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AGE AND COMPOSITION OF MAFIC DIKES WITHIN THE WYOMING PROVINCE: A WINDOW INTO THE EVOLUTION OF THE SUBCONTINENTAL LITHOSPHERE
By
JOSHUA LEE RICHARDS
A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE
UNIVERSITY OF FLORIDA
2007
2
© 2007 Joshua Richards
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To Robert Earl Roach —you will forever be in my heart
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ACKNOWLEDGMENTS
At this time, I would like to acknowledge my friends and family for their constant
persistence and belief that I can accomplish anything. I would also like to thank my committee
members, Dr. David Foster and Dr. Michael Perfit, especially Dr. Paul Mueller for his patience
throughout this process. I would like to thank Dr. Jim Vogl, Dr. David Mogk, and Kelly Probst
for the necessary help in the field. I would also like to thank Dr. George Kamenov and Warren
Grice for the assistance in data collection and interpretation. An extended thanks goes to all the
faculty, staff and fellow graduate students who have made my time here some of the best years
of my life. Lastly, I would like to acknowledge the organizations which provided the necessary
funding which made this project a reality: Department of Geological Sciences at the University
of Florida, Tobacco Root Geological Society, United States Geological Survey (Grant #
05HQGR0156), and National Science Foundation (EAR 0106592).
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TABLE OF CONTENTS page
ACKNOWLEDGMENTS ...............................................................................................................4
LIST OF TABLES...........................................................................................................................7
LIST OF FIGURES .........................................................................................................................8
ABSTRACT.....................................................................................................................................9
CHAPTER
1 INTRODUCTION ..................................................................................................................11
2 PRECAMBRIAN MAFIC DIKES OF THE BEARTOOTH-BIGHORN MAGMATIC ZONE (bbmz) .........................................................................................................................15
Geologic Setting and Samples ................................................................................................15 Results.....................................................................................................................................16 Discussion...............................................................................................................................18
3 PRECAMBRIAN MAFIC DIKES OF THE MONTANA METASEDIMENTARY PROVINCE (MMP) ...............................................................................................................35
Geologic Setting and Previous Work .....................................................................................35 Age Relations..........................................................................................................................36 Tobacco Root Mountains........................................................................................................37 Ruby Range ............................................................................................................................38 Highland Mountains ...............................................................................................................39 Results.....................................................................................................................................40 Discussion...............................................................................................................................41
4 EOCENE MAFIC DIKES OF THE MONTANA ALKALI PROVINCE (MAP).................46
Geologic Setting and Previous Work .....................................................................................46 Castle Mountains ....................................................................................................................47 Crazy Mountains.....................................................................................................................48 Results.....................................................................................................................................48 Discussion...............................................................................................................................49
5 CONCLUSION.......................................................................................................................52
APPENDIX MATERIALS AND METHODS..............................................................................53
Sampling Strategy...................................................................................................................53 Sample Processing ..................................................................................................................53
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Zircon Separation....................................................................................................................55
LIST OF REFERENCES...............................................................................................................57
BIOGRAPHICAL SKETCH .........................................................................................................63
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LIST OF TABLES
Table page 2-1 General information on mafic dikes sampled. ...................................................................29
2-2 Major, Trace, and Isotopic concentrations of mafic dikes within the Wyoming Province and Great Falls tectonic zone (GFTZ). ...............................................................31
2-3 U-Pb data for BT01 sample from the Southeastern Beartooth Mountains ........................34
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LIST OF FIGURES
Figure page 1-1 Field map of the Wyoming Province .................................................................................13
1-2 This field schematic comprises samples collected for this research..................................14
2-1 SiO2 vs. FeO*/MgO discrimination diagram.....................................................................21
2-2 The Ti-Zr-Y discrimination diagram .................................................................................22
2-3 Trace element plots for all three areas. ..............................................................................23
2-4 REE plots for all three areas ..............................................................................................24
2-5 U-Pb concordia diagram for BBMZ sample BT01............................................................25
2-6 Sm/Nd plot for samples within the BBMZ........................................................................26
2-7 Diagram 206Pb/204Pb vs. 207Pb/204Pb for dikes within the BBMZ. .....................................27
2-8 Age vs. initial εNd ...............................................................................................................28
3-1 Sm/Nd plot for samples within the MMP..........................................................................44
3-2 Whole rock 206Pb/204Pb vs. 207Pb/204Pb diagram for samples within the MMP. ................45
4-1 Whole rock 206Pb/204Pb vs. 207Pb/204Pb isotopic diagram for MAP samples. ....................51
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Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science
AGE AND COMPOSITION OF MAFIC DIKES WITHIN THE WYOMING PROVINCE: A WINDOW INTO THE EVOLUTION OF THE SUBCONTINENTAL LITHOSPHERE
By
Joshua Lee Richards
August 2007
Chair: Paul Mueller Major: Geology
Mafic dikes provide geochemical and isotopic insight into processes affecting mantle
evolution. Two specific areas within the Wyoming Province, the Beartooth-Bighorn Magmatic
Zone (BBMZ), and the Montana Metasedimentary Province (MMP), and one area within the
Great Falls tectonic zone (GFTZ), the Montana Alkali Province (MAP), each contain mafic
intrusions, primarily as dikes. One of three dikes sampled from the BBMZ has a U-Pb zircon age
of 2.8 Ga. Geochemical and isotopic similarities for the other dikes suggest a Late Archean
emplacement for all three dikes. No dikes sampled from the MMP yielded reliable, datable
phases, however, geochronology and paleomagnetic data suggest that dikes were either emplaced
at 1450 or 780 Ma. The MAP, a petrologic province, formed ~50 Ma ago within the GFTZ, a
largely Proterozoic feature.
Major and trace element geochemistry for all samples exhibit features, such as a relative
depletion in high field strength elements, characteristic of modern convergent margin volcanism.
Nd and Pb isotopic data, however, suggest that the geochemical features were derived from the
source of the mafic magmas and that these signatures were established well before the time that
the magmas formed. In particular, Sm/Nd isotopic data from whole rocks suggest metasomatic
enrichment in the mantle in the BBMZ at ~3.4 Ga and ~2.0 Ga for the MMP, whereas Pb/Pb
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whole rock analysis suggests metasomatism of the mantle in the BBMZ at ~3.2 Ga, ~1.9 Ga for
the MMP, and ~1.8 Ga for the MAP. The ages quoted above are based on secondary isotopic
ratios and are only loosely constrained (i.e., the 3.2 Ga and the 3.4 Ga “ages” from the BBMZ
samples are not statistically distinct). Similarly, the 1.9 Ga and 1.8 Ga “ages” for the MMP and
MAP samples are also indistinguishable from each other. What is distinct, however, is that
samples from the Proterozoic GFTZ (MMP and MAP) suggest that the underlying mantle was
modified (metasomatized) at this time and has remained largely undisturbed. The mantle-altering
events inferred for all three areas appear to be similar to modern day island arc settings. We
propose that the evolution recorded in the isotopic data suggests that metasomatism of the dike
sources occurred at different times throughout the area and we conclude that the source of this
metasomatism is from subduction related processes similar to modern day island arc settings.
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CHAPTER 1 INTRODUCTION
Mafic dikes are useful geologic tools for investigating processes such as continental
breakup (Harlan et al., 2003), ancient mantle plumes (i.e., Heaman et al., 1992; Park et al.,
1995), magmatic underplating (Chamberlain et al., 2003), and the evolution of the subcontinental
lithosphere in general. Study of mafic dikes within the Wyoming Province, therefore, can
enhance our understanding of the subcontinental lithosphere/ mantle system below this Archean
terrain.
Within the Wyoming province, dikes were sampled from two sub-provinces, the
Beartooth-Bighorn Magmatic Zone (BBMZ) and the Montana Metasedimentary Province
(MMP) (Figure 1-1 and 1-2). Dikes within the BBMZ and MMP are thought to represent the
only magmatic events that occur between the cratonization of the Wyoming Province at ~2.8 Ga
(Mueller and Frost, 2006) and sedimentation during the Paleozoic (Harlan et al., 2003), with the
exception of the formation of the Belt Basin (1.45 Ga) in the MMP. Dikes, therefore, are integral
for understanding processes which produced this mafic magmatism.
Mafic magmatism within the MAP is considerably younger than that in the other two
field areas. These magmas intruded Phanerozoic rocks exposed in the Great Falls tectonic zone
(GFTZ) (Figure 1-1). In the past, the origin of the GFTZ has been argued as either an Archean
continental suture zone (i.e., Boerner et al., 1998) or as a Proterozoic collisional zone (i.e.,
O’Neill and Lopez, 1985; Mueller et al., 2002; Mueller et al., 2006; Foster et al., 2006), but the
latter is currently the most accepted hypothesis. Dikes within this area provide information about
the subcontinental lithosphere/ mantle beneath the MAP and subsequently can help constrain age
and origin of the GFTZ.
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Dikes within the Wyoming Province, Great Falls tectonic zone, and in other locals are
postulated to result from mantle plumes, asthenosphere upwelling, generation by decompression
melting of the subcontinental lithosphere, and/or subsequent magmatism due to subduction
related processes (LeCheminant and Heaman, 1989; Heaman et al., 1992; Harlan et al., 2003). In
this paper, we present major element, trace element, and isotopic data for mafic dikes within the
Wyoming Province and the Great Falls tectonic zone. These data provide new insights into the
development of the subcontinental lithosphere/ mantle beneath the Northern Wyoming Province
and Great Falls tectonic zone.
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Figure 1-1. Field map of the Wyoming Province and associated geochronological events occurring along its boundaries. The box denotes the specific field area for
this project and is enlarged in Figure 1B. [Reproduced with the direct permission from Foster, D.A., Mueller, P.A., Mogk, D.W., Wooden, J.L., and Vogl, J.J., 2006, Proterozoic evolution of the western margin of the Wyoming craton: implications for the tectonic and magmatic evolution of the northern Rocky Mountains: Canadian Journal of Earth Sciences (page 1604, Figure 1).
