drilling project hole 453 on the western side of the … · 31. petrography and mineral...

31. PETROGRAPHY AND MINERAL COMPOSITIONS OF GABBROS RECOVERED IN DEEP SEADRILLING PROJECT HOLE 453 ON THE WESTERN SIDE OF THE MARIANA TROUGH1

James H. Natland, Deep Sea Drilling Project, Scripps Institution of Oceanography, La Jolla, California

ABSTRACT

The compositions, mineralogies, and textures of gabbros recovered in polymict breccias in Hole 453 indicate thatthey are the cumulus assemblages of calc-alkalic crystal fractional on that occurred beneath the West Mariana Ridge.They are among a class of gabbros known only from other calc-alkalic associations (e.g., the Lesser Antilles and thePeninsular Ranges batholith of Southern California) and differ from gabbros of stratiform complexes, ophiolites, andthe ocean crust. Particularly abundant in the Hole 453 breccias are olivine-bearing gabbros with extremely calcic Plagio-clase (An94_97) but with fairly iron-rich olivines (Fo76_77). Other gabbros contain biotite and amphibole and occur inbreccias with fairly high-grade greenschist facies (amphibole-chlorite-stilpnomelane) metabasalts. One unusual gabbrohas experienced almost complete subsolidus recrystallization to an assemblage of aluminous magnesio-hornblende,anorthite, and green hercynitic spinel. This reaction, the extremely calcic Plagioclase, the occurrence of biotite and am-phibole, and the association with greenschist facies metamorphic rocks suggest that crystallization of the gabbros oc-curred at elevated P(H2O). Comparisons with other calc-alkalic gabbro suites suggest pressures in excess of 4 kbar(about 12 km depth). The gabbros were exposed by the early stages of opening of the Mariana Trough and imply thatconsiderable uplift may have attended rifting. They were also subjected to hydrothermal alteration after breccia forma-tion, resulting in formation of chlorite, epidote, actinolite, and prehnite. Temperatures of at least 200°C—and prob-ably 350°C—were reached, and most likely could not have been attained without extrusion or intrusion of magmasnearby, even though no such rocks were cored.

INTRODUCTION

One of the major surprises of Deep Sea Drilling Proj-ect Leg 60 was the recovery of gabbros in a complex poly-mict breccia in Hole 453 on the western side of the Mari-ana Trough (Fig. 1). Also in the breccias are metamor-phosed calc-alkalic basalts, andesites, and dacites (Woodet al., this volume). The breccias were subjected to in-tense hydrothermal alteration after their formation, withtemperatures locally exceeding 200 °C (Lawrence andNatland, this volume). This caused significant changes inthe chemistry and mineralogy of the rocks (Sharaskin,this volume).

The objective of this chapter is to explore the petro-logic significance of the gabbros with respect to mag-matic processes. For this purpose, I make use of thechemical data of Wood et al. (this volume) and some ofthe data of Sharaskin (this volume) and Bougault et al.(this volume). Mainly, however, the discussion will fo-cus on mineral analyses determined by electron micro-probe, in the context of the whole-rock chemical data.The metamorphic history of the gabbros and associatedrocks is complex, involving an early stage of greenschistfacies metamorphism, retrograde prehnite-pumpellyitefacies metamorphism before the breccias were formed,and hydrothermal metamorphism after they were formed.Apart from petrographic information, consideration ofthe effects of metamorphism on primary rock chem-istry, and some data on metamorphic minerals, the de-tails of metamorphism will not be discussed here. I will,however, describe subsolidus reactions that occurred ina few gabbros.

Initial Reports of the Deep Sea Driling Project, Volume 60.

The mineralogical study was initiated because thechemistry of the gabbros suggested that they may havecrystallized under conditions of elevated water pressureand because they were associated with calc-alkalic meta-volcanic rocks derived from the West Mariana Ridge.Unmetamorphosed calc-alkalic lavas were drilled at Site451 on the summit of the West Mariana Ridge about 44km to the west of Site 453 during Leg 59 (Kroenke et al.,1980; Mattey et al., 1980). The gabbros therefore seemedlikely to shed light on the processes of calc-alkalic dif-ferentiation on the West Mariana Ridge.

SETTINGFrom airgun profiles, the breccias appear to be flat-

lying and about 400 meters thick. They occur in a nar-row, steep-walled, north-trending basin near the westernside of the Mariana Trough (Fig. 1, inset). They areoverlain by 455.5 meters of turbiditic vitric tuffs, theoldest of which are early Pliocene (approx. 5.0 m.y. old;see Site 453 chapter, this volume; Bleil, this volume;Ellis, this volume).

