in more detail in Chapter One.
Basalts of the uppermost Gopher Ridqe Formation, in
Figure 38, span the fields of ARB and BAB. The development
of the diagram in Figure 38 was given in Chapter One. In
the extended REE plot, Figure 39, they exhibit the variable
Th/Ta of ARB and BAB, and they exhibit variable LREE enrich
ment and depletion, more typical of BAB. Chemically, they
span a range suggesting formation in an arc rift zone to
early back-arc basin setting. Samples of other suspected
sUbduction-associated rift basin sequences in the Foothills
terrane also are plotted in Figure 38.
Subaqueous silicic explosive and effusive volcanism of
the upper Gopher Ridge Formation took place in an arc rift
zone. The basin containing the central pelitic belt must
have begun subsidence and expansion prior to these felsic
eruptions, as the eastern and western metavolcanic belts
apparently were separated by enough distance that very
little of the subaqueously erupted ash miqrated to
-----------------------------~ .. -.. -.... --- .... " .... " .... .. ............ -.- ...... .
r ~.:
(V/TI)x100
o
(Tb/Ce)x50
ExplanatIon
* Upper Gopher Ridge Fm. basalt
• Copper Hili Fm. bas./bas.andes.
A Logtown Ridge Fm. basalt
• Green Mountain altered basalt
o Guadalupe Intrusive Complex gabbro, d7.abase, diorite
Th/Ta
Figure 38. ~be VTRT diagram witb samples from subduction-associated rift basin units in tbe soutbern Footbills terrane.
--------------------_.--_. __ ._ ... _--_. - .. _---- ,,- ....
154
CD :!: ~ 10· c o
.s;. o ~
CJ 10 o
«t:
1
Basalts, Upper Gopher Ridge Formation in study area
o N5-55 * N7-07 A N7-09
lSS
10 -I To Th Ce TbTr v
Figure 39. Simplified extended coryell-Matsuda plot of high-Al basalts of the upper Gopher Ridge Formation in the study area.
---~.- ... ~----...... -... -.-... , .. -.. ' ... -- .......... _ ... _- ... -_ ..
I~
156
depositional sites in the eastern belt. The lowest part of
the Mariposa Formation, represented by the mudstone-tuff
member, formed in this arc rift zone (intra-arc) basin.
Back-arc/interarc Basin, Assemblage 5
Assemblage 5 comprises the bulk of the Mariposa
Formation in the central pelitic belt plus the Guadalupe
intrusive complex (see Figure 23). The mudstone-tuff member
of the Mariposa Formation, placed in the arc rift zone
assemblage, probably unconformably overlies lower Gopher
Ridge volcanic rocks and it contains thin felsic tuffs which
may have been deposited while upper Gopher Ridge felsic
volcanism was taking place to the west. It is likely that
the overlying bulk of the Mariposa Formation in the central
pelitic belt (Assemblage 5) formed in a basin between sepa
rated segments of an arc complex. The bulk of the Mariposa
Formation contains no felsic tuffs and overlaps the western
felsic volcanic rocks. This suggests that as the basin
enlarged, arc volcanism became extinct in the western meta
volcanic belt in the southern part of the Foothills terrane
and the basin represented by the central pelitic belt may
have been an interarc basin between remnant arcs. The term
"back-arc basin" as used in this report includes all basins
behind an arc edifice, whether the arc is active or extinct.
I ~:
W··
157
The nature of this basin is elucidated by stratigraphic
characteristics of the Mariposa Formation. On the west side
of the eastern metavolcanic belt, cutting across mUdstone of
the mudstone-tuff member of the Mariposa Formation is an
embedded lens of epiclastic breccia containing boulders and
cobbles of intermediate volcanic rocks. The epiclastic lens
is suggested to be a canyon-fill deposit and probably repre
sents resedimented breccia deposited by debris flows in a
feeder channel to a deeper water submarine fan. Other
coarse breccias with volcanic and sedimentary clasts are
locally present at the base of the Mariposa Formation around
the southern closure of the antiform in the eastern metavol
canic belt. The coarse epiclastic breccias reflect steep
escarpments bordering the sedimentary basin and suggest
active high-angle faulting at the time of deposition of the
basal Mariposa Formation. Synsedimentary faulting in the
central pelitic belt also is suggested by common intra
clastic mUdstone conglomerates intercalated with coherent
mUdstone.
Other than the lenses of various basic rocks in the
northern part of the central pelitic belt, there are two
types of basic rock suites intrusive into the Mariposa
Formation. One is a LREE-enriched basalt-type associated
with the Green Mountain Besshi-type deposit in the southern
---_ ...... _.'-, ... --- -.-. .. , ..
part of the belt. The other is the northern facies of the
Guadalupe complex.
158
A basic sill/dike intrudes the mine stratigraphy of the
Green Mountain deposit (Mattinen and Bennett, 1986). This
body has been rather extensively hydrothermally altered, and
is now depleted in Na20, K20, and many trace elements in
cluding V and Sc (Table 4). The basalt has high CaO and
moderately high MgO, which also may be modified. However,
it does not appear to have significantly altered REE, Th,
and Ta contents. In the REE plot (Figure 40A) the basalt
exhibits strong LREE enrichment, a very slight negative Eu
anomaly, and a middle to heavy REE range that is much flat
ter than the light REE range. This REE pattern is quite
similar to arc alkaline basic rocks such as the high-MgO
alkaline basic suite of Aoba and Ambryn Islands, Vanuatu
(Gorton, 1977). The basalt is plotted in extended Coryell
Matsuda form in Figure 40B. The high Th/'l'a and enriched
LREE yield a plot again similar to arc-related alkaline
basic rocks, which are plotted with ARB in Figure 38. The
altered alkali contents make a definite classification
impossible, but the REE, Th, Ta, and MgO contents suggest
the rock is a moderatE~ly high-Mg alkaline basic rock.
Another Callovian-age suite of dominantly basic volcanic
rocks to the north in the Foothills terrane, the Logtown
I~;
A.