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Figure 1-2. This field schematic comprises samples collected for this research. The Beartooth-Bighorn Magmatic Zone (BBMZ), the Montana Metasedimentary Province (MMP) and the Montana Alkali Province (MAP) are represented. Samples are represented by numbers as followed: 1-4 =BTL01, BTL02, BT01 and BT02; 5=CA04; 6-7= CZ01 and CZ02; 8=TR01; 9=RR01; 10-12=HM03, SJM-10, and SJM-18; 14-15= KG and
247FM. Geographic locations are noted in Table 2-1.
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CHAPTER 2 PRECAMBRIAN MAFIC DIKES OF THE BEARTOOTH-BIGHORN MAGMATIC ZONE
(BBMZ)
Geologic Setting and Samples
The Beartooth-Bighorn magmatic zone (BBMZ) is located in the north central portion of
the Wyoming Province and encompasses the Beartooth Mountains and the Bighorn Mountains
(Figure 1-1 and 1-2). The BBMZ has experienced multiple periods of both felsic and mafic
magmatism between ~3.5 Ga, earliest gneisses, and 0.8 Ga, latest mafic dikes (Mogk et al., 1992;
Wooden and Mueller, 1988; Harlan et al., 2003; Mueller et al., 2006), but is composed
dominantly of Late Archean igneous to metaigneous rocks that are tonalitic to granodioritic to
trondhjemitic (TTG) in composition (e.g., Mogk et al., 1992; Chamberlain et al., 2003).
Precambrian mafic dikes intruded the basement of the BBMZ and have been studied by
multiple workers (Prinz, 1964; Mueller, 1971; Wooden, 1975; Harlan et al., 2003). These dikes
range in composition from Archean orthoamphibolites, metadolerites, and late Precambrian
quartz dolerites to Neoproterozoic olivine dolerites (Prinz, 1964; Mueller and Rogers, 1973;
Baadsgaard and Mueller, 1973; Wooden, 1975; Harlan et al., 1997).
Samples BTL02, BT01 and BT02 are fine grained samples from the chilled margins of
three separate dikes. These dikes are tholeiitic with ophitic to sub-ophitic texture. Plagioclase
and clinopyroxene are primary minerals, but amphibole is also common and interpreted as a
product of deuteric alteration along with seritization of plagioclase (Winter, 2001). In another
dike (BTL), BTL01 is from a zone of intermediate texture and contains An60-An40 plagioclase
(labradorite) phenocrysts measuring up to 2 cm in length and 1 cm in width. It is located between
the chilled margin (BTL02) and the central zone of the dike. The central zone of the dike
contains even larger plagioclase (up to 4cm in length) phenocrysts than BTL01, and, therefore,
was not sampled due to concerns for having a homogeneous, representative sample. The relative
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proportion of plagioclase in the groundmass of BTL01 is less (~25% labradorite) than that of the
other three samples (~50% plagioclase).
Based on chilled margin compositions, all samples are from Group I-II of Mueller and
Rogers (1973). These groups were designated based on TiO2 content, i.e., Mueller and Rogers
(1973) defined Group I as having the lowest wt % TiO2 (< 2.0) of all dikes in their study. This
group made up ~64 % of the dikes in their study and are compositionally similar to the
metadolerite samples of Prinz (1964). Group I dikes were estimated to have a coherent minimum
age of 2.55 Ga based on K-Ar and Rb-Sr whole-rock analyses (Baadsgaard and Mueller, 1973).
Results
Major element, trace element, and isotopic data are listed in Table 2-2. Dikes from the
BBMZ plot near the boundary between tholeiitic and calc-alkaline compositions (Figure 2-1).
Samples range in SiO2 from 53- 57 wt % and have low-to-moderate TiO2 contents equivalent to
Group I of Mueller and Rogers (1973). This similarity extends into the mineral assemblages also,
with both having plagioclase and clinopyroxene (augite) as primary minerals. BTL01, a variation
of Prinz’s (1964) leopard rock, can be classified as an anorthositic gabbro because of its
porphyritic texture and composition. Some shearing fabric is noted in thin sections of all BBMZ
samples with the exception of BTL01. This fabric is likely associated with post emplacement
movement along the fractures filled by the dikes and associated hydrothermal alteration for at
least sample BTL01 (e.g., seritization of the large plagioclase phenocrysts), in addition to
deuteric alteration seen in most dikes of this age (Prinz, 1964).
Trace element abundances normalized to primitive mantle for all BBMZ dike samples
show depletion of Nb in relation to U and K (Figure 2-3a), similar to island arc calc-alkaline
basalts (Wilson, 1989; Winter, 2001). BTL01, however, has much lower LREE content and
yields a pattern that more closely resembles those of primitive, arc type basalts. An important
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difference in the patterns is that the rare earth element (REE) abundances in the chilled margin
samples BT01, BT02, and BTL02 show pronounced enrichment of the light REE (e.g., La =
>100X) with respect to chondritic values, while BTL01 (phenocryst-rich sample) shows
distinctly lower abundances (Figure 2-4a). The HREE contents of all samples, however, are
slightly enriched (Lu=~10X) with little to no Eu anomaly in any sample (Figure 2-4a).
Mueller and Rogers (1973) used the data of Baadsgaard and Mueller (1973) to assign an
age of ~2.5 Ga to these Group I (low TiO2) mafic dikes. In an effort to obtain a more
constrained age for this group, U-Pb data were obtained from one sample (Table 2-3). Small
zircons (<50 microns) were extracted from BT01 and analyzed using the Sensitive High
Resolution Ion Micro Probe- Reverse Geometry (SHRIMP-RG) at the Stanford-U.S.G.S. Micro-
analytical Center. Data plot close to concordia and suggest a crystallization age of 2.8 Ga
(Figure 2-5). The Quad Creek Metanorite, a small pluton equated to Group I dikes by Mueller
and Rogers (1973), also yields a U-Pb age of ~2.8 Ga (Mueller unpublished), consistent with
BT01 and within error of the 2.55 Ga whole-rock Rb-Sr age of Mueller and Rogers (1973).
Overall, these zircon data suggest the Group I dikes of Mueller and Rogers (1973) are better
assigned an age of ~2.8 Ga.
Whole-rock Sm-Nd isotopic data (Table 2-2) for samples BTL02, BT01 and BT02 yield
present day εNd values of -33.1 to -43.2 and calculated initial εNd values of -2.6 to -3.6 using 2.8
Ga as an approximate crystallization age (Table 2-3). 147 Sm/144 Nd ratios for these samples
(BTL02, BT01, and BT02) range from 0.08836 to 0.11132 and produce a range of TDM from 3.0-
3.1 Ga. Regression line for Sm/Nd for the dikes in the BBMZ produce an “age of ~3.4 Ga
(Figure 2-6) and is within error of the TDM and TCHUR. Sample BTL01 yields a present day value
of -7.4, an initial εNd of 2.64, and has a 147Sm/144Nd value of 0.16784 (Table 2-2). Though its
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higher εNd value is commensurate with its less evolved trace element chemical composition
compared to the other BBMZ samples, it is difficult to calculate reliable TDM and TCHUR for this
sample because of the high 147Sm/ 144Nd ratio (i.e., close to the values of model mantle
compositions). The relatively high 147 Sm/ 144 Nd does, however, allow for a comparatively more
reliable estimate of initial εNd because there is little change relative to the evolving mantle over
time. Figure 13 depicts the initial εNd values from all four samples and the initial εNd for 2.8 Ga
felsic rocks within the BBMZ (Wooden and Mueller, 1988) and shows that the dike
compositions are strongly overlapping with the initial εNd compositions of the more felsic rocks.
Whole-rock Pb isotopic compositions (Figure 2-7) of all BBMZ samples produce a linear array
equivalent to an age of ~3.2 Ga, though they are clearly younger because they intrude 2.8 Ga
gneisses and granitoids (Wooden and Mueller, 1988).
Discussion
Major element data for dike samples BTL01, BTL02, BT01, and BT02 show relatively
high SiO2 (54-57 wt. %), have moderately high total iron (10-11.5 wt. %), and are interpreted as
calc-alkaline bordering on tholeiitic (Figure 2-1).
Trace elemental data are particularly useful in determining the source of (e.g., primitive
mantle vs. enriched mantle) and modes of magmatism (e.g., subduction related vs. plume-
related) of mafic rocks. For sample BTL01, a less enriched pattern of LREE compared to
BTL02, BT01, and BT02 (20X chondritic values versus 200-600X chondritic values), indicates a
more primitive signature for the intermediate zone of the leopard rock sample than that of the
chill margin samples within the BBMZ (Figure 2-4a). This is likely due to analysis of a higher
percentage of groundmass, which is possibly depleted in plagioclase relative to the large
phenocrysts within sample BTL01. Alternatively, the dike may be a composite intrusion.
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The relative depletion in HFSE (Nb, Ta, Ti) evident in all samples from the BBMZ
(Figure 2-3a) suggests the source material is more similar to the metasomatized mantle (mantle
wedges) in modern island arc environments, as opposed to plume type magmatism proposed for
dikes such as the Mackenzie or Franklin mafic dike swarms of Northern Canada (e.g. Park, 1981;
Armstrong et al., 1982; Fahrig and West, 1986; LeCheminant and Heaman, 1989; Park et al,
1994). It is important to note that these samples do not plot within field D (within plate basalts),
indicating a lack of evidence for plume related magmatism (Figure 2-2).
The time at which this signature was acquired, however, is more difficult to specify.
Isotopic data (Sm/Nd, U/Pb) are useful in understanding the evolution, mixing, and/or
contamination of dike magmas (e.g., contamination of mafic intrusions when emplaced in more
felsic crust) and, therefore, help constrain the time at which the HFSE signature was acquired.