LITHOLOGYA summary lithology of the Hole 453 breccias is

shown on Fig. 2. The diagram also summarizes theproportions of clasts of different composition in eachcore and the proportions of clasts to breccia matrix.Three lithologic sequences were cored: (1) Cores 49-58,85.5 meters of gabbro-metabasalt polymict breccia; (2)Cores 59-62, 28.5 meters of metavolcanic polymict brec-cia; and (3) Core 63, sheared, altered mafic gabbro ofwhich 1.5 meters were recovered. One additional corebrought up only material that had fallen down the hole.

A full description of these cores is presented in theSite 453 chapter (this volume), and detailed rock-by-

579

J. H. N ATL AND

WEST MARIANA RIDGE ^*4 MARIANA TROUGH

18°N

SPREADINGQ•t V Z O N E

efi Site \ 40, ,40

17° 30'

# Leg 60 Sites + Leg 58 SitesO Leg 59 Sites J f Leg 31 Sites Ridges (all types) Frenches with related subduction

Proposed spreading centers(dashed where no well-defined rift)

Figure 1. Location of DSDP sites drilled in the Philippine Sea. Site survey targets (SP numbers) are also indicated. Inset showsbathymetry of a portion of the Mariana Trough and West Mariana Ridge and locations of Sites 451, 453, and 454. Contoursspecified are in hundreds of meters (from Packham and Williams, this volume.)

580

GABBRO PETROGRAPHY AND MINERAL COMPOSITION

CoreNumber of Clasts 3 cm Longest Diameter (%)

20 40 60 80

Recovered Interval (%)

60 80

Recovery (%)

20 40 60 80

Breccia Matrix(Cemented Pieceswith Clasts < 3 c mLongest Diameter)

Recovery Too Low Cores 59 to 61for Meaningful Statistics

Figure 2. Stratigraphy, schematic lithology, and recovery per core of gabbro polymict breccias in Hole 453.

rock and thin section descriptions are on the igneousrock description forms at the end of that chapter. Addi-tional information can be found in Sharaskin (this vol-ume), Wood et al. (this volume) and Lawrence and Nat-land (this volume). For our purposes, it is sufficient tonote that in the upper breccia, gabbro clasts form 50-90% of all clasts larger than 3 cm diameter in any givencore, that the average gabbro clast is 7.6 cm long butthere are gabbro boulders up to 46 cm in longest coreddiameter, that metabasalt clasts are smaller on the aver-age (5.5 cm) than gabbros and do not exceed 10 cm inlongest cored diameter, and that there are two zones ofparticularly intense hydrothermal alteration in the hole.These are Cores 55 through 57 Section 2 in the upperbreccia and Cores 58 through 63 in the lower brecciaand sheared mafic gabbro. The 1.5 meters of shearedgabbro in Core 63 was evidently from a single largeboulder and was nearly continuously cored.

PETROGRAPHYThe terminology for gabbros used herein is that of

McCallum et al. (1980), except that for convenience Ialso allow mineralogic modifiers to be descriptive of themode—as I shall explain momentarily. In the upperpolymict breccia, there are three chemically and petro-graphically distinct types of gabbro and two types of

metabasalt. Most gabbros are olivine-bearing, and thereare four subtypes: troctolites (olivine anorthite gabbrowith rare clinopyroxene), olivine gabbro, olivine anor-thositic gabbro, and anorthositic olivine gabbro. Thelast three subtypes all have clinopyroxene, Plagioclase,and olivine but are distinguished on the basis of theabundance of these minerals, respectively. From handspecimens it is evident that gradations among thesetypes can occur within the same rock. This probably re-flects cumulus layering in the original plutonic mass. Ofthe subtypes, olivine anorthositic gabbros are most abun-dant and troctolites least abundant.

The minor gabbro types are gabbro — which hasabundant clinopyroxene and Plagioclase and essentialbiotite but no olivine—and fine-grained amphibole gab-bro which is plagioclase-rich but lacks both olivine andclinopyroxene. These two types are distinctly more sodicand less magnesian in composition than the olivine-bearing gabbros and are also finer grained. They areminor lithologies scattered throughout the upper brecciaand include the sheared mafic gabbro of Core 63.

In addition to all these primary gabbro types, thereare several distinctive metamorphosed and partiallymetamorphosed gabbros, plus rare recrystallized tona-lites and quartz diorites (Sharaskin, this volume). In theupper breccia, the matrix consists of individual angular

581

J. H. NATLAND

mineral grains broken from the coarser grained rocksset in an extremely fine-grained red or green cement.Virtually no breccia matrix was recovered in the lowerbreccia; what there was is soft, friable, and dark green.