.. -to •
~ to' c o
.c U
~ u 10 o
0:
Altered basalt, Green Mountain deposit
10 .t-'--_~~_-.,..,~_~~~.....,..,.,.-__ ~~..,.."..~ __ La Ce Nd Sm Eu Tb Tm Yb Lu
8.
to •
.. :0--5 to' c o .c (J
~ g to 0:
to·' Ta Th Ce
Atomfc Number
Altered basalt, Green Mountain deposit
TbTI
o Gt.4-01
v
159
Figure 40. coryell-Matsuda plot (A) and simplified extended coryell-Matsuda plot (D) of a basio hypabyssal rock adjaoent to the Green Mountain deposit.
-----------------~--------..• --.. --.----.... ----..... - ........ _ ......... -.-._ ...... .
160
Ridge Formation, has similar alkaline facies (analyses of
Newton, unpublished, and Kemp, 1982) and may be correlative.
This type of arc volcanic rock is rift-related, in either
intra-arc or back-arc settings. On the basis of the
compositional differences between these basic rocks and
essentially coeval arc rift zone subalkaline basalts of the
upper Gopher Ridge Formation to the west of the central
pelitic belt, the basic rocks of the Green Mountain area
were probably emplaced to the rear of the main Gopher Ridge
arc, possibly in a transitional stage from arc rifting to
early back-arc basin extension. Bearing in mind thg altered
nature of the sill, it plots near the Logtown Ridge basalts
in the ARB field of Figure 38.
The Guadalupe complex is a bimodal basic and acidic
intrusive complex consisting of an older gabbroic facies
intruded by a quartz monzonitic to granitic facies prior to
Nevadan deformation. The subalkaline basic rocks of the
northern facies have slightly enriched LREE patterns (Figure
29) and variable Th and Ta relationships (Figure 41), typi
cal of back-arc basalts (cf. Figure 340) and common in arc
rift zone basalts. In Figure 38, samples of basic hypabys
sal rocks of the Guadalupe northern facies plot almost
entirely in the field of BAB. These basic rocks are similar
to the basalts of the upper Gopher Ridge Formation, but they
.. --~. -.-.. _-, ._ .. ." ,.' .,.
Basic hypabyssal rocks, northern part of Guadalupe Intrusive Complex
10 -1 To Th Ce TbTI
o MP-03 * NS-60 A N7-S2 w N7-S3 + N7-S4 )( NS-782
161
v
Figure 41. Simplified extended coryell-Matsuda plot of basic hypabyssal rooks of the northern part of the Guadalupe Intrusive complex.
-------- ------------------ ---.... -_._----.. .--.-.P .... _ .• _, . "". .. __ • __ . _. ,_ ._, . ___ ._. __ ...
162
are lower in A1203 (cf. Tables 4 and 3) and higher in Ti/V,
indicating a shift toward MORB compositions. Their chemis
try is consistent with emplacement in a back-arc basin at a
more advanced stage of rifting than the tectonic setting of
the upper Gopher Ridge basalts.
Work presented in Chapter Two indicates that during Late
Jurassic Nevadan deformation the southern Foothills terrane
underwent sinistral transpression. The area of the G1lada
lupe complex behaved as a restraining bend, causing the
transfer of movement between long linear strands of an
oblique fault system. By association with the later trans
pressive oblique shear zones, earlier fractures which may
have controlled basin subsidence, magmatism, and hydrother
mal activity may have developed during transtension. They
may have been growth faults that were active during develop
ment of a pull-apart zone. This pull-apart zone would have
formed within the larger linear back-arc/interarc basin.
Forearc/proto-trench, Assemblage 6
The high-Mg basalt lenses in the central pelitic belt
have simplified extended Coryell-Matsuda patterns (Figure
42A) which exhibit low REE content with flat REE slopes, low
Ti/V, and variable Ta/Th correlation. These patterns should
be compared with the forearc patterns of Figure 34E. The
two analysed samples plot in the forearc field in the TVRT
A.
• :5 to' c o .c
~ u to
High-tAg basalts, lenses In the Central Pelitic Belt
o N6-355 '* N7-57
& ~~~--=======41~ _______________ •
10" To Th Ce Tbn v
(TI/V)/5
B.
(Tb/Co)xSO Th/To
163
Figure 42. Plots of high-Mg basic rocks cropping out as lenses in the Central Pelitic Belt. (A) Simplified extended Coryell-Matsuda plot. (B) TVRT diagram with the same samples as in Figure 42A. Although individually they plot in overlap fields, they share only the field of FAB (forearc basic rocks).
164
diagram, Figure 42B. These rocks are compositionally simi
lar to high-Mg basalts of r~te cretaceous age (McGreary and
Ben-Avraham, 1985) on Gorgona Island, Colombia (cf. Figure
31B). Those basalts overlie ultramafic cumulates and exhib
it pillow structures and spinifex textures (Echeverria,
1982). That mafic-ultramafic complex, in the forearc region
adjacent to the Colombia Trench, is a block in a folded
uplifted zone overlying a trench-vergent thrust fault
(McGreary and Ben-Avraham, 1985). This modern forearc
environment may have been the final emplacement setting of
the Gorgona Island basalts, but the greater age of that
mafic-ultramafic complex leaves tectonism during magma gene
sis undetermined. Theories on high-Mg volcanism in forearc
settings rangE! from early-arc primitive magma genesis
(Meijer, 1980) to late-arc back-arc basin related magma
genesis (Crawford, et al., 1981).
The high-Mg basalts in the central pelitic belt are
present as lenses in broken formation and melange. West of
Lake McClure, high-Mg bodies in melange are serpenitized and
have mineral assemblages that formed at pressures commonly
attributed to a trench setting. The inferred temperature of
formation is higher than that of the glaucophane schist
stability field and a normal trench setting is not indi
cated. structural evidence presented in chapter Two
--------------------_ .. _- -.-..... --" ... - - .. - - '" ..