Sm/Nd data for dikes within the BBMZ form an array at ~3.4 Ga (Figure 2-6), which is clearly
older than the dominant age of the country rock in the eastern Beartooth Mountains, but is within
error of whole rock lead isotopic data, which produce an array suggesting an age of ~3.2 Ga
(Figure 2-7).
For the Beartooth Mountains, isotopic Nd and Pb values from the mafic dikes are best
interpreted to indicate that the mantle source of the dike magmas was modified prior to
formation of the dike magmas. The LREE enrichment, HFSE depletion, and isotopic systematics
seen in the dikes suggest this modification was likely to have been a metasomatic event
associated with convergence and subduction about 3.1-3.4 Ga ago. Due to the differences in
concentration of Pb in the crust and mantle, the Nd value of 3.4 Ga is likely more representative
of the metasomatic event which occurred in the source BBMZ magmas. This metasomatic event
resulted in resetting of parent/daughter ratios in the U/Pb and Sm/Nd isotopic systems and
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significant homogenization of isotopic compositions to yield the isochron-like arrays. This
proposed metasomatic event is compatible with evidence for 3.1-3.5 Ga felsic magmatism in the
eastern Beartooth Mountains (Mueller et al., 2006) indicating that a series of crust forming
events in this interval were substantial enough to impact the crustal and mantle systems coevally.
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0
1
2
3
4
42 47 52 57 62
SiO2 wt. %
Fe
O*/
Mg
O w
t. %
BBMZ
MMP
Calc-Alkaline
Tholeiitic
BTL01
Figure 2-1. SiO2 vs. FeO*/MgO discrimination diagram for island-arc basalts. Field designation line extrapolated from diagram in Winter (2001). FeO* is calculated total iron as FeO. This diagram represents an approximation of values. Samples from the MAP were not plotted due to their high alkalic and silica undersaturated composition.
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Figure 2-2. The Ti-Zr-Y discrimination diagram for basalts (after Pearce and Cann, 1973). Fields are A= island-arc tholeiites; B= MORB, island-arc tholeiites, and calc-alkali basalts; C= calc-alkali basalts; D= within-plate basalts. Coordinates for the designated fields (A, B, C, and D) are listed in both Pearce and Cann (1973) and Rollinson (2003).
23
1
10
100
1000BTL01BTL02BT01 BT02
Sam
ple/
Prim
itive
Man
tle
0.1
1
10
100
1000SJM-10SJM-18TR01 HM03247FMKG RR01
RbBa Th U Nb Ta K La Ce Pb Pr Sr P Nd Zr HfSmEu Ti Dy Y HoYb Lu1
10
100
1000
CA04CZ01CZ02
A
B
C
BBMZ
MMP
MAP
Figure 2-3. Trace element plots for all three areas. A) BBMZ. B) MMP. C) MAP. The “spider” diagrams are all normalized using primitive mantle values from Sun and McDonough (1989).
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1
10
100
1000BTL01 BTL02 BT02 BT01
Sa
mp
le/
Ch
on
dri
te
1
10
100
1000 SJM-10 SJM-18 TR01 HM03 247FM KG RR01
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu1
10
100
1000
10000
CA04 CZ01 CZ02
A
B
C
BBMZ
MMP
MAP
Figure 2-4. REE plots for all three areas. A) BBMZ. B) MMP. C) MAP. The REE plots are normalized using chondritic values from Sun and McDonough (1989).
25
28802840
28002760
27202680
26402600
2560
0.44
0.48
0.52
0.56
0.60
11 12 13 14 15 16207Pb/235U
206 Pb
/238 U
data-point error ellipses are 2σ
Intercepts at 2802.4 +5.5/-4.4 Ma
MSWD = 0.99
Figure 2-5. U-Pb concordia diagram for BBMZ sample BT01, a southern Beartooth sample located just east of Beartooth Butte (Prinz, 1964) and correlates with other ~2.8 Ga dikes also located within this area of the BBMZ.
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0.5100
0.5104
0.5108
0.5112
0.5116
0.5120
0.5124
0.5128
0.06 0.08 0.10 0.12 0.14 0.16 0.18
147Sm/144Nd
143N
d/14
4N
d
Age = 3393±190 MaInitial 143Nd/144Nd =0.50847±0.00015
Figure 2-6. Sm/Nd plot for samples within the BBMZ with a calculated array “age” of ~3.4 Ga using Isoplot 3.2 (Ludwig, 2004).
27
15.2
15.4
15.6
15.8
16.0
16.2
16.4
15.5 16.5 17.5 18.5 19.5 20.5206Pb/204Pb
207 Pb
/204 Pb
Approximate "Age" = 3.2 Ga
Figure 2-7. Diagram 206Pb/204Pb vs. 207Pb/204Pb for dikes within the BBMZ. Using U/Pb geochronology from zircon grains within BT01, emplacement age for the BBMZ dikes is 2.8 Ga.
28
-20.00
-15.00
-10.00
-05.00
00.00
05.00
10.00
15.00
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Age (Ga)
Epsi
lon
Nd
BBMZ
MAP
MMP
Dudas et al., 1987
Mueller et al., 2004 MMDS
CHUR
Depleted Mantle
MMDS in MMP
Generalized Evolution of Northern Wyoming Province Crust
Wooden and Mueller, 1988
Figure 2-8. Age vs. Initial εNd for the BBMZ, MMP, MAP and a Metamorphosed Mafic Dikes and Sills (MMDS) sample from within the MMP. MMDS value was taken from Mueller et al., 2004 and dated at 2.06 Ga using U/Pb geochronology of zircon. There are two samples plotted for the MMP; TR01 and SJM-18. These samples represent the only dikes with known ages for the samples in the MMP for this project. Both samples are ~1.45 Ga in age (refer to MMP section for clarification). Samples added from Dudás et al., 1987 are alkalic to subalkalic in composition and within the MAP. Wooden and Mueller (1988) samples are felsic 2.8 Ga crustal samples from the BBMZ and defined by the light gray field overlying the BBMZ mafic dike samples.
29
Table 2-1. General information on mafic dikes sampled. Coordinates for the latitude and longitude are in WGS 84. Age determinations are as follows; 1 zircon , SHRIMP, U-Pb; 3 Prinz, 1964 and Wooden, 1975, K-Ar; 4 Dudás et al., 1987; Harlan et al., 1988; 5 Johnson and Swapp, 1989, Harlan et al, 2005; 6 Wooden, 1975; 7 Mueller, 1971. 8 Harlan et al., 1990, BBMZ refers to the Bighorn-Beartooth Magmatic Zone and the MMP refers to the Montana Metasedimentary Province. Both are represented in figure 1. Textural descriptions were compiled in this research. All observations are of the author. Sample Location Coordinates Name Age (Ma) Strike Size Composition General Key Notes Alteration
BT01 Beartooth/ BBMZ
44.97932 N 109.5934 W
Diabase 2802±5.51 N 75º W 3 m width >1 km length
Plag + pyx + opx Fine to medium grained containing three coexistsing pyroxenes
minimal
BT02 Beartooth/ BBMZ
45.00760 N 109.5213 W
Diabase 2800 N 45º W 10 m width >1 km length
Plag + pyx + opx + bi Fine to medium grained with orthopyrexene crystals
minimal
BTL01 Beartooth/ BBMZ
42.24055 N 109.6630 W
Diabase 2800-25003 N 25º W 6 m width <1 km length
Plag+ opx+ cpx Porphyritic 60% large (up to 8 inches) euhedral and subeuhedral plagioclase phenocrysts
minimal
BTL02 Beartooth/ BBMZ
42.24055 N 109.6630 W
Diabase 2800-25003 N 25º W 1 m width <1 km length
Plag + opx + cpx with metadolerite and Metanorite within the groundmass
Nonporpyhritic fine grained, metadolerite groundmass
minimal
TR01 Tobacco Root/ MMP
45.42093 N 112.1561 W
Diabase 1450 5 N 75º W 6 m width ~500 m length
Plag + cpx+ mafic groundmass Fine grained fresh diabasic intrusion
none
RR01 Ruby Range/ MMP
45.1743 N 112.4239 W
Diabase 14506 N 23º W 4 m width not exposed at surface
Kspar + cpx + neph Fine to medium grained amphibolite lens
Amphibolite grade
HM03 Highland/ MMP
45.59607 N 112.4870 W
Diabase * N 68º W 9 m width >2 km length
Plag + cpx + hornblende + biotite
Dark greenish-gray to black, Fine to medium grained diabase. Ophitic to subophitic cpx and plag. Chloritization has been noted as minor hydrothermal alteration
chloritization of pyroxene
SJM-10 Highland/ MMP
45.67377 N 112.3491 W
Diabase * N/A N/A Plag + cpx+ biotite + hornblende
No textural description available N/A
SJM-18 Highland/ MMP
45.69483 N 112.3659 W
Diabase 1450 N/A N/A Plag + cpx+ biotite + hornblende
No textural description available N/A
KG Gravelly/ MMP
44.88678 N 111.7346 W
Diabase Not Available
NW strike As wide as 30 m
Plag+ opx+ cpx Fine to medium grained black to greenish fresh
minimal
247FM Gravelly/ MMP
N/A Diabase Not Available
NW strike As wide as 30 m
Plag+ Hbl + Kspar + Qtz Fine to medium grained black to greenish fresh
minimal
30
Table 2-1. Continued
Sample Location Coordinates Name Age (Ma) Strike Size Composition General Key Notes Alteration
CA04 Castle/ GFTZ
46.41131 N 110.5519 W
Diabase 60-50 4 N 64º W 30 m width >3 km length
Cpx + Kspar+ phl+ nepheline + sodalite
Fine to medium grained, porphyritic with sodalite reported but none noted
none
CZ01 Crazy/ GFTZ
46.17021 N 110.5214 W
Diabase 48± 2 4 N 18º W 5 m width 50 m length
Cpx + Kspar+ phl + nepheline fine to medium grained with primary phase phlogopite and mafic groundmass
none
CZ02 Crazy/ GFTZ
46.16797 N 110.4925 W
Diabase 48± 2 4 N 79º W 15 m width ~750 m length
Cpx + Kspar+ phl + varying sedimentary xenoliths
Fine to medium grained, amphibolite occurs in the groundmass along with phlogopite
none
31
Table 2-2. Major, Trace, and Isotopic concentrations of mafic dikes within the Wyoming Province and Great Falls tectonic zone (GFTZ). Major element analyses was accomplished using x-ray fluorescence (XRF). Fe2O3 and FeO were calculated by assuming FeO accounts for 85% of Fe in the system. * in the TDM field of Sm/Nd indicates difficulty expressing a valid age due to slope of sample evaluation line with respect to CHUR's evolution. * for CZ01 and RR01 signifies that the major element chemistry was normalized due to low total.