Olivine-bearing Gabbros (Plate 1)

The olivine-bearing gabbro subtypes can best be de-scribed as a continuum of petrographic types with dif-ferent modes but nearly identical mineral compositions.The major whole-rock geochemical differences dependprimarily on the proportion of Plagioclase or clino-pyroxene and to a much lesser extent on olivine, whichrarely makes up more than 20% of the mode except inthe troctolites, which are rare and in which olivine maybe 50% of the mode. The olivine is typically alteredto clays, iron hydroxides, or serpentine and in somesamples is completely altered. Orthopyroxene, if oncepresent, is now completely altered and was not founddespite diligent searching in any samples, by either op-tical or electron microprobe examination.

The olivines are granular to euhedral in outline andwere evidently the first minerals to form in these rocks.The crystals are 3-5 mm in diameter. Many are sur-rounded by thin coronas of somewhat aluminous palegreen magnesiohornblende that appears to have formedby a subsolidus reaction between olivine and Plagioclasein the presence of water (see Mineralogy section).

Both clinopyroxene and Plagioclase form large (0.5-10.0 cm) mosaics of crystals. Plagioclases adjacent toeach other have generally anhedral outlines, but Plagio-clase adjacent to clinopyroxene has regular, often eu-hedral crystal outlines. The clinopyroxene is almost in-variably anhedral in outline and is commonly twinned.Many grains enclose smaller crystals of amphibole, simi-lar in composition but somewhat less aluminous thanthe amphibole rims on the olivines just described. Theinclusions are related to crystallographic features of theenclosing clinopyroxene crystals, such as cleavage.

In addition to the major silicate phases, these gab-bros contain scattered, small euhedral magnetite crys-tals, usually forming less than 1% of the mode. Themagnetite is generally partly replaced by a wormy, lessreflective (cation-deficient?) oxide mineral, but in onesample it is quite abundant (over 10%) and is associatedwith traces of green spinel.

The texture of these rocks does not seem entirelyamenable to application of conventional terminology forcumulus rocks (Wager et al., 1960). Since olivine, the firstmineral to form, is not sufficiently abundant to havetrapped intercumulus material, the rocks cannot be con-sidered orthocumulates. Locally, either Plagioclase orclinopyroxene forms interlocking mosaics within whichno other minerals occur; hence the rocks represent atype of adcumulus growth. For rocks in which two phaseshave adcumulus textures, the term heteradcumulate hasbeen proposed (Brown, 1956). Rocks that combine thefeatures of orthocumulates and adcumulates—that is,having abundant primocrysts surrounded by an adcumu-lus phase—are called mesocumulates. This term reason-ably describes the troctolites. In the other olivine-bearinggabbros, however, the olivine primocrysts are not abun-

dant and there are two adcumulus phases. Most of thegabbros thus seem to be intermediate between meso-cumulates and heteradcumulates, with heteradcumulustexture predominating.

Gabbros (Plate 2)

Gabbros consist predominantly of pale green clino-pyroxene and Plagioclase, either of which can predomi-nate, with minor biotite, ilmenite, and magnetite. Sec-ondary clays, iron hydroxides, and chlorite can be quiteabundant.

The clinopyroxene commonly shows a fine criss-crossed pattern of cleavage or parting (diallage). In onesample, the clinopyroxene has been completely alteredand only this gridwork, highlighted by secondary ironhydroxides, remains.

All samples of this type examined petrographicallyshow the effects of ductile deformation to a greater orlesser degree. The plagioclases in one sample are bent,and the clinopyroxenes in all of them show 100 twinning(e.g., Rayleigh, 1965). The sheared gabbro in Core 63has a granoblastic texture where it has not been cata-clastized, and its textural similarity to gabbros higher inthe hole suggests that they had a similar source and thatCore 63 does not represent a shear or fault zone in Hole453, as suggested in the site chapter.

The biotite occurs in clumps of crystals between feld-spars and pyroxenes and is most abundant in gabbroswith predominant Plagioclase. It surrounds large, an-hedral opaque minerals. These are mainly ilmenite withlesser magnetite that has traces of ilmenite exsolutionlamellae. The biotite and opaques were the last mineralsto crystallize and in one moderately deformed sampleare not themselves recrystallized, bent, or granulated.This suggests that the gabbros were not entirely crystal-lized before deformation.