165
indicates that regional deformation and shearing during
metamorphism were sinistral oblique with an east side to the
northwest sense of shear. The zone of melange is suggested
to have formed in a dilatant back-flow zone during
southeast-directed oblique underthrusting.
Assemblages indicative of forearc and proto-trench
facies were superimposed on a back-arc/interarc basin assem
blage. The high-Mg rocks with forearc basalt chemical
affinities and the melange and broken formation zones with
blocks bearing high-pressure mineral assemblages were gene
rated as the former back-arc/interarc basin closed by
oblique underthrusting. High pressures and moderate temper
atures generated epidote-amphibolite facies metamorphic
assemblages rather than glaucophane-schist facies assem
blages. The moderate temperatures may ~e attributed to
higher than normal heat flow beneath the basin. This high
heat flow at the onset of deformation and shearing is indi
cated by syntectonic intrusive rocks such as the Guadalupe
intrusive complex.
DISCUSSION OF TECTONIC ENVIRONMENTS AND
ORE DEPOSIT SETTINGS
Lateral displacement along the Bear Mountains fault zone
appears to have been relatively small as strata have been
correlated across shear zones composing it. Even shear
..... ~- •• - -_. -_ .. - .IIII!.-_. - ..
166
zones containing melange bearing high-pressure facies rocks
die out to the northwest (see Chapter Two), indicating that
underthrust zones had little displacement. Therefore, the
rocks in their present positions, give some sense of the
spatial distribution of tectonic environments in which they
formed.
The earliest tectonic environment was a mid-ocean ridge
spreading center at which Triassic(?) ocean floor (lower
Penon Blanco Formation) formed. In Early Jurassic time,
this ocean floor was in the vicinity of a subduction zone
and a primitive volcanic island arc (middle to upper Penon
Blanco Formation) developed on top of it.
A stratigraphic gap, unrelated to shearing and repre
senting approximately 30 million years, is present between
the Penon Blanco and Gopher Ridge arc sequences. This is
consistent with Edelman and Sharpls (1989) interpretation
that the Penon Blanco and equivalent rocks in the Sierra
Nevada foothills were amalgamated prior to 165 Ma ago, and
that Callovian and younger sequences formed an overlap
assemblage.
In Middle to Late Jurassic time, a second episode of
island arc volcanism (lower Gopher Ridge Formation)
developed on top of and to the west of the Penon Blanco
Formation. This extensional arc rifted to form first an arc
167
rift zone (upper Gopher Ridge Formation, lowermost Mariposa
Formation) and second a back-arc/interarc basin (Mariposa
Formation in the central pelitic belt). In latest Jurassic
time, this basin became a forearc/proto-trench setting as
high-Mg basalts were emplaced into the basin and melange
with high-pressure facies rocks developed during basin
closure and underthrusting.
The host basalts of the cyprus-type deposits in the
lower Penon Blanco Formation have mid-ocean ridge geochem
ical affinities rather than subduction-related rift basin
affinities. The Cyprus-type deposits are part of Petrotec
tonic Assemblage 1 - ocean floor. The assemblage probably
formed in open ocean and drifted into what was to become a
site of subduction-related volcanism.
Kuroko-type deposits such as the Blue lofoon deposits are
associated with rhyolitic lava facies in a bimodal assem
blage of rhyolite and basalt at the top of the Gopher Ridge
Formation. The ore deposits are part of Petrotectonic
Assemblage 4 - arc rift zone. They formed at the transition
from Gopher Ridge arc volcanism to back-arc basin activity
in the central pelitic belt.
Various tectonic settings have been suggested for
Besshi-type deposits. As summarized by Fox (1984), sug
gested settings include volcanic arcs in distal depositional
I <.
168
environments and earlier in arc evolution than Kuroko depos
its, forearc basins and/or trenchs, back-arc basins and/or
epicratonic rift basins.
The Besshi-type deposits in the southern Foothills
terrane, exemplified by Green Mountain and related deposits
are part of Petrotectonic Assemblage 5 - back-arc/interarc
basin. The altered basalt at the Green Mountain deposit has
REE and Th/Ta characteristics which suggest that prior to
alteration, it may have been an alkaline basalt. Such rocks
constitute an alkaline subset of ARB and are found in rift
related intra-arc and back-arc settings. The setting of the
Green Mountain deposit most likely was an early back
arc/interarc basin.
The structural and stratigraphic evidence suggests that
within a larger elongate back-arc/interarc basin, the
Besshi-type deposits formed in a pull-apart zone similar to
the Guaymas basin, but in a more juvenile stage of deve
lopment and associated with sUbduction activity.
In the southern Foothills terrane, there appears to be
no genetic linkage between the Kuroko-type deposits and the
Besshi-type deposits, other than that they formed in differ
ent parts of the same basin system. This is consistent with
Fox's (1984) conclusion of a general lack of correlation
between these two deposit types. Likewise, there is no
I~
169
genetic association between the Foothills Besshi-type depos
its or Koroko-type deposits and the lower Penon Blanco
Cyprus-type deposits, which are separated by tens to pos
sibly hundreds of millions of years.
Submarine polymetallic massive sulfide deposits in
general are suggested to be indicative of a rifting environ
ment (Scott, 1983). Kuroko-type felsic volcanic associated
deposits specifically have been suggested to indicate a
rifting environment (Sillitoe, 1982). In particular, this
study indicates that southern Foothills Kuroko-type deposits
formed in subduction-related rift settings that are distin
guishable on the basis of volcano/sedimentary stratigraphy
and basalt/basaltic andesite chemistry from preceding open
ocean settings and subsequent back-arc/interarc basin set
tings now closely juxtaposed.
The recognition of such tectonic settings in the rock
record may be significant for mineral exploration. Con
versely, characteristic ore deposit-types may elucidate
tectonic settings, and the deposits themselves may be con
sidered part of a diagnostic petrotectonic assemblage.