Sub-province BBMZ MMP MAP Sample BTL01 BTL02 BT01 BT02 TR01 RR01* HM03 SJM-10 SJM-18 KG 247 FM CA04 CZ01* CZ02 SiO2 (%) 53.39 54.01 56.92 54.54 50.02 47.38 51.32 50.99 48.98 49.18 54.36 46.52 44.85 49.48
TiO2 0.49 1.14 0.63 1.06 1.25 0.34 1.32 1.37 1.25 0.65 1.12 0.53 0.45 2.06 Al2O3 14.97 15.82 15.92 16.01 14.21 13.31 13.79 12.89 13.36 23.75 15.3 13.97 16.47 13.23 Fe2O3 1.47 1.70 1.26 1.53 2.02 1.44 2.40 2.54 2.26 1.10 1.66 1.42 1.57 2.40 FeO 8.34 9.65 7.17 8.67 11.44 8.16 13.60 14.40 12.80 6.24 9.40 8.06 8.90 13.60 MnO 0.15 0.14 0.13 0.14 0.2 0.16 0.25 0.25 0.25 0.11 0.15 0.17 0.17 0.23 MgO 7.8 4.43 3.61 4.3 7.41 7.61 5.69 5.18 7.02 3.71 4.98 5.98 4.05 6.65 CaO 8.2 6.8 5.52 6.54 11.24 10.46 9.7 9.45 11.46 12.54 8.26 8.59 7.08 7.65 K2O 1.55 1.73 3.01 2.2 0.21 3.68 0.39 0.4 0.28 0.3 1.15 2.48 2.43 1.01
Na2O 2.52 3.11 3.46 3.35 2.13 4.80 2.18 2.2 2.02 2.14 3.03 6.37 8.68 0.96 P2O5 0.08 0.78 0.81 0.6 0.12 1.48 0.11 0.12 0.09 0.06 0.15 0.92 1.14 0.22 LOI 0.73 0.74 1.05 0.66 0.56 1.18 0.24 0.36 1.08 0.9 0.97 4.04 4.17 3.12
Total 99.68 100.03 99.5 99.6 100.81 100.00 100.99 100.15 100.86 100.68 100.54 99.05 100.00 100.61
Normative Qz 0.96 4.63 5.88 3.31 3.00 3.54 1.00 4.00 6.92 Or 9.15 10.21 17.77 12.99 1.00 21.19 2.00 2.36 2.00 2.00 7.00 14.64 14.05 5.96 Ab 21.30 26.29 29.25 28.32 18.00 2.46 18.00 18.60 17.00 18.00 25.00 11.25 5.80 8.11 An 24.95 24.09 19.01 22.14 28.00 3.82 26.00 24.11 26.00 54.00 25.00 2.21 28.79 Ne 20.15 23.09 34.53 Di 12.44 3.83 2.60 5.34 22.00 30.72 17.00 18.54 25.00 6.00 13.00 28.41 22.79 6.43 Hy 26.86 23.78 18.99 21.18 24.00 26.00 26.06 20.00 16.00 20.00 33.33 Ac 1.00 15.59 Ol 3.00 12.22 4.00 10.27 9.89 Mt 2.14 2.47 1.83 2.22 2.00 2.05 3.00 3.69 3.00 2.00 2.00 2.06 1.24 3.49 Il 0.93 2.17 1.20 2.02 0.63 2.00 2.61 2.00 1.00 2.00 1.01 0.84 3.92
Ap 0.17 1.70 1.77 1.31 3.17 0.26 2.01 2.42 0.48
32
Table 2-2. Continued Sample BTL01 BTL02 BT01 BT02 TR01 RR01 HM03 SJM-10 SJM-18 KG 247 FM CA04 CZ01 CZ02 Element (ppm)
Sc 17 21 18 20 42 25 41 43 43 27 24 15 7 19 V 136 225 168 189 297 227 319 353 348 154 187 195 246 193 Cr 111 73 79 94 193 64 70 36 95 255 115 84 9 163 Co 24 28 23 27 49 34 47 49 57 46 39 29 28 32 Ni 55 54 51 48 81 47 49 40 69 156 69 37 14 54 Cu 59 46 34 1 163 40 114 107 417 51 148 107 248 133 Zn 49 112 93 106 86 83 102 111 117 72 90 93 106 99 Ga 17 20 20 19 16 12 17 19 17 12 18 15 13 14 Rb 7 45 108 105 2 18 9 8 2 76 54 64 66 84 Sr 153 1056 1036 469 125 91 72 59 92 140 290 2400 3691 3396 Y 15 29 30 26 23 24 28 33 26 20 25 24 26 29 Zr 52 297 252 254 78 99 45 41 40 82 130 188 163 345 Nb 2 9 12 10 5 7 2 3 2 3 9 47 47 51 Ba 67 1537 2034 1217 56 143 84 112 30 357 405 2921 3985 4346 La 5 111 127 44 6 9 5 7 3 11 22 120 174 197 Ce 9 215 273 91 18 21 15 19 12 21 44 211 327 350 Pr 1 23 26 11 2 3 2 3 2 3 5 24 40 38 Nd 6 89 103 53 11 14 9 12 9 11 21 95 165 155 Sm 2 13 15 9 3 4 3 4 3 3 5 13 20 21 Eu 0.57 3.26 3.50 2.39 1.03 1.09 1.01 1.18 1.00 0.72 1.27 3.38 4.63 5.43 Gd 1.79 10.73 11.80 6.30 3.98 3.82 4.03 4.32 3.90 2.65 4.40 10.17 14.48 14.30 Tb 0.37 1.19 1.25 0.99 0.65 0.67 0.71 0.82 0.68 0.50 0.71 1.22 1.72 1.77 Dy 2.23 5.31 5.53 4.83 3.88 4.01 4.62 5.34 4.29 3.00 4.16 4.70 5.50 6.09 Ho 0.45 1.00 1.05 0.93 0.84 0.82 1.02 1.16 0.92 0.62 0.86 0.86 0.96 1.03 Er 1.11 2.96 3.27 2.58 2.15 2.13 2.75 3.22 2.41 1.65 2.38 2.67 3.55 3.39 Tm 0.24 0.39 0.39 0.39 0.39 0.35 0.47 0.52 0.42 0.32 0.38 0.35 0.37 0.36 Yb 1.22 2.32 2.45 2.26 2.28 2.01 3.00 3.41 2.53 1.75 2.31 1.95 2.03 2.04 Lu 0.20 0.35 0.37 0.33 0.34 0.30 0.44 0.50 0.38 0.28 0.35 0.29 0.29 0.30 Hf 1.25 6.63 5.64 6.23 2.25 2.73 1.49 1.45 1.40 2.06 3.37 4.58 3.69 7.41 Ta 0.14 0.40 0.52 0.47 0.36 0.47 0.20 0.24 0.25 0.23 0.69 1.63 1.63 1.83 Pb 2.48 17.52 19.48 12.56 1.12 3.30 1.13 0.87 2.17 7.60 10.57 19.48 34.81 29.53 Th 0.17 20.13 11.97 4.72 0.84 0.87 1.11 1.83 0.32 2.30 5.85 14.74 18.53 23.39 U 0.17 1.88 1.50 1.96 0.27 0.25 0.40 0.52 0.13 0.58 1.60 3.36 5.16 5.06
33
Table 2-2. Continued
Sample BTL01 BTL02 BT01 BT02 TR01 RR01 HM03 SJM-10 SJM-18 KG 247 FM CA04 CZ01 CZ02
Whole Rock Isotopes
Wr Rb-Sr
87 Rb/86 Sr 0.1028 0.2159 0.2935 0.6001 0.1038 0.5492 0.3938 0.3949 ND 1.7651 0.4736 0.0493 0.0539 0.0300
87Sr/86Sr 0.7064 0.7122 0.7136 0.7248 0.7044 0.7200 0.7138 0.7136 ND 0.7648 0.7209 0.7059 0.7053 0.7057
WR Sm-Nd
147Sm/144Nd 0.1678 0.0889 0.0864 0.1113 0.1690 0.1622 0.1953 0.1728 0.2030 0.1378 0.1242 0.0824 0.0758 0.0859
143Nd/144Nd 0.5122 0.5105 0.5104 0.5109 0.5126 0.5122 0.5126 0.5123 0.5129 0.5114 0.5113 0.5120 0.5125 0.5120
TCHUR (Ga) 2.2 3.1 3.0 3.0 0.5 1.8 * 2.2 * 3.2 2.8 0.8 0.2 0.8
TDM 2.7 3.2 3.2 3.2 1.7 2.5 * 2.8 * 3.4 3.0 1.2 0.7 1.2
ЄNd Present -7.78 -42.41 -43.34 -33.12 -1.66 -8.10 -1.62 -6.55 5.13 -24.25 -25.63 -11.59 -3.60 -12.35
ЄNd Initial 2.64 -3.56 -3.59 -2.36 3.11 -1.92 -1.37 -2.28 4.01 -13.73 -12.67 -10.86 -2.78 -11.64
WR Pb
206/204 17.522 19.937 16.472 18.720 20.116 17.697 21.764 25.407 17.924 16.252 18.639 16.831 17.377 16.901
207/204 15.624 16.293 15.437 15.835 15.741 15.670 16.055 16.387 15.513 15.268 15.740 15.359 15.416 15.360
208/204 37.159 45.984 39.679 46.583 39.849 37.350 41.319 44.925 37.777 36.274 37.665 37.333 37.557 37.271
34
Table 2-3. U-Pb data for BT01 sample from the Southeastern Beartooth Mountains (Figure 1-2). U-Pb age calculated from twelve spots on a single zircon grain analyzed by SHRIMP in Menlo Park, Ca.