Amphibole Gabbros (Plate 3)

These samples, of which two were examined petro-graphically, show the most characteristic cumulus tex-tures of all the gabbros. One of them consists of aboutequal proportions of Plagioclase and amphibole, withlesser ilmenite and magnetite. The plagioclases are eu-hedral primocrysts, many showing oscillatory zoning,and are partially overgrown with postcumulus Plagio-clase. There is a considerable range in compositions ofthe Plagioclase (An55_4o), reflecting the zoning and over-growths. The feldspars are surrounded by anhedral greenamphibole, which is commonly twinned. The magnetitesare scattered along the amphibole grain boundaries,whereas ilmenite most commonly occurs as roundedgrains enclosed by the amphiboles. Texturally, this rockis a mesocumulate.

The other amphibole gabbro is coarser grained, con-tains unzoned plagioclases interlocking with pale greenamphibole, and has no opaque minerals. This rock is aheteradcumulate.

Spinel-bearing Gabbro (Plate 4, Figs. 1 and 2)

Of the recrystallized gabbros, one specimen in partic-ular seems to have been subjected to particularly ex-

582


treme conditions of temperature and P(H2O). This is agabbro consisting of about equal parts of Plagioclaseand green amphibole, with abundant smaller crystals(which are nevertheless up to 1 mm diameter) of greenspinel occurring in clusters in the rock. The sample,which was from the zone of intense hydrothermal al-teration (Core 55), also experienced partial retrogrademetamorphism. It is veined with chlorite and lesser epi-dote, which replace mainly amphibole and some of thespinel. The compositions of the plagioclases are similarto those in the olivine-bearing gabbros, and the amphi-boles are similar in composition to the amphibole coro-nas around olivines in those rocks (see Mineralogy sec-tion). Other gabbros outside the hydrothermal zone haveoptically similar green spinel, although it is much rarerthan in this rock. For these reasons, the spinels in thisgabbro do not appear to have crystallized in response tothe hydrothermal conditions but to have formed undersubsolidus conditions similar to those that affected lesstransformed olivine-bearing gabbros.

Metavolcanic Rocks (Plate 3)

There are two types of metavolcanic rocks in the up-per breccia: originally aphyric and plagioclase-phyricmetabasalts. Before metamorphism, the Plagioclase phe-nocrysts were strongly zoned individual crystals andglomerocrysts up to 1 cm in diameter. They are nowmosaics of smaller crystals aligned along original twinplanes. The groundmasses of these rocks have beencompletely recrystallized to fairly coarse granoblasticmosaics of green amphibole, Plagioclase, brownishorange stilpnomelane, magnetite, chlorite, and minorsphene. The aphyric rocks are similar except they lackrecrystallized phenocrysts.

Although the metabasalts are dull gray and suffi-ciently coarse grained to be mistaken for diabase inhand specimens, there is no mistaking their meta-morphic textures in thin section. No relict pillow mar-gins or any other features of the original flows havebeen preserved, apart from the phenocrysts, althoughevidently the rocks were quite fine grained before meta-morphism.

In the lower breccia, extremely fine-grained green-stones, some speckled with calcite and pyrite, were re-covered. Some of these were breccias, now completelyindurated by metamorphism. The rocks have been re-constituted to albite-chlorite-epidote assemblages, thelatter mineral forming veins in some of them. Fromtheir high SiO2 (Wood et al., this volume), we knowthey were originally andesites and dacites. Chemically,they have been more greatly modified than any of themetabasalts of the upper breccia, since they are now al-most completely albitized and chloritized (see Chemistrysection).

Minor Lithologies

Minor lithologies include hydrothermally altered gab-bros in Cores 56 and 57, upper breccia metabasalts thatexperienced retrograde metamorphism to prehnite-pum-pellyite facies (Sharaskin, this volume), and biotite-bearing, partially deformed quartz diorite (Sharaskin,

this volume). The hydrothermally altered gabbros havebeen completely demagnetized (Bleil, this volume), withplagioclases largely replaced by a pale micaceous min-eral and mafic minerals replaced mostly by chlorite, ser-pentine, epidote, and actinolite. One gabbro in this zonehas been almost entirely recrystallized to a plagioclase-prehnite-epidote(?) assemblage and could representeither metamorphism prior to breccia formation or thehighest grade of hydrothermal alteration in the upperbreccia (see Plate 4, Figs. 3-7).