However, as a caveat, a certain mineralization style may
form in different plate-tectonic settings with similar
environmental factors. For example, Besshi-type deposits
have a characteristic style of mineralization in thick
-
170
tabular sheets, may be associated with sill-like intrusive
bodies in sediment-charged basins with variable components
of mafic igneous activity, and have higher zn/cu ratios than
other mafic associated massive sulfide deposits. These
deposits may be related to formation in semi-restricted rift
basins somewhat distal to either major open ocean spreading
centers, back-arc basin spreading centers, or arc volcanic
centers.
SUMMARY
cyprus-type deposits lie at the top of a Triassic(?)
metabasalt sequence, the oldest volcanic rocks in the area,
which make up the lower part of the Penon Blanco Formation.
Pyrite-chalcopyrite dominated ore is associated with pillow
basalts and Mn-Fe cherts which are overlain by chert (con
taining radiolaria), mUdstone and tuff. The tholeiitic
basalts have the depleted LREE and low Th/Ta signatures of
mid-ocean ridge basalts rather than marginal basin basalts.
The Cyprus-type deposits are part of an ophiolitic sequence
that appears to have formed in an open-ocean spreading
center environment.
Overlying the basalts and cherts is a thick sequence of
intermediate to mafic crystal tuffs and breccias, the Early
Jurassic middle Penon Blanco Formation. Relatively high Mg
contents, mildly depleted to mildly enriched LREE patterns,
I ~. [
and negative Ta and Ce anomalies of these tholeiitic meta
volcanic rocks suggest they formed in a primitive volcanic
arc setting. This arc developed on top of the ophiolitic
rocks.
171
The uppermost part of the Penon Blanco Formation is
dominated by metasedimentary rocks, and it is apparently
unconformably overlain by meta-breccias of basaltic andesi
tic composition of the Middle to Late Jurassic lower Gopher
Ridge Formation. The lower Gopher Ridge breccias have
predominantly rounded amygdaloidal porphyritic lava frag
ments and appear to be the deposits of hydroclastic
eruptions in an extensional arc environment.
The Late Jurassic upper Gopher Ridge Formation is domi
nantly a bimodal sequence of meta-rhyolitic lavas and tuffs
and meta-basaltic lavas. Felsic lava facies host the
Kuroko-type deposits, which have sphalerite dominated sul
fide facies and gypsum/anhydrite dominated sulfate facies.
The tectonic setting appears to have been an arc-rift zone
that formed during the transition from arc volcanism forming
the lower Gopher Ridge Formation to younger basinal sedimen
tation forming the Mariposa Formation.
The Besshi-type pyrrhotite-chalcopyrite-sphalerite
deposits developed within the sediment-dominated basin
forming the Late Jurassic Mariposa Formation. The sulfide
deposits are hosted by metasedimentary rocks, and they are
.... - ....... ~._ ..... _.IIIIIfI...... • _, . 4"~ •• - .~. --'--"'~-"
172
associated with minor metabasaltic rocks. This basin was
intruded by a mafic to felsic intrusive complex, the basic
members of which have the variable Th/Ta and LREE relation
ships of back-arc basin basalts. The Besshi-type deposits
appear to have formed in the median part of a long linear
basin between rifted arc segments. The inferred tectonic
setting of the sulfide deposits was an early back-arc or
interarc basin, which may have been related to transten
sional tectonics.
The basin later became the site of high-Mg basalt intru
sion/extrusion, suggesting a forearc affinity. During Late
Jurassic early Nevadan oblique underthrusting, blocks with
high-pressure metamorphic assemblages were emplaced in a
melange zone.
S~x petrotectonic assemblages are recognized in the
southern Foothills terrane: Triassic(?) mid-ocean ridge,
Early Jurassic primitive volcanic arc, Middle to Late Juras
sic extensional volcanic arc, Late Jurassic arc rift zone,
Late Jurassic back-arc/interarc basin, and Late Jurassic
forearc/proto-trench environment.
173
APPENDIX A
Data Souroes for the TVRT Diagram
Bougault H, Joron JL, Maury RC, Bohn M, Tardy M, Biju-Duval B (1984) Basalts from the Atlantic crust west of the Barbados Ridge (site 453, leg 78A):geochemistry and mineralogy. In Orlovsky (ed) Init'Rep Deep Sea Drill Proj 78:401-408
Bougault H, Joron JL, Treuil M, Maury R (1985) Local versus regional mantle heterogeneities: evidence from hygromagmaphile elements. In Whalen (ed) Init Rep Deep Sea Drill Proj 82:459-482
Bougault H, Maury RC, El Azzouzi M, Joron JL, cotton J, Treuil M (1982) Tholeiites, basaltic andesites, and andesites from Leg 60 sites: geochemistry, mineralogy, and low partition coefficient elements. In Lee, Powell (eds) Init Rep Deep Sea Drill Proj 60:657-677
Briggs RM, Goles GG (1984) Petrological and trace element geochemical features of the Okete volcanics, western North Island, New Zealand. contrib Mineral Petrol 86:77-88
Cambon P, Bougault H, Joron JL, Treuil M, Curie M (1983) Basalts from the East Pacific Rise: an example of typical oceanic crust depleted in hygromagmaphile elements. In Amidei (ed) Init Rep Deep Sea Drill proj 65:623-633
Cambon P, Joron JL, Bougault H, Treuil M (1980) Leg 55, Emperor seamounts: trace elements in transitional tholeiites, alkali basalts, and hawaiites - mantle homogeneity or heterogeneity and magmatic processes. In Shambach (ed) Init Rep Deep Sea Drill Proj 55:584-597
Castillo P, Batiza R, Stern RJ (1986) Petrology and geochemistry of Nauru Basin igneous complex: large-volume, offridge eruptions of MORB-like basalt during the Cretaceous. In orlovsky (ed) Init Rep Deep Sea Drill Proj 89:555-576
Clague DA, Frey FA (1980) Trace-element geochemistry of tholeiitic basalts from site 433C, Suiko seamount. In Shambach (ed) Init Rep Deep Sea Drill Proj 55:559-569
....... -.. -..... __ ._"....... .......• ., ' .. - '" ~ " .. ' .. -.-~-..... -"'
Claque DA, Frey FA (1982) Petrology and trace element geochemistry of the Honolulu volcanics, Oahu: Implications for the oceanic mantle below Hawaii. J Petrol 23:447-504
Crawford AJ, Beccaluva L, Serri G, Dostal J (1986) Petrology, geochemistry and tectonic implications of volcanics dredged from the intersection of the Yap and Mariana trenches. Earth Plan Sci Lett 80:265-280
Dietrich V,Emmermann R, Keller J, Puchelt H (1977) Tholeiitic basalts from the Tyrrhenian sea floor. Earth Plan Sci Lett 36:285-296
174
Dupuy C, Dostal J, Marcelot G, Bougault H, Joron JL, Treuil M (1982) Geochemistry of basalts from central and southern New Hebrides arc: implication for their source rock composition. Earth Plan Sci Lett 60:207-225.