Spot Name
U (ppm)
Th (ppm)
Corr 206 /238
% err
ppmRad 206P
b
204corr
206Pb/238UAge
1serr
204corr
207Pb/206Pb
Age 1serr
204corr208Pb/232Th
Age 1s err
% Dis- cor- dant
4corr
208r/232
%err
238/ 206r
%err
207r/206r
%err
207r/235
% err
BT01-1 158.7 134.8 0.5335 1.1 72.7 2756.1 25.2 2806.8 7.3 2712.9 237.7 1.8 0.1436 8.8 1.8745 1.1 0.1976 0.4 14.54 1.2
BT01-2 252.5 27.2 0.5465 1.1 118.5 2810.5 24.1 2804.0 6.0 2918.3 48.4 -0.2 0.1553 1.7 1.8299 1.1 0.1973 0.4 14.87 1.1
BT01-3 174.7 210.6 0.5044 1.1 75.7 2624.3 23.9 2790.7 15.8 2775.9 36.5 6.3 0.1472 1.3 1.9902 1.1 0.1957 1.0 13.56 1.5
BT01-4 224.3 25.2 0.5295 1.1 102.1 2739.1 23.8 2813.1 6.3 2691.8 48.2 2.7 0.1425 1.8 1.8888 1.1 0.1984 0.4 14.48 1.1
BT01-5 245.2 71.7 0.5600 1.1 118.0 2866.8 24.6 2797.8 6.0 2715.3 36.7 -2.4 0.1438 1.4 1.7856 1.1 0.1966 0.4 15.18 1.1
BT01-6 224.1 68.1 0.5447 1.1 104.9 2802.8 24.4 2793.2 6.4 2870.4 66.2 -0.3 0.1526 2.3 1.8361 1.1 0.1960 0.4 14.72 1.1
BT01-7 475.2 419.6 0.4736 1.0 193.3 2491.7 20.4 2710.5 5.6 2407.0 37.7 8.8 0.1265 1.6 2.1193 1.0 0.1864 0.3 12.13 1.0
BT01-8 253.1 61.3 0.5419 1.1 117.8 2791.0 24.1 2809.7 8.5 2679.5 112.3 0.7 0.1418 4.2 1.8457 1.1 0.1980 0.5 14.79 1.2
BT01-9 232.3 42.7 0.5316 1.1 106.1 2747.4 23.9 2806.2 8.1 2512.6 71.2 2.1 0.1324 2.8 1.8818 1.1 0.1976 0.5 14.48 1.2 BT01-10 169.1 160.1 0.5108 1.1 74.2 2656.7 24.4 2793.3 8.2 2756.0 104.5 5.1 0.1461 3.8 1.9608 1.1 0.1960 0.5 13.78 1.2 BT01-11 163.3 103.8 0.5503 1.1 77.2 2825.3 26.1 2804.8 7.5 2854.7 38.8 -0.7 0.1517 1.4 1.8181 1.1 0.1974 0.5 14.97 1.2 BT01-12 176.7 93.0 0.5474 1.1 83.1 2814.6 25.6 2800.3 7.1 2880.4 38.6 -0.5 0.1532 1.3 1.8267 1.1 0.1969 0.4 14.86 1.2
4corr = 204 common Pb correction applied r and rad = radiogenic after common Pb correction
35
CHAPTER 3 PRECAMBRIAN MAFIC DIKES OF THE MONTANA METASEDIMENTARY PROVINCE
(MMP)
Geologic Setting and Previous Work
The Montana Metasedimentary Province (MMP) is located on the northwestern part of
the Wyoming Province (Figures 1-1 and 1-2). The MMP is characterized by metasupracrustal
sequences comprised of quartzofeldspathic gneisses with distinctive marble-iron formation-
quartzite associations, calcareous gneisses, pelitic to mafic schists, amphibolites, and meta-
ultramafic rocks (Mogk et al., 1992; Mueller et al., 1993). The protoliths for the high grade
metamorphic rocks in the MMP are believed to be from deposition of volcanic and sedimentary
rocks along a passive continental margin in Archean time (Harlan, 1992; Mogk et al., 1994).
Gneisses range from 3.2 to 3.5 Ga and detrital zircons from the metasupracrustal sequences yield
ages up to 3.9 Ga, comparable to published dates within quartzites in the BBMZ (e.g. Mogk et
al., 1992; Stevenson and Patchett, 1990, Mueller et al., 1982; Mueller et al., 1998). Magmatism
in the MMP occurred at multiple times between 3.5-1.8 Ga. The 1.8 Ga magmatism is coeval
with a significant metamorphic event that affected the rocks and led to their inclusion within the
GFTZ (Wooden et al., 1978; Johnson and Swapp, 1989; Harlan, 1992; Harlan et al., 2005;
Mueller et al., 2002).
Mafic dikes from four ranges (Tobacco Root Mountains, Ruby Range, Highland
Mountains, and Gravelly Range) within the MMP were analyzed for this project. The Tobacco
Root Mountains, Highland Range, and Ruby Range all contain country rock lithologies that are
typical of the MMP (i.e., Wooden, 1972; Wooden et al., 1978; Harlan et al, 2005; Johnson and
Swapp, 1989). The Gravelly Range is compositionally similar to the other ranges but lack pelitic
rocks found within the MMP (O’Neill and Christiansen, 2004). The Gravelly Range has exposed
36
low-grade metamorphic rocks, ranging from greenschist to lower amphibolite facies, whereas,
the Tobacco Root, Ruby Range, and Highland Mountains have experienced upper amphibolite
facies metamorphism last recorded at ~1.8 Ga (Johnson and Swapp, 1989; Harlan, 1992; Harlan
et al., 2005; Mueller et al., 2005).
Age Relations
Dike swarms of the Tobacco Root and Ruby Range were separated into three groups (A,
B, and C) by earlier workers (Wooden et al., 1978, Johnson and Swapp, 1989, Harlan et al,.
2005) based on major elements (e.g., FeO*, MgO, and CaO), whole rock Rb/Sr (Wooden et al.,
1978), Sm/Nd mineral isochrons (Harlan et al., 2005), correlation with known regional tectonic
events (Johnson and Swapp, 1989; Harlan et al., 2005), and correlating paleomagnetic data
(Harlan et al., 2005).
Group A dikes are slightly altered, fine-to-medium grained diabase, with clinopyroxene
(augite) and plagioclase as primary minerals. Group A dikes from the Ruby Range gave a whole
rock Rb/Sr isochron of 1455±125 Ma with an initial 87Sr/86Sr ratio of 0.7019±0.0008 (Wooden,
1975; Wooden et al., 1978). Group A and C dikes from the Tobacco Root Mountains gave both
whole rock and individual mineral (plagioclase and pyroxene) Rb/Sr and Sm/Nd isochrons of
1448±49 Ma (Harlan et al., 2005). Because of these similarities and because the paleomagnetic
data for Group A and Group C dikes are essentially identical, it has been suggested that the
Group C dikes were also emplaced at ~1450 Ma (Harlan et al., 2005).
For reference, Group B dikes have clinopyroxene and plagioclase as primary minerals,
but also contain biotite as a minor primary mineral. Group B dikes have also experienced more
alteration. The most commonly altered mineral is plagioclase that is partially replaced by clays
and micas and chloritization of biotite and pyroxene (Wooden et al., 1978). Group B dikes also
have generally lower MgO wt %, strong enrichment of FeO*, and whole rock Rb/Sr parallel
37
isochrons of 1130 to 1160 Ma (Wooden et al., 1978; Harlan et al., 2005). The paleomagnetic
pole direction for Group B dikes is not compatible with pole directions established for ca. 1100
and 1450 Ma (Harlan et al., 1997; Harlan et al., 2005). There is correlation, however, of poles
from Group B dikes with well dated Neoproterozoic (ca. 780-723 Ma) igneous rocks from the
Cordillera. In addition to these samples, Harlan et al. (2005) reports paleomagnetic data reported
by Harlan et al. (1996) and Harlan et al. (1997) “strongly” suggest Group B dikes are
Neoproterozoic in age.
Samples SJM-18 and TR01 plot within Group A, with respect to fields (i.e., MgO vs
CaO, K2O, Ni, Cr) lain out by earlier workers (Wooden et al., 1978; Johnson and Swapp, 1989;
Harlan et al., 2005) and are, therefore, inferred to have an emplacement age of ~1450 Ma.
Published U/Pb data (sample TR-42; Harlan et al., 2005) from baddeleyite and zircon grains also
indicate 1448 Ma emplacement age for sample TR01.
Group A and C dikes are attributed to extension accompanying the formation of the Belt
Basin (ca. 1450 Ma) (Johnson and Swapp, 1989; Harlan et al., 2005). The ca. 780 Ma mafic
magmatism of Group B dikes is attributed to the breakup of Rodinia along the western boundary
of Laurentia (Harlan et al., 2005; Mueller et al., 2006).