Upper Breccia Matrix (Plate 5)

Mineral identifications in samples of the matrix sur-rounding gabbro and metabasalt clasts in the upperbreccia were based on microscopic examination (in bothtransmitted and reflected light), X-ray diffraction, andscanning electron microscopy. Originally the matrixconsisted solely of small angular rock and mineral frag-ments derived from crushing and granulation of thelarger clasts and boulders. The matrix has been con-siderably recrystallized and cemented. The very topmostpiece of cemented breccia in Core 49 is the only one withany remaining matrix void space, which is lined withdrusy quartz (see fig. 1 of Lawrence and Natland, thisvolume). No other quartz occurs in the matrix. Belowthis, through two cores (corresponding to 19 m cored),the principal cementing mineral in the matrix is calcite,colored faint pink by iron hydoxides. To about Core 55,the breccia matrix is red and consists of chlorite, ironhydroxides, and loosely packed clays coating feldsparsand other minerals. The iron hydroxides are derivedfrom transformation of ferromagnesian silicates andmagnetite and are particularly abundant where the orig-inal components of the matrix were broken, mainlyfrom nearby metabasalts. Chips of Plagioclase, and thincoronas of plagioclases at the edges of the gabbro andmetabasalt clasts, have a creamy bleached appearanceresulting from transformation to K-feldspar (Lawrenceand Natland, this volume).

Similar bleached chips and coronas are quite promi-nent in the hydrothermally altered zone of Cores 55 and56, where the breccia matrix is green and consists almostentirely of compactly layered chlorite. Below the hydro-thermally altered zone, the matrix is again red, althoughthere are patches of green chloritic cement.

CHEMISTRY

The average of 13 analyses of olivine-bearing gabbrosand representative analyses of gabbro and amphibolegabbro are presented in Table 1 and are based on data inWood et al. (this volume), Sharaskin (this volume) andBougault et al. (this volume). Among the analyses usedto compile the composition of average olivine-bearinggabbro are several with significant enrichments in K2O,Rb, and Ba, redistributed from the breccia matrix byformation of chlorite during hydrothermal alteration.The covariances of Rb, Ba, and Na2O with K2O amongolivine-bearing gabbros are shown on Figure 3, A-C.Since the trace elements show strong positive correla-tions with K2O, their abundance in any given olivinegabbro also reflects mainly hydrothermal metasomat-

583

J. H. NATLAND

Table 1. Average and representative analyses of Hole 453 and com-parative gabbros.

SiCnTiO2A12O3Fe2O3TMnOMgOCaONa2OK 2OP2O5LOI

Totals

NiCrRbSrYZrBaNb

QCOrAbAnNe

WoDi En

FsEn

H y Fs

Ol I0

FaMt11Ap

1

44.45 ± 1.930.12 ±0.08

21.93 ±4.147.33 ± 1.620.10 ±0.03

10.22 ± 2.7313.88 ±2.790.83 ±0.64(<0.40)0.65 ± 0.67 (<0.05)0.00 ±0.00

—

99.51

40 ± 18lOO± 110

6 ± 7 ( < 1 )372 ± 88< l

8 ± 2241 ±286(<20)< l

0.000.00

(0.30)(3.38)57.860.004.573.061.159.143.449.283.861.360.230.00

2

44.430.24

23.237.140.107.70

16.980.740.030.00—

99.99

3

45.30.17

19.17.040.10

10.3717.170.430.360.00—

100.08

4

41.80.04

21.78.290.11

12.8112.820.510.290.00—

98.32

Trace Elements (ppm)

39122

5314

18

72< l

5883

413< l

768

< l

CIPW Normsb

0.000.000.182.74

59.941.91

10.126.413.050.000.008.984.711.320.460.00

0.000.002.120.74

49.091.57

15.0310.233.610.000.00

10.954.271.300.320.00

0.000.001.711.46

56.031.553.142.100.800.000.00

20.898.841.710.080.00

5

43.600.03

28.996.320.116.64

13.380.710.620.03—

100.45

2504

438< l

9230< l

0.002.903.666.03

66.140.000.000.000.003.881.968.934.981.170.060.07

6

48.40.62

11.8214.45a

0.2710.1111.681.620.180.052.07

100.97

21197

335162032

0.000.001.06

13.7024.44

0.0013.837.425.959.717.795.845.172.981.180.12

7

45.80.85

18.712.30a

0.165.91

12.311.940.170.051.64

99.88

13

4460

123134

0.000.001.00

16.4041.79

0.007.893.664.152.452.776.157.692.531.620.12

8

55.90.74

17.19.570.154.717.823.510.820.17—

100.55

4

8

2541

2314

5.640.004.84

29.6828.47

0.003.841.911.859.999.710.000,001.781.410.39

Notes:1. Average of 13 olivine-bearing gabbros (Wood et al., this volume).2. Average composition of Agrigan subvolcanic cumulus body (Stern, 1979).3.c Sample 453-53-3, 0-2 cm (Wood et al., this volume). Olivine-bearing gabbro.4.c Sample 453-55-3, 25-28 cm (Wood et al., this volume). Olivine-bearing gabbro.5.c Sample 453-55-4, 118-123 cm (Bougault et al., this volume). Contiguous to spinel-bearing

gabbro.6. Sample 453-63-1, 68-70 cm (Sharaskin, this volume). Gabbro.7. Sample 453-56-2, 123-128 cm (Sharaskin, this volume). Amphibole gabbro.8.c Sample 453-49-3, 52-58 cm (Wood et al., this volume). Amphibole gabbro.