Ewart A, Bryan WB, Gill, JB (1973) Mineralogy and geochemistry of the younger volcanic islands of Tonga, SW Pacific. J Petrol 14:429-465
Fodor RV, vetter SK (1984) Rift-zone magmatism: Petrology of basaltic rockstransitional from CFB to MORB, southeastern Brazil margin. contrib Mineral Petrol 88:307-321
Frey FA, Gerlach OS, Hickey RL, Lopez-Escobar C, MunizagaVillavicencio F (1984) Petrogenesis of the Laguna del Maule volcanic complex, Chile (36 S). contrib Mineral Petrol 88:133-149
Garcia MO, Frey FA, Grooms DG (1986) Petrology of volcanic rocks from Kaula Island, Hawaii: Implications for the origin of Hawaiian phonolites. contrib Mineral Petrol 94:461-471
Gorton MP (1977) The geochemistry and origin of quaternary volcanism in the New Hebrides. Geochim Cosmochim Acta 41:1257-1270
Joron JL, Bougault H, Maury RC, Bohn M, Desprairies A (1984) strongly depleted tholeiites from the Rockall Plateau margin, north Atlantic: geochemistry and mineralogy. In Backman (ed) Init Rep Deep Sea Drill proj 81:783-794
Joron JL, Bougault H, Maury RC, Stephan JF (1982) Basalt from the Cocos plate, site 487, leg 66: petrology and geochemistry. In Lee (ed) Init Rep Deep Sea Drill Proj 66:731-734
Joron JL, Briqueu L, Bougault H, Treuil M (1980) East Pacific rise, Galapagos spreading center and Siqueiros fracture zone, deep sea drilling project leg 54: hygromagmaphile elements- a comparison with the North Atlantic. In Powell (ed) Init Rep Deep Sea Drill Proj 54:725-735
175
Kirkpatrick RJ, Clague DA, Freisen W (1980) Petrology and geochemistry of volcanic rocks, DSDP leg 55, Emperor seamount chain. In Shambach (ed) Init Rep Deep Sea Drill Proj 55:509-557
Leeman WP, Budahn JR, Gerlach DC, smith DR, Powell BN (1980) origin of Hawaiian tholeiites: trace element constraints. Am J Sci 280-A:794-819
Lopez-Escobar L, Frey FA, Vergara M (1977) Andesites and high-alumina basalts from the central-south Chile high Andes: geochemical evidence bearing on their petrogenesis. Contrib Mineral Petrol 63:199-228
Luhr JF, Carmichael ISE (1985) Jorullo volcano, Michoacan, Mexico (1759-1774): The earliest stages of fractionation in calc-alkaline magmas. contrib Mineral Petrol 90:142-161
Mann AC (1983) Trece element geochemistry of high alumina basalt-andesite-dacite-rhyodacite lavas of the main volcanic series of santorini volcano, Greece. contrib Mineral Petrol 84:43-57
Nelson SA, Carmichael ISE (1984) Pleistocene to recent alkalic volcanism in the region of Sanganguey volcano, Nayarit, Mexico. contrib Mineral Petrol 85:321-335
Newsom HE, White WM, Jochum KP, Hoffman AW, (1986) siderophile and chalcophile element abundances in oceanic basalts, Pb isotope evolution and growth of the Earth's core. Earth Plan Sci Lett 80:299-31
---~-- .. -.,.---.--.......... . ... . -- ...... " ..• __ .•.... , ..
176
Pearce JA, Rogers N, Tindle AJ, watson JS (1986) G,eochemistry and petrogenesis of basalts from deep sea drilling project leg 92, eastern Pacific. In Bailey (ed) Init Rep Deep Sea Drill Proj 92:435-457
Peccerillo A, Taylor SR (1976) Geochemistry of Eocene calcalkaline volcanic rocks from the Kastamonu area, northern Turkey. contrib Mineral Petrol 58:63-81
Perfit MR, Fornari DJ, Malahoff A, Embley, RW (1983) Geochemical studies of abyssal lavas recovered by DSRV Alvin from eastern Galapagos rift, Inca transform, and Ecuador rift 3. Trace element abundances and petrogenesis. J Geoph Res 88B:10551-10572
Phelps OW, Gust DA, Wooden JL (1983) Petrogenesis of the mafic feldspathoidal lavas of the Raton-clayton volcanic field, New Mexico. contrib Mineral Petrol 84:182-190
Rogers NW, Hawkesworth CJ, Parker RJ, Marsh JS (1985) The geochemistry of potassic lavas from Vulsini, central Italy and implications for mantle enrichment processes beneath the Roman region. contrib Mineral Petrol 90:244-257
Saunders AD (1986) Geochemistry of basalts from the Nauru basin, deep sea drilling project legs 61 and 89: implications for the origin of oceanic flood basalts. In orlovsky (ed) Init. Rep. Deep Sea Drill Proj 89:499-517
Saunders AD, Fornari DJ, Joron JL, Tarney J, Treuil M (1982) Geochemistry of basic igneous rocks, Gulf of california, deep sea drilling project leg 64. In Blakeslee, Platt, stout (eds) Init Rep Deep Sea Drill proj 64:595-642
Seifert KE, Vallier TL, Windom KE, Morgan SR (1981) Geochemistry and petrology of igneous rocks, deep sea drilling project leg 62. In stout (ed) Init Rep Deep Sea Drill Proj 62:945-953
Taylor SR, Capp AC, Graham AL, Blake DH (1969) Trace element abundances in andesites, II. Saipan, Bougainville, and Fiji. contrib Mineral Petrol 23:1-26
Venturelli G, Thorpe RS, DalPiaz GV, Moro AD, Potts PJ (1984) Petrogenesis of calc-alkaline, shoshonitic and associated ultrapotassic volcanic rocks from the Northwestern Alps, Italy. contrib Mineral Petrol 86:209-220
...... -.. --, .. - ... - ' •.....