Tobacco Root Mountains
The Tobacco Root Mountains are a north-trending, east-tilted block raised along a high
angle normal fault located on the western margin of the range (Burger, 2004). The majority of
exposed rocks are high-grade Archean metamorphic rocks that include quartzofeldspathic
gneisses, but there are some Paleozoic and Mesozoic rocks in the northern part of the range. The
central and eastern portion of the range contains granitic to dioritic intrusions of the Late
Cretaceous Tobacco Root batholith (Burger, 2004). The southern part of the range is contains
dike swarms that show a uniform strike of WNW and range in composition from unaltered to
38
metamorphosed mafic and ultramafic rocks (i.e., Wooden, 1975; Wooden et al., 1978; Harlan et
al., 2005).
The metamorphosed dikes have been extensively studied by earlier workers (Reid, 1963;
Cordua, 1973; Brady et al., 2004) and in summary; the MMDS (metamorphosed mafic dikes and
sills) of the Tobacco Root Mountains are described as diabase intrusions were metamorphosed at
~1.8 Ga metamorphic event of the MMP/ GFTZ. Compared to samples from this project from
the same region, the MMDS has a similar SiO2 contents (48% and 52%), similar TiO2 (~.5 to
2%), general mineralogy (clinopyroxene and plagioclase as primary minerals) to the
unmetamorphosed dikes of the MMP (Brady et al., 2004; Burger et al., 2004). Trace element
chemistry is reported for the MMDS in Brady et al. (2004) and is almost identical (i.e., range of
LREE enrichment is 10-100 times chondritic values; enrichment of HREE is ~10 times
chondritic values) to values for the MMP dikes (Figure 5b).
Ruby Range
The Ruby Range is an uplifted Precambrian block whose exposed core is comprised of
deformed, high grade, metamorphic rocks (i.e., quartzofeldspathic gneisses) (Garihan, 1979), in
which three major stratigraphic divisions were recognized; Cherry Creek Group, the Dillon
quartzofeldspathic gneiss, and the pre-Cherry Creek rocks (oldest). Garihan (1979) reports a
complex history of recurring intrusions, metamorphism, deposition, and reactivation of faults.
The dike sampled from the Ruby Range is located in the southern portion of this range
(Figure 1b) and its field characteristics are listed in Table 2-1. Early workers (Wooden, 1975;
Garihan, 1979 and references within) report that metabasites are the primary mafic intrusion in
the area. These metabasites are compositionally similar to RR01 and have the same mineral
assemblages (Garihan, 1979). Garnet is present in both the reported metabasites and RR01,
although RR01’s garnet is attributed to post emplacement metamorphic process. RR01’s exposed
39
at the surface was very limited and therefore difficult to interpret its field relations as a dike or a
possible mafic pod or lens. Due to the high total alkali content (Na2O + K2O=8.5 wt. %) (Table
2-2) and the presence of garnet, it is not considered likely that this sample is a pristine intrusion,
but and its bulk composition likely reflects alteration to a significant degree.
Highland Mountains
The Highland Mountains are similar to the Tobacco Root Mountains and Ruby Range,
i.e., a block of mainly Archean rocks uplifted during the Laramide Orogeny of Late Cretaceous
and early Cenozoic time (Johnson and Swapp, 1989; Harrison et al., 1974). The southern
Highland Range consists of Archean quartzofeldspathic gneiss similar to what is present
throughout the MMP, although rocks such as marbles and pelitic schists that are present in the
Tobacco Root Mountains and Ruby Range have not been reported in the Highland Mountains
(Harlan, 1992).
Dikes occur in two separate groups distinguished by strike; an east-west group and a
northwest-southeast group. The east-west trending dikes are more numerous than the northwest
trending dikes and Johnson and Swapp (1989) examined their intersection in an attempt to
determine relative age. They noted that no petrologic or geochemical differences were evident
and proposed that both groups were from the same emplacement event and source.
Unfortunately, the age of the Highland Range dikes is poorly constrained (Harlan, 1992; Johnson
and Swapp, 1989), but is proposed to be Mesoproterozoic (~1.45 Ga) based on two arguments
presented above. Sample SJM-18 is believed to be ~1.45 Ga due to its geochemical similarities
to Group A dikes as discussed above. Also, dikes in the Highland Mountains cut basement rocks
that have been metamorphosed at 1.8 Ga (i.e., Harlan, 1992; Johnson and Swapp, 1989) implying
an age of less than 1.8 Ga. Although moderate alteration of HM03 is reported in Table 2-1, it
was not metamorphosed during the 1.8 Ga metamorphic event (Mueller et al, 2004, Brady et al,
40
2004), but was more likely affected by localized hydrothermal alteration (Johnson and Swapp,
1989).
Results
Major, trace element, and isotopic ratios are reported in Table 2-2. Samples from the MMP
have a range of 46-51 wt. % SiO2 and, except for RR01 and KG, are similar with respect to other
oxides. All samples fall near the tholeiitic-calc alkaline transition line in Figure 2-1. Sample
RR01 contains potassium feldspar and minimal plagioclase, whereas no potassium feldspar was
noted in any other MMP samples (Table 2-1). RR01 is the only MMP sample with metamorphic
garnet porphyoblasts. Further examination in thin section indicates that this is post emplacement
metamorphism due to the orientation of clinopyroxene around the porphyoblasts of garnet. These
observations, along with unclear field observations make it difficult to interpret the composition
or age of RR01 with any certainty. Sample KG from the Gravelly Range is also distinct in having
twice as much Al2O3, but only half of the total iron and TiO2 as the other samples in the MMP.
Trace element abundances (Figure 2-3b) for samples within the MMP show variable
depletion of high field strength elements (HFSE) when normalized to primitive mantle. The
Highland samples (HM03, SJM-10, and SJM-18) are the most depleted in some HFSE,
especially in Zr relative to other samples in the MMP (Figure 2-3b). The Gravelly Range
samples (247FM and KG) show enrichment of U as well as Pb relative to primitive mantle and
other MMP samples. Sample RR01 shows HFSE depletions, but also show large (100x)
enrichment of K with respect to primitive mantle.
All samples from the MMP have HREE concentrations ~10-20 times and LREE of 10-50
times chondritic values (Figure 2-4b), with the exception of the two samples from the Gravelly
Range (247FM and KG). Both of these samples show a slightly greater enrichment of LREE (80-
100 times chondritic values), but similar HREE abundances to the rest of the MMP samples. All
41
samples from the MMP show a pattern similar to calc-alkaline basalts (Figure 2-4b) (Wilson,
1989) and also similar to samples from the MMP in Burger et al. (2004). Figure 2-2 indicates
that samples from the MMP, using an incompatible trace element ternary diagram, show
variation across different compositional/tectonic settings (i.e., MORB, IAB, and calc-alkaline).
The three samples that fall into field A (island arc tholeiites) are all from the Highland Range,
whereas the other four (KG, 247FM, RR01, and TR01) samples scatter between fields B and C.
It is important to note that none of the MMP samples plot into field D of Figure 2-2, indicating
that they are not likely to be within-plate basalts (i.e., plume related).
Although discussed in the same context due to some geochemical and geographic
similarities, there are differences in isotopic systematics between the samples within the MMP.
The 147Sm/ 144Nd values of the MMP dikes (Table 2-2) are high and pose a difficulty for
calculating TDM and TCHUR values. As with TDM and TCHUR, both samples HM03 and SJM-18
exhibit initial εNd values that are unlikely to be accurate. Both show a change of just one epsilon
unit from initial to present day values (Table 2-2). Initial εNd values for samples in the MMP
range from 5 to -8, whereas present day εNd values yield a range of -1.62 to -25.63. Due to the
range and high Sm/Nd ratios, only two initial εNd values (SJM-10 and TR01) were plotted for the
MMP (Figure 8) because the age assignments can be made more comfortably. A sample from the
MMDS of the Tobacco Root Mountains reported in Mueller et al., (2004) was also plotted on
Figure 8 to provide initial εNd values for 2.06 Ga from U/Pb zircon ages. Whole rock Pb data for
samples within the MMP are listed in Table 2-2. 206Pb/204Pb ratios vary between 16.252 and
25.407, 207 Pb/204Pb ratios vary from 15.268 to 16.387.
Discussion
Samples from the MMP show a wide compositional variation and resemble compositions
of basalt and basaltic andesite due to the wt % of SiO2, TiO2, Al2O3, and total iron, with the
42
exception of RR01, which has distinctly higher total alkali of 8.5 wt. % and lower TiO2 than the
other MMP samples. Due to these differences, the presence of garnet (likely post-emplacement),
and a poor understanding of field relations, RR01 is not used in the source evolution
interpretations for the MMP.
Due to the depletion in HFSE (Nb, Ta, Ti) for all samples from the MMP (Figure 2-3b),
the source is interpreted to be similar to the metasomatized mantle created in the mantle wedges
in modern island arc environments. REE patterns for the MMP have slight enrichments in LREE,
but not to the extent of average calc-alkaline basalt (Figure 2-4b and 2-4c). This is also shown in
Figure 2-1 where dikes from the MMP plot near the transition line for tholeiitic/calc-alkaline.
Island-arc tholeiitic basalts usually have LREE depleted patterns whereas calc-alkaline basalts
are typically LREE enriched (Wilson, 1989).
Sm/Nd ratios and εNd values indicate the source may have had parent/daughter ratios reset
ca. 2.0 Ga (Figure 3-1). Although this age is consistent with the proposed Paleoproterozoic
metamorphic event within the western MMP (Johnson and Swapp, 1989; Mueller et al., 2004;
Harlan et al., 2005), difficulties calculating TDM and TCHUR values arise due to high 147Sm/ 144Nd
ratios for 5 of the 7 MMP samples (0.1622-.2030) (TR01, HM03, SJM-10, SJM-18, and RR01).