a Total iron is reported as FeO.b Norms are calculated assuming Fe2 + /Fe2 + + Fe3 + = 0.86.c Samples are from same breccia clasts as samples examined by electron microprobe, this study.

ism. Na2O is present in many gabbros at higher levelsthan one might anticipate from its low abundance(0.33-0.77%) in plagioclases, the principal sodium-bearing phase (see Mineralogy section). Because it doesnot show so strong a positive correlation with K2O,however, its behavior during hydrothermal alterationeither was not the same, or it was variably enriched pre-viously by other (competing?) processes.

Using this information, estimated original (prehydro-thermal alteration) abundances of K2O, Na2O, Ba, andRb in "average olivine-bearing gabbro" are indicated inparentheses in column 1 of Table 1, and the norm iscomputed on that basis (with normative albite and or-thoclase in parentheses). Variations in MgO, CaO, andFeO resulting from formation of chlorite and epidoteduring hydrothermal alteration are more difficult toassess. The altered gabbros are probably somewhatricher in MgO and lower in CaO than originally, basedon comparisons to chloritized deep sea basalts (Hum-phris and Thompson, 1978), but in the gabbros these ef-fects are probably not so great as in basalts. In most ofthe gabbros, the ferromagnesian silicates are still largelyintact.

The analyses in Table 1 for gabbro and amphibolegabbro (columns 6 and 7) were selected because they ap-

2.0 -

Ko0

1.0 -

2.0 -

Ko0

1.0 -

2.0-

KoO

-

200 400 600 800Ba (ppm)

5 10 15Rb (ppm)

1000

20

1200

25

A

1400

B

C

1.0 2.0 3.0Na9O

Figure 3. A. Ba versus K2O. B. Rb versus K2O. C. Na2O versus K2Ofor 13 olivine-bearing gabbro samples reported in Wood et al. (thisvolume).

pear to have minimal enrichment in K2O, hence presum-ably were not greatly affected by hydrothermal altera-tion. There were too few analyses of each of these gab-bro types to attempt the systematic evaluation of K, Rb,and Ba enrichment that was done for the olivine-bearinggabbros.

The most important chemical attribute of the olivine-bearing gabbros, which are the most abundant, is theirextremely calcic compositions. Because gabbro cumulatesdo not represent former liquid compositions, their chem-istry can reflect local concentrations of particular miner-als—especially, in these samples, Plagioclase. Many ofthe analyses of olivine-bearing gabbros in Wood et al.(this volume) and Sharaskin (this volume) are of nearlyanorthositic rocks. The chemical similarity of all the oliv-ine-bearing gabbros, however, can be demonstrated bythe use of parameters of "cryptic variation" such asMg#(Mg/Mg + Fe) × 100) and normative An/(An + Ab)× 100. On Figure 4A, the consistent highly calcic compo-sitions of the gabbros are revealed by the high proportionof normative An they contain. They are far more calcicthan abyssal gabbros from either the Mid-Atlantic Ridge(Tiezzi and Scott, 1980) or the Mid-Indian Ocean Ridge(Engel and Fisher, 1975) when compared at comparableMg#. They are more calcic than gabbros of stratiformcomplexes such as the Stillwater (Bowes et al., 1973) andSkaergaard intrusions (Wager and Brown, 1967) and

584


80

§ 6 0

20

20 40 60 80

Normative An/(An + Ab) x 100 (mol %)

Calc-Alkalic Gabbros and Ejected Blocks• Site 453

t possible albitization duringhydrothermal and higher-gradegreenschist facies metamorphism

Φ Average Agrigan cumulus mass(Stern, 1979)

+ North Peak, Peninsular Ranges,San Diego County, California(Nishimori, 1976).

j ) Soufriere Volcano, St. Vincent(Lewis, 1973)

Ocean-Floor Gabbros and Other Plutonic Rocksx Mid-Atlantic Ridge, 26 N

(Tiezzi and Scott, 1980)0 Gabbroic to granitic rocks,

Indian Ocean Ridge(Engel and Fisher, 1975)