I>'
Weaver BL, Tarney J, Saunders AD (1985) Geochemistry and mineralogy of basalts recovered from the central north Atlantic. In Whalen (ed) Init Rep Deep Sea Drill Proj 82:395-419
Wedepohl, KH (1985) Origin of the Tertiary basaltic volcanism in the northern Hessian depression. contrib Mineral Petrol 89:122-143
177
Wood DA, Marsh NG, Tarney J, Joron JL, Fryer P, Treuil M (1982) Geochemistry of igneous rocks recovered from a transect across the Mariana trough, arc, fore-arc, and trench, sites 453 through 461, deep sea drilling project leg 60. In Lee, Powell (eds) Init Rep Deep Sea Drill Proj 60:611-645
Wood DA, Mattey DP, Joron JL, Marsh NG, Tarney J, Treuil M (1981) A geochemical study of 17 selected samples from basement cores recovered at sites 447, 448, 449, 450, and 451, deep sea drilling project leg 59. In Orlovsky (ed) Init Rep Deep Sea Drill Proj 59:743-752
Wood DA, Tarney J, Varet J, Saunders AD, Bougault H, Joron JL, Treuil M, Cann JR (1979) Geochemistry of basalts drilled in the North Atlantic by IPOD leg 49: implications for mantle heterogeneity. Earth Plan Sci Lett 42:77-97
Worner G, Beusen JM, Duchateau N, Gijbels R, Schmincke HU (1983) Trace element abundances and mineral/melt distribution coefficients in phonolites from the Laacher See volcano (Germany). Contrib Mineral Petrol 84:152-173
Worner G, Schmincke HU (1984) Mineralogical and chemical zonation of the Laacher See tephra sequence (East Eifel, West Germany). J Petrol 25:805-835
-----------------------------~-~--.-.. ---.-- .... -............ -.... _ ....... ----_ ... ----.
=')""
178
APPENDIX B
Rotated Foliation in Domain lA
The dominant foliation in Domain lA is NW-striking in
rocks of lower greenschist grade. A small subset of folia
tion in Domain lA strikes northeast and dips moderately
steeply southeast (see Figure 13). In many cases these
foliations are in upper greenschist to lower amphibolite
grade country rocks and meta-intrusive rocks as in Domain 2.
These fabrics developed while temperatures were still
elevated in the contact zones of intrusive rocks. The NE
striking higher-grade foliation is an extension n~ toe
Domain 2 structural trend into the NW-trending shear zone of
which Domain lA is a part, and the earlier fabrics are in
varying degrees of rotation toward conformity with the NW
trending fabrics. This rotation probably took place during
progressive shear in the development of the southwest strand
of the Bear Mountains fault zone.
Flattening foliation data in this domain pass a test for
rotational symmetry of non-uniform data at a 95% confidence
level using the eigenvalues of the direction cosine matrix,
defining a girdle rather than a random distribution (method
of Bingham, 1964; commercial equal-area analysis program by
Darton software). The best-fit girdle through the poles to
foliation was calculated by eigenvector analysis, and the
pole to this plane plots in the center of the cluster of
stretching lineation data (Newton, unpublished data).
179
Assuming simple shear, the poles to a passively rotated
planar feature would be expected to approach the pole to the
shear plane at high shear strains (Wheeler, 1987). There
fore, assuming in the present case that the earlier-formed
NE-trending foliation rotated passively, the pole to the
shear plane (the kinematic shortening axis) will be located
somewhere on the best-fit girdle of poles to rotated folia
tion, presumably somewhere in the vicinity of the modal
foliation. The pole to the girdle of rotated foliations is
a line whose orientation does not change during shear as the
foliation rotates around this fixed element. This pole
therefore must lie in the·shear plane, in which all lines
are unchanged by shear, and it represents the intersection
of the shear plane and the general orientation of the planar
feature prior to rotation. A shear plane striking 340°
intersects the pole to the girdle of rotated foliations at a
dip on the shear plane of 78°. Assuming passive rotation of
the competent relict high-grade foliation, the fact that
lineation in these L-S fabrics shows little deviation from a
cluster distribution while foliation forms a well-defined
girdle, confirms that the axis of rotation (fixed during
shear) was subparallel to stretching lineation. Progressive
-----------------------------~.-"- .. -.-,,-,, .............. - ... -..... .
I ,!
180
deformation was coaxial with early deformation. The loca
tion of the line of slip on the general shear plane remains
to be determined.
Wheeler (1987) suggested a method with which planar or
linear markers that have been passively rotated toward
conformity with a shear plane can be used to indicate the
sense of shear when the slip direction and shear plane
orientation are known. In the present case, where the sense
of shear and the general shear zone orientation are known,
the method can be used in reverse to indicate whether the
slip line is closer to strike or dip of the shear plane. As
suggested by Wheeler (1987), the line of slip is labeled the
kinematic x direction, the kinematic shortening axis is
labeled z, and the intermediate axis is labeled y. It is
assumed that at high shear strains these axes would approach
their longitudinal strain counterparts, the X,Y, and Z axes
of the strain ellipsoid.