The two highest values of 147Sm/144Nd are for samples HM03 (0.1953), which produces a TDM of
6.2 Ga, and SJM-18 (0.2030), which produces a TDM of 8.5 Ga. Although this should cause
concern for the integrity of the REE and trace element data, HM03 and SJM-18 show close
similarities to the Highland sample (SJM-10 has a TDM and TCHUR of 2.8 Ga and 2.2 Ga,
respectively) in major, trace, and isotopic ratios (Sm/Nd and U-Pb). Consequently, the rather
extreme TDM for these samples more likely reflects the difficulty in extrapolating evolution of
samples with Sm/Nd near that of CHUR and depleted mantle (DM).
43
Whole rock lead isotopes for the MMP produce a ~1.9 Ga “age” (Figure 3-2). This “age”
is indistinguishable from either the proposed ~1.8 Ga metamorphic event within the MMP or the
Sm/Nd reference line of ~2.0 Ga. Metasomatism of source (s) for the MMP dikes is likely
constrained to this time frame of ~1.9-2.0 Ga.
44
0.5120
0.5122
0.5124
0.5126
0.5128
0.5130
0.15 0.16 0.17 0.18 0.19 0.20 0.21
147Sm/144Nd
143N
d/14
4N
dReference Lines are ~ 2.0 Ga
RR01
SJM-10
HM03
SJM-18
TR01
Figure 3-1. Sm/Nd plot for samples within the MMP form two closely parallel reference lines that represent a ~2.0 Ga “age”, which were calculated using Isoplot 3.2 (Ludwig, 2004). The Gravelly Range samples, KG and 247 FM, were not included in this graph due to the different history of the Gravelly Range from the rest of the MMP with respects to Sm/Nd isotopic systematics.
45
15.0
15.4
15.8
16.2
16.6
14 16 18 20 22 24 26 28
206Pb/204Pb
207 Pb
/204 Pb
Approximate "Age" = ~ 1.9 Ga
Figure 3-2. Whole rock 206Pb/204Pb vs. 207Pb/204Pb diagram for samples within the MMP. “Age” for these samples is actually a reference line and although there is variation of composition within the dikes of the MMP, the data suggest that the source material(s) within the MMP is isotopically similar with respect to Pb.
46
CHAPTER 4
EOCENE MAFIC DIKES OF THE MONTANA ALKALI PROVINCE (MAP)
Geologic Setting and Previous Work
The Crazy Mountains and the Castle Mountains are part of the high-potassium igneous
province of Montana (referred to as the Montana Alkali Province (MAP)) (Hearn et al, 1992;
Dudás et al., 1987) (Figure 1-2). This petrologic province contains unique Mid-Eocene,
feldspathoidal, mafic, alkalic rocks that are characterized by a strong enrichment in incompatible
elements with respect to primitive mantle and have isotopic compositions (Rb/Sr, Sm/Nd, and
Pb/Pb) that reflect an ancient source (Hearn et al., 1989). In addition to these unique mafic rocks,
there are also felsic varieties of the feldspathoidal rocks which are texturally and compositionally
different from their mafic counterparts.
The MAP magmas, at one time, were thought to be penetrating Archean basement
(Dudás and Eggler, 1989), however, U-Pb zircon ages from mafic and felsic igneous and
metamorphic rocks within the Little Belt Mountains give dominantly Paleoproterozoic ages (1.86
Ga; Mueller et al., 2002; Vogl et al., 2005) indicating little to no Archean component. These
rocks within the Little Belt Mountains were also analyzed for trace elements and Sm/Nd isotopic
ratios, which show characteristics of petrogenesis in a convergent zone where juvenile
lithosphere was subducted (i.e., Mueller et al., 2002; Vogl et al., 2005; Foster et al., 2006). This
convergent boundary is referred to as the Great Falls tectonic zone (O’Neill and Lopez, 1985;
Mueller et al., 2002; Vogl et al., 2005; Foster et al., 2006)
The GFTZ is postulated to be Paleoproterozoic collisional orogen between the Archean
Wyoming and Medicine Hat provinces (O’ Neill and Lopez, 1985; Mueller et al., 2002; Foster et
al., 2006). This is a zone containing NE –trending faults, intrusions, and depositional patterns
which range in age from Proterozoic to Tertiary (O’Neill and Lopez, 1985). Even though
47
completely covered by Proterozoic and Phanerozoic sedimentary rocks, the GFTZ provides
insight into Proterozoic accretionary processes along the western margin of Laurentia (Dahl et
al., 1999; Mueller et al., 2005). The western section (i.e. Tobacco Root and Highland Mountains)
of the GFTZ (Figure 1a) includes Archean rocks with metamorphic zircon and monazite ages ~
1.77 Ga (Harlan, 1996; Mueller et al., 2004; Mueller et al, 2005; Foster et al., 2006), while the
northwestern portion of the GFTZ appears to be Paleoproterozoic (see above). With the
exception of the uplifted exposed cores in the Little Belts and Little Rocky Mountains, the
basement is concealed in MAP. It is this concealment that causes inherent difficulty in
understanding where Archean basement (i.e., Beartooth Mountains) to the south ends and where
Proterozoic (i.e., Little Belt Mountains) basement to the north begins.
Between 69 and 27 Ma, the MAP experienced alkalic magmatic activity focused in eight
major centers and multiple minor centers (Hearn et al., 1989). The main periods of magmatism
took place between 69-60 Ma and 54-50 Ma. In the early Eocene, numerous locations
experienced this alkalic igneous activity (i.e., Crazy, Castle, and Little Belt Mountains; Figure 1-
1 and 1-2) (Hearn, 1989; Dudás and Eggler, 1989). Mafic dikes were sampled from the Crazy
and Castle Mountains, which are described in McDonald et al (2005).
Castle Mountains
The Eocene Castle Granite, Blackhawk Diorite, and a porphyritic dacite are exposed in
the Castle Mountains, Montana, where they intrude Precambrian Belt series rocks and younger
Paleozoic and Mesozoic sedimentary rocks. Sample CA04 is a malignite dike from the eastern
side of the Castle Mountains that radiates from the Comb Creek Laccolith, a nepheline syenite,
and intrudes the Bearpaw shale (Dudás and Eggler, 1989). This laccolith lies ~10km south of this
exposure and includes dikes and sills that have compositions of phonolite, trachyte, malignite,
and lamprophyre (Dudás and Eggler, 1989).
48
Crazy Mountains
The mafic intrusions of the Crazy Mountains were emplaced into Cretaceous and
Paleocene sedimentary strata; adjacent basement cored uplifts include the Archean-cored blocks
of the Beartooth Mountains to the south, and the Proterozoic rocks of the Little Belt Mountains
to the northwest. Rb/Sr mineral isochrons for the mafic igneous activity in the area produce an
~48±2 Ma age for the Crazy Mountains (Dudás et al., 1987) and is used as the reference age for
these samples.
The mafic alkalic rocks (including malignite, nepheline syenite, analcite, syenite,
theralite, trachyte porphyry, and quartz latite) are sodium-rich, silica-undersaturated, strongly
alkaline, and were emplaced as dikes, sills, phaccoliths, and laccoliths that are generally located
in the northern half of the Crazy Mountains. Samples CZ01 and CZ02 are dikes located in the
northwestern portion of the Crazy Mountains (Figure 1b) and are discussed in Dudás and Eggler
(1989). Sample CZ01 is a malignite with the reported highest 143Nd/144Nd of any sample
analyzed in the Crazy Mountains (Dudás and Eggler, 1989). Sample CZ02 is a minette with
reported xenoliths ranging in composition from amphibolite to clinopyroxenite with some
sedimentary xenoliths occurring (Dudás and Eggler, 1989), although none were observed.
Results
The field characteristics and geochemical data for samples from the MAP are presented
in Tables 2-1 and 2-2, respectively. The samples from this area range from 44-49.5% SiO2, but
CZ02 has lower values for Na2O, K2O, and P2O5 wt. % (0.96, 1.01, and 0.22, respectively) than
CA04 (6.4, 2.48, and 0.92) and CZ01 (8.6, 2.43, and 1.14). CZ02 (2.06 and 13.60) has the
highest TiO2 and total iron wt % compared to CA04 (0.53 and 8.06) and CZ01 (0.45 and 8.90)
(Table 2). These differences reflect variations in mineralogy. For example, CZ02 has twice as
49
much magnetite and contains no nepheline compared to CA04 and CZ01. Normatively, CZ02
has 7% olivine (Table 2-2), but olivine is not present in thin section or hand sample.
For the normalized trace element diagram (Figure 2-3c) samples from the MAP show a
tendency for depletion in the high field strength elements (HFSE) (i.e. P, Ti, Zr, Hf). CZ02 has
the largest depletion of P, but has the smallest depletion of Ti compared to the other two samples
(Figure 2-3c). The REE plot (Figure 5c) for the MAP shows a strong enrichment in the LREE
(~800x chondritic values) for CZ01 and CZ02 and a slight enrichment in the HREE (20x
chondritic values). CA04 shows a slightly less enriched LREE pattern (~500x chondritic values)
in comparison to CZ01 and CZ02.
Sm-Nd isotopic data show a variation in initial εnd of -3 to -12 and a present day value of
-2.75 to -11.6. It is interesting to note that the low 147Sm/ 144Nd ratio of CZ01 (0.07584) is paired
with the high 143Nd/144Nd (0.51246) (Table 2-2). Samples CA04 and CZ02 have similar
207Pb/204Pb and 206Pb/204Pb ratios, whereas, CZ01 has the highest ratios of the MAP samples
(Table 2-2).
Discussion
Due to the high alkalic compositions of the MAP dikes, presentation within Figure 2-1
proved to be misleading. The high Na2O wt % for sample CZ01 (8.5) compared to CA04 and
CZ02 is attributed to the amount of nepheline present in CZ01 (~30 %). CA04 does have
nepheline present (~15-20 %) but at lesser extent than CZ01. The presence of feldspathoidal
rocks within this petrographic province is consistent with the observed high Na2O relative to
K2O reported by Dudás et al., (1987), Dudás and Eggler (1989), and Hearn et al., (1989).