® Ferrogabbros, Mid-AtlanticRidge (Mryashiro et al., 1970;Bonatti et al., 1971)

Layered IntrusionsS Skaergaard averages

(Wager and Brown, 1967, table 6, p. 158)

§j)Stil lwater complex, fieldof analyses (Bowes et al., 1973)

Ophiolitic GabbrosM Masirah ophiolite, Oman,

data in Wood et al.(thisvolume)

Figure 4. A. Mg/(Mg + Fe) × 100 versus normative An/(An + Ab) × 100 for calc-alkalic gabbros compared with ocean crust, ophiolitic, andlayered gabbro intrusions. B. Mg/(Mg + Fe) × 100 versus normative An/(An + Ab) × 100 for West Mariana Ridge and Agrigan Island, Marianaarc, lavas, compared with fields for Site 453 and the Peninsular Ranges, southern California batholith (Nishimori, 1976), and the averagecumulus mass beneath Agrigan calculated by Stern (1979) to be responsible for the Agrigan fractionation trend (solid line). The effects ofalbitization on Hole 453 lower breccia metavolcanic samples is indicated by dashed arrows.

than gabbros and ultramafic rocks of the Masirah ophio-lite in Oman (Wood et al., this volume), which is con-sidered to have originated at an ocean ridge spreadingcenter (Coleman, 1981; Hopson et al., 1981). This rela-tionship is obvious despite probable Na enrichment ofseveral of the gabbros as a consequence of hydrothermalalteration and applies also to the gabbros and amphibolegabbros.

Instead, the gabbros resemble those from a variety ofcalc-alkalic associations, as plotted on Figure 4A. Theseinclude ejected gabbroic blocks from Soufriere volcano,St. Vincent, Lesser Antilles Island arc (Lewis, 1973);zoned ultramafic gabbroic plutons in the PeninsularRanges batholith of Southern California (Nishimori,1976; Walawender, 1976; Walawender and Smith, 1980);and gabbroic xenoliths in calc-alkalic lavas of Agriganvolcano in the Mariana Island arc (Stern, 1979). Thegabbros of Hole 453 also closely match the compositionof a cumulus mass Stern (1979) inferred to underlieAgrigan, which he calculated by least-squares computertechniques to be responsible for the fractionation trendobserved in Agrigan lavas (compare average composi-tions in Table 1, columns 1 and 2, and see Fig. 4B).

The metamorphosed basalts, andesites, and dacitesof both the upper and lower breccias are considerably

more sodic than their original igneous precursors (noteon Figure 4B that they plot far to the left—they havelow An/(An + Ab × 100)—of little-altered calc-alkalicbasalts and andesites of Site 451 on the West MarianaRidge [Mattey et al., 1980] and the Mariana arc [Stern,1979]). The most extreme Na enrichment is in the lowerbreccia where there is also extensive formation of chlo-rite. The combined effects of transformation of lowerbreccia lavas to a dominantly albite-chlorite mineralassemblage is shown on Figure 4B by the dashed arrowslinking fresh and altered lavas compared at approxi-mately the same SiO2 contents. The coarser-grainedmetabasalts of the upper breccia, which have experi-enced a higher grade of greenschist facies metamor-phism—shifted only slightly to the left of the unmeta-morphosed basalt composition on Figure 4B—retainmore of their primary chemical compositions. Neverthe-less, their original calc-alkalic lineage can more con-fidently be inferred from trace and minor element abun-dances (i.e., high Sr, Ba, and K) and low TiO2 thanfrom other major oxide abundances (Wood et al., thisvolume).

In summary, all of the rocks of Hole 453, includingthe gabbros, have calc-alkalic attributes. None of themare related in any way to intrusive or extrusive activity

585

J. H. N ATL AND

x Site 453 metavolcanicrocks

o Site 451 West MarianaRidge volcanic clasts(Mattey et al., 1980)

A Agrigan volcano lavas(Stern, 1979)

A Agrigan volcano lavas(Dixon and Batiza, 1979)

^ Average cumulus mass^ beneath Agrigan (Stern, 1979)

Vi possible albitizationand chloritizationduring hydrothermalalteration and green-schist facies meta-morphism. Arrowsare traced back tounaltered Agriganlavas with similarSi0o .

Solid line is least- squaresfit to Agrigan lavas usingdata of Stern (1979).‰ 0.9867

20 40 60 80

Normative An/(An + Ab) x 100 (mol %)

Figure 4. (Continued).

attending the opening of the Mariana Trough, in whichbasalts are much closer in composition to abyssal oce-anic tholeiites (Fryer et al., this volume; Wood et al. thisvolume).