The pole to the shear plane (x-y plane) is z. Poles to
a passively rotated planar marker converge toward z, here
with a counterclockwise sense of rotation. The exact x and
y orientations are not known in this case, but x can be
expected to be in the vicinity of X, in the cluster of
stretching lineation, in regions of high shear strain. The
___ ..... _ ... _ •• ~ •• _ ... _ •• _ ••••••• ~... •• .... • __ " •• _N "'R" ~ •• _., __ ... _._ ••
maximum strain axis can be expected to approach the slip
direction with increasing shear strain.
181
Paterson and others (1987) suggest.ed the Bear Mountains
fault zone has a predominant reverse sense of slip with a
small left-lateral component. In that case the x direction
would be close to the dip direction of the shear zone and
the y direction' would be close to horizontal. with a more
dominant strike-slip component, the x direction would be
close to the strike direction, and with true transpression,
the x direction would be approximately at a rake of 45° to
the general shear plane.
In the Wheeler method, the stereographic or equal-area
trace of the y-z plane separates two domains. In one of
these domains, an arrow in the direction of rotation of the
poles points toward z, and in the other domain, the arrow
poin'cs away from z. The x-z plane will indicate the true
sense of shear across the shear plane and the arrows indi
cating this movement along the x-z trace will point the same
as the arrows in the domains separated by y-z through which
the x-z trace passes. In the present case, the sense of
shear has been determined as sinistral-reverse independently
by other kinematic indicators. Therefore the proper direc
tion of the arrows along the x-z trace is known, and the
trace of the y-z plane has to be on the side of the girdle
-_.------------------------
I .: I
182
of poles of rotated foliation, such that th~ arrows in the
domain through which x-z passes have the same orientation as
the x-z arrows. If the x direction was close to the dip of
the shear plane, the y-z plane would be on the north side of
the rotated pole girdle and the arrows in the domain con
taining x-z would be opposite to the arrows on x-z. In
order for the arrows to be in the same direction, the y-z
plane must be on the south side of the girdle. Therefore
the y direction has to be closer to vertical than the
intersection of the x-y plane and the best-fit girdle to
foliation poles, and the x direction has to have a rake on
the x-y plane smaller than the rake of the pole to the
girdle. For the shear zone in Domain lA, the slip direction
has to be at a raJce of less than 58 0 to the general shear
plane. Therefore, increasing shear strain was not rotating
linear structures toward the dip direction, but rather
toward the strike direction, suggesting a larger strike-slip
component than dip-slip component, the reverse of the rela
tionship for the field area suggested by Paterson and others
(1987).
In Domain lA, the intersection of the shear zone great
circle and the midpoint of the modal cluster of stretching
lineation (the greatest number of lineation readings from a
high shear strain area) as an approximation of the slip
.... _ •• _ •••• _ •• - •• a.I •.•••. - ....• ••.••..•. '_~ ".' ,_, _. ··. ___ 0"-·"
I ~.
183
line, would give a maximum rake and plunge. This possible
slip line has a rake of 45°SE with the general shear plane
(Figure 13), and it would indicate either subequal compo
nents of strike-slip and dip-slip or a greater component of
strike-slip than dip-slip along this segment of the Bear
Mountains fault zone, consistent with transpression.
At a shear strain greater than 6.5 and subequal
components of wrench shear and reverse shear, stretching
lineation and fold axes would be within 10· of the line of
slip (Ridley, 1986). If these stretching lineations did
still remain oblique to the slip direction, then they must
have remained steeper than the slip line.
----------------------------~--~ .. -.. ---.............. -...... ...- .. - ....... _---_ ... -_ ..
184
APPENDIX C
Instrumental Neutron Aotivation Analytioal Teohniques
Twenty samples were analyzed by the author for trace
elements by instrumental neutron activation analysis (INAA).
Rocks were crushed to cobble size in a steel jaw crusher,
crushed to pea size in ~ ceramic jaw crusher, powdered in a
ceramic shatterbox, and split. Approximately one gram
samples were sealed in polyeurethane tubes and irradiated
for three hours at a thermal neutron flux of 7 x 1011
neutrons/cm2/sec in the TRIGA reactor in the Nuclear
Engineering Laboratory at the University of Arizona, which
is under the direction of Dr. George Nelson.
counting was performed in the Gamma Ray counting
Facility in the Lunar and Planetary Laboratory at the
University of Arizona, which is under the direction of Dr.
William V. Boynton. samples were counted, using a fast
timing anti-compton spectrometer (FACS), for 2 hours each
5-6 days after irradiation and 4 hours each 30-35 days after
irradiation. counting statistics, reported as one standard
deviation in Tables 1-4, were derived from multiple determi
nations of single or multiple peaks for each element.
------------------------,----~ .. --- .... --................. ,. ... . ..•........... __ .....•..
I~'
185
APPENDIX D
petrographio Desoriptions of Representative Analyzed samples
Basaltic lava
Aphanitic with minor relict clinopyroxene phenocrysts.
Phenocrysts:
Clinopyroxene - strongly altered to chlorite.
Matrix: Fine mat of chlorite + minor quartz. Rock cut by minor epidote veinlets.
N5-10 Clinopyroxene-plagioclase crystal tuff
Broken to whole, anhedral to euhedral crystals plus volcanic lithic fragments in a meta-ash matrix.
Phenocrysts:
Plagioclase - weakly altered to epidote and sericite.
Clinopyroxene - dark brown, weakly altered to chlorite and epidote.
Matrix: Fine feldspar-quartz mixture, epidote, chlorite, trace hematite.
N5-141 Basaltic andesitic greenschist, similar to N5-140.
Well-foliated sericite-chlorite schist with relict plagioclase phenocrysts.
Phenocrysts:
Plagioclase - laths, mildly to moderately altered to sericite + epidote.