The REE patterns (Figure 2-4c) for the MAP show strong enrichment in LREE relative to
HREE. This pattern is similar to modern alkali basalt (Wilson, 1989; Winter, 2001).Variations in
the concentration of Cr and Ni, the presence of phologopite in CZ01 and CZ02, and lack of
50
garnet within the MAP samples, suggests that the source(s) of these dikes was likely to be a
metasomatized, clinopyroxene rich mantle and less likely to be predominately peridotitic type
source (Hearn et al., 1989).
Sm/Nd data for samples in the MAP, when combined with samples reported in Hearn et
al. (1989), show a diverse range of ratios for Sm/Nd with less variance in 143Nd/144Nd ratios.
Values for CZ01 and CZ02 are within error of published ratios for samples within the Crazy
Mountains (Dudás et al., 1987; Dudás and Eggler, 1989). Consequently, there is no secondary
array for these samples.
Whole rock U/Pb data form an array indicative of an age of ~1.8 Ga (Figure 4-1). This
“age” represents a time of resetting of parent/daughter isotopic ratios of the source for the MAP
magmas, likely from metasomatism of the source material initiated in a convergent environment.
This “age” is also undistinguishable from the Sm/Nd and U/Pb data from the MMP (Figure 3-1
and 3-2, respectively). The whole rock U/Pb data for the MAP allow us to propose that the
subduction related metasomatism is likely attributed to the Great Falls collision between the
Wyoming Province and the Hearne-Medicine Hat Block at ~1.86 Ga (Mueller et al., 2002;
Mueller and Frost, 2006; Foster et al., 2006).
51
15.34
15.36
15.38
15.40
15.42
16.7 16.9 17.1 17.3 17.5206Pb/204Pb
207 Pb
/204 Pb
Approximate "Age"= ~1.8 Ga
Figure 4-1. Whole rock 206Pb/204Pb vs. 207Pb/204Pb isotopic diagram for MAP samples. MAP Pb/Pb data form an array with an “age” of ~ 1.8 Ga.
52
CHAPTER 5 CONCLUSION
The Wyoming Province has a diverse and complicated geologic history that includes
intrusion of both Archean and Proterozoic mafic dikes. All samples for this study are borderline
tholeiitic to calc alkaline in composition. Normalized trace element diagrams for samples within
all three areas show a range of depletions in high field strength elements (i.e. Nb, Ta, Ti) that is
characteristic of modern convergent margin magmatism. REE patterns differ, particularly in
terms of LREE enrichment, suggesting distinct sources and/or petrogenesis. These distinct
patterns are consistent with modern magmatic arc signatures, suggesting that the source of these
dikes was the subcontinental lithosphere and that this lithosphere was modified by distinct
episodes of Archean and Proterozoic subduction and mantle metasomatism. This metasomatism,
in turn, reset isotopic parent/daughter ratios for the source material of the dikes within the
BBMZ, MMP, and MAP. Pb and Nd isotopes from mafic dikes within the BBMZ indicate that
this metasomatic event occurred at ~3.1-3.4 Ga, whereas Pb and Nd isotopic data for the MMP
and MAP are indistinguishable from one another at ~1.9-2.0 Ga and ~1.8 Ga, respectively.
53
APPENDIX MATERIALS AND METHODS
Sampling Strategy
Diabase dikes and sills were sampled using locations within the citied literature coupled
with additional samples located during fieldwork. These dikes and sills were examined to
determine physical weathering and metamorphism of the rock units. In addition to these above
mentioned samples, samples SJM-10, SJM-18, and KG were collected by The Keck Group and
processed along with the samples that I collected.
Sample Processing
Samples were all processed using standard crushing procedures (i.e. jaw crusher, disk
mill), and mineral separation using water table techniques with the heavy mineral separates used.
This fraction was then sieved using a number 50 (350 microns) pan. From this sieved fraction,
heavy liquids were used to further separate higher density minerals from lower densities. The
heavy liquids used were Tetramethylbromide (TBE), which has a reported density of 3.06g/cc3,
and Methyl Iodide (MeI), which has a reported density of 3.26 g/cc3. Zircons were then
extracted at the smallest degree possible at 1.5 amps using a Frantz 300M Magnetic Separator.
These nonmagnetic fractions were then separated into the best least fractured zircons using a
microscope.
All collected samples were also processed in the ball mill for fine rock powders. These
powders were used for major, trace element, and whole rock isotope determination. For major
elemental analysis, Rock powders were sent off-premise for x-ray fluorescence and are reported
in Table 2. The remaining rock powders were processed at the University of Florida. For trace
elements and isotopic data, these powders were dissolved in sealed Teflon vials for several days
54
at 100˚C in HF-HNO3 mixture. Trace element data was acquired using Element ICP-MS and
were corrected for standard and machine drift.
Radiogenic isotopic analyses were performed at the Department of Geological Sciences
at the University of Florida. Sr, Pb, and Nd were separated using standard chromatographic
methods. Once separated, whole rock Pb, Sm-Nd, and Rb were analyzed using Nu Plasma
Multiple-Collector magnetic separator Inductively Coupled Mass Spectrometer (MC-ICP-MS),
whereas Sr was analyzed using a Micromass Sector 54 Thermal Ionization Mass Spectrometer
(TIMS).
Samples and standard solutions analyzed on the (MC-ICP-MS) were aspirated into the
plasma source either via a Micromist nebulizer with GE spray chamber (wet plasma) or through
DSN-100 desolvating nebulizer (dry plasma), depending on the Nd concentration. The
instrument settings were carefully tuned to maximize the signal intensities on a daily basis.
Preamplifier gain calibration was performed before each analytical session. Nd isotope
measurements were conducted for 60 ratios in static mode simultaneously acquiring 142Nd on
low-2, 143Nd on low-1, 144Nd on Axial, 145Nd on high-1, 146Nd on high-2, 147Sm on high-3, 148Nd
on high-4, and 150Nd on high-5 Faraday detectors. The measured 144Nd, 148Nd, and 150Nd beams
were corrected for isobaric interference from Sm using 147Sm/144Sm = 4.88, 147Sm/148Sm = 1.33,
and 147Sm/150Sm = 2.03. All measured ratios were normalized to 146Nd/144Nd = 0.7219 using an
exponential law for mass-bias correction. The mean value of 143Nd/144Nd for our Ames Nd in-
house standard based on 23 repeat analyses during the samples analyses was 0.512140 (2σ =
0.000012). Three repeat analyses of the JNdi-1 and LaJolla Nd standards during the same time
interval produced mean values of 0.512106 (2σ = 0.000013) and 0.511856 (2σ = 0.000013),
respectively. Three separate dissolutions of USGS SRM BCR-1 were prepared and analyzed for
55
Nd isotopes together with the samples in order to further evaluate the analytical protocol. The
mean value of 143Nd/144Nd for the analyses of BCR-1 was 0.512645 (2σ = 0.000011), which is
indistinguishable from the published TIMS value of 0.51264 (Gladney et al., 1990).
Pb isotopic analyses were conducted on a Nu Plasma multi collector ICP-MS using the Tl
normalization technique on fresh mixtures to prevent oxidation of thallium to Tl3+ (for more
details see Kamenov et al. 2004, and Kamenov et al, 2005). Analyses of NBS 981 conducted in
wet plasma mode together with the sample analyses gave the following results:
206Pb/204Pb=16.937 (+/-0.004 2σ), 207Pb/204Pb=15.490 (+/-0.003 2σ), and 208Pb/204Pb=36.695 (+/-
0.009 2σ). Due to the low Pb content the ultramafic xenoliths were analyzed in dry plasma mode
and the reported data are relative to the following NBS 981 values (n=29): 206Pb/204Pb=16.937
(+/-0.001, 2σ), 207Pb/204Pb=15.491 (+/-0.001, 2σ), and 208Pb/204Pb=36.694 (+/-0.004, 2σ).
The TIMS is equipped with seven Faraday collectors and one Daly collector. Sr samples
were loaded on oxidized W single filaments and run in dynamic collection mode. Data were
acquired at a beam intensity of 1.5V for 88Sr, with corrections for instrumental discrimination
made assuming 86Sr/88Sr=0.1194. Errors in measured 87Sr/86Sr are better than +/- 0.00002 (2σ)
based on long-term reproducibility of NBS 987 (87Sr/86Sr=0.71024). Rb was analyzed by both
mass spectrometers to develop an accuracy curve and determine whether a certain machine
produces higher precision data.
Zircon Separation
Zircons were separated with the aid of a microscope. Using the lowest magnetic angle at
1.5 amps, the zircons were hand picked on the basis of shape, size, and degree of inclusions,
although due to the scarcity of zircons within these samples, any grain thought to be a zircon was
extracted. The grains that were smaller than 50 microns were analyzed on the Sensitive High
56
Resolution Ion Micro Probe (SHRIMP), by Dr. Paul Mueller and Dr. Joe Wooden. The grains
larger than 50 microns were mounted on epoxy disks and polished to expose grain centers. The
disks also contained the standard Forest Center (FC-1). Forest Center is a gabbro that has been
precisely dated at 1099 Ma and is located in Duluth Minnesota (Paces and Miller, 1993).
Titanite Separation
Titanite was extracted from one sample (BTO2) and analyzed using the ICP-MS as
mentioned above. This procedure was administered by Sam Coyner and used jointly for his
research.
57
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BIOGRAPHICAL SKETCH
Joshua Richards was born in Indianapolis, Indiana in 1974. He attended Center Grove
High School in Greenwood, Indiana in 1992. After high school, he joined the United States Navy
in 1994. After serving his tour of duty, he began his undergraduate studies in chemistry at
Indiana University-Purdue University at Indianapolis (IUPUI) in 1997. In 2000, he changed his
major to geology and graduated with his Bachelor of Arts in 2004. In August of 2004, he began
his graduate studies at the University of Florida in the department of geological sciences. In
August 2007, he received his Master of Science in geology.