Clinopyroxene(?) - replaced by chlorite.
Matrix: Fine chlorite + sericite.
---------------------.---~.- ... -----.-........ -........... ,._. _ ........... __ ......... .
! .:..'
N5-214 Amphibole-plagioclase porphyritic lava
Subhedral to euhedral phenocrysts in a trachytic textured matrix.
Phenocrysts:
Amphibole - 8-10 %, pleochroic in shades of brown and green, weakly altered to epidote + quartz.
Plagioclase - 5 %, highly altered to calcite + epidote.
Matrix: Fine mat of altered plagioclase lathes + small amphiboles.
N5-331 Rhyolitic lava
Plagioclase-alkali feldspar aphanitic porphyritic lava
Phenocrysts:
186
Plagioclase - subhedral, mildly altered to sericite + epidote.
Alkali feldspar - sUbhedral, untwinned, weak perthite, moderately altered to sericite.
Matrix: Very abundant feldspar microlites in a trachytic fabric, mild to moderate alteration to sericite + epidote.
N5-347 Basaltic andesitic lava
Clinopyroxene aphanitic porphyritic lava.
Phenocrysts:
Clinopyroxene - large, euhedral, weakly altered to chlorite.
Matrix: Very fine grained (hyalopilitic) quartzofeldspath~c material with epidote alteration, irregular network of clinopyroxene needles.
N5-646 Amphibole-plagioclase crystal lithic tuff
Lithic fragments of porphritic lava in a crystal tuff matrix.
Phenocrysts in lava:
Amphibole - 6 %, mildly altered to chlorite ± epidote ± quartz.
Plagioclase - 2 %, strongly altered to sericite + epidote.
187
Matrix: Small broken altered amphibole and plagioclase crystals in fine feldspar + quartz material with epidote, chlorite, trace hematite.
Basaltic lava
Aphanitic chloritic greenstone with relict plagioclase and pyroxene.
No phenocrysts.
Matrix: Fine chlorite + quartz + epidote, relict plagioclase laths moderately altered to sericite + epidote, minor relict clinopyroxene.
N7-54 Diorite
Fine-grained, equigranular plutonic rock.
Igneous minerals:
Plagioclase - 50 %, weakly altered to sericite. Amphibole - 25 %, brown, pleochroic, rimming
pyroxene. Clinopyroxene - 20 %, strongly replaced by igneous
amphibole, very weakly altered to chlorite. Opaque - 5 %, disseminated.
---------------------------~-.~-.---... " ..... " ........... "- .- .... '-""-'"--'''
I'"~
Gabbro
Fine-grained, equigranular plutonic rock.
Igneous minerals:
188
Plagioclase - 45-50 %, weakly altered to sericite. Clinopyroxene - 25-30 %, strongly replaced by
igneous amphibole, no chlorite/epidote alteration.
Amphibole - 20 %, hornblende, rimming pyroxene. Opaque - 4 %, disseminated.
---------------------------•.. ---~--.---..... -... -.-....... _ .. , .. -_ ................... _ ... -_ .. ..
I .;
APPBNDIX B
Footnotes for Chapter Three
189
Taliaferro (1933) informally called the thin-bedded
sequence, the Hunter Valley cherts and tuffs. Bogen (1985)
noted that the chert-dominated base is gradational into the
upper tuff-dominated part of the unit, and he placed the
cherty base in a lower formation with the basalts, which he
named the Jasper Point Formation.
separating the chert facies from the rest of the thin
bedded sequence into a formal lower formation is impracti
cal. The thin-bedded sequence is completely internally
gradational and is recognizably different as a unit from the
underlying basalts and the overlying coarse pyroclastic
rocks. There is no mappable break, even a transitional
boundary, within the unit. Additionally, the cherty facies
is only locally present (Taliaferro, 1943b), and probable
correlatives of the cherty facies along strike would be
impossible to differentiate from the rest of the thin-bedded
sequence. To separate only the cherty facies into a lower
formation would appear to be somewhat based on the interpre
tation that it is an ocean floor sequence genetically
associated with the underlying pillow basalts, which is not
a valid criterion for naming a stratigraphic unit.
190
The thin-bedded sequence as a whole either should be
differentiated as a separate unit, or kept within a larger
formation. As the bulk of the sequence is tuffaceous and
intermediate tuffs and crystal tuffs at the top of the
sequence are gradational into massive crystal tuffs and
breccias of the Penon Blanco Formation, it is here retained
in the Penon Blanco Formation. The lowest basalts also are
retained as the base of the Penon Blanco Formation, and the
basalts and the thin-bedded sequence are grouped together as
the lower Penon Blanco Formation. As there exists a dis
tinct mappable contact between the basalts and the overlying
thin-bedded sequence, the basalts have been separated as a
unit, here called the Basalt Member of the Penon Blanco
Formation. The Penon Blanco Formation in this study
includes all rocks in ca. 200 Ma and older volcanic/sedimen
tary sequences in the field area, as distinguished from
160-170 Ma volcanic sequences such as the Gopher Ridge
Formation. This is similar to the usage of saleeby (1982),
and facilitates large-scale correlations and tectonic inter
pretations.
In this study, the basal cherty facies of the thin
bedded sequence and the underlying MORB-type basalts are
grouped together as a petrotectonic assemblage that spans
.... _ •.. _ .-.. ---,1IIf"~""'" .
stratigraphic unit boundaries. This petrotectonic assem
blage is referred to as Assemblage 1.
2 Bogen (1985) incorrectly identified this sequence as
191
being older than the volcanic rocks adjacent to the west and
included it in the top of Penon Blanco Formation. The rocks
are east-dipping, normally-graded, locally conglomeratic,
epiclastic rocks with crystal and lithic detritus indistin
guishable from the western volcanic rocks. The epiclastic
sequence clearly depositionally overlies the volcanic rocks
to the west and is the younger sequence.
192
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---.-.... _------------_.--_._._. - - .--....
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