MINERALOGY

To elucidate the conditions of formation of the gab-bros, electron microprobe analyses of mineral composi-tions were obtained at Scripps Institution of Oceanog-raphy using an automated Cameca electron microprobe.Instrumental corrections were made using standard pro-grams. Reference standards for particular oxides weredifferent, but well-analyzed olivine, pyroxene, and pla-gioclase standards were used for most oxides and wererun as unknowns to determine final corrections—gener-ally small—for some oxides. No corrections of this typewere applied to spinels, micas, and opaque minerals.

The samples analyzed were two olivine-bearing gab-bros, a recrystallized spinel gabbro, a gabbro, an am-phibole gabbro, and a metabasalt—all from the upperbreccia. They were selected from among the originalcollection of polished thin sections made on board Glo-mar Challenger. Petrographic descriptions of the spe-cific samples, which are identified on the tables of min-eral data, are included on the igneous rock descriptionforms at the end of the Site 453 chapter. The mineral-ogies of the samples are described by rock type, in thesame sequence as in the preceding section.

Olivine-bearing Gabbros

The high normative An/(An + Ab) × 100 of the oliv-ine-bearing gabbros is borne out by the extremely calciccompositions of their plagioclases, which are anorthitic(An94_97) (Table 2). These highly calcic plagioclases areall the more unusual considering the surprisingly highiron contents of olivines (Fo76_77) in the same samples(Table 3). Typical tholeiitic layered gabbroic intrusionsrarely have such calcic Plagioclase, yet what Plagioclasethey have is usually associated with more magnesianolivines. For example, the most calcic Plagioclase in theRhum layered complex (Anç>o) coexists with olivine ofcomposition F090 (Brown, 1956). In the Bushveld com-plex, intercumulus Plagioclase of composition An80 oc-curs in the basal peridotites in which olivine has compo-sitions between Fo93 and Fo85 (Wager and Brown, 1967;Buchanan, 1975). Olivine is absent throughout much ofthe main mass of the intrusion but recurs near the top,where it is quite iron-rich (Fo2i_5i) and coexists withplagioclases of compositions An45_53 (von Gruenewalt,1973; Buchanan, 1975). At Cuillin, Isle of Skye, Plagio-clase of composition An84_88 coexists with olivine Fo84_87

(Weedon, 1965). Similarly, in the Samail ophiolite, gab-bros have An80_85 coexisting with olivines of composi-tion Fo72_80 (Pallister and Hopson, 1981). In short, theHole 453 gabbros have mineralogical affinities withneither gabbros of stratiform complexes nor with an"oceanic" ophiolite complex.

586


Coexisting plagioclases and olivines in the variouscalc-alkalic gabbro associations shown on Figure 4A,however, resemble those of the Hole 453 olivine-bearinggabbros. Lewis (1973) and Arculus and Wills (1980) re-port olivines with compositions of Fo60_80 in the sameLesser Antilles ejected gabbroic blocks as plagioclasesof compositions An^c^. Nishimori (1976) reports oliv-ines with compositions Fo70_79 coexisting with plagio-clases of compositions An86_93 in peridotites and olivine-bearing gabbros of the Peninsular Ranges batholith.Stern (1979) found that olivine of composition Fo74_82coexists with Plagioclase of compositions An88_96 in hisxenoliths from Agrigan in the Mariana arc.

Pyroxenes in the olivine-bearing gabbros have thecompositions of diopside and salite (Table 4). They havelow TiO2 and tetrahedrally coordinated aluminum (A1IV),indicating that they did not crystallize from an alkalicmagma (LeBas, 1962) and are similar to pyroxenes inthe Soufrière plutonic ejecta (Lewis, 1973; Arculus andWills, 1980), the Peninsular Ranges gabbros (Nishi-mori, 1976), and the Agrigan xenoliths (Stern, 1979). Iwas unable to identify orthopyroxene either optically orby means of the microprobe in the two olivine-bearingsamples analyzed. Arculus and Wills (1980) found or-thopyroxene to be concentrated in samples from theLesser Antilles having plagioclases much less calcic thanin olivine-bearing gabbros (An83_47). Nishimori (1976)reports orthopyroxene as a reaction rim in olivine gab-bros from the Peninsular Ranges batholith, but found itto be much more abundant in gabbros and noritic gab-bros in the same zoned plutons. I found an extremelymagnesian clay mineral adjacent to an olivine primo-cryst in one of the olivine-bearing gabbros from Hole453 which may have been a similar orthopyroxene reac-tion rim, now entirely altered. Stern (1979) reported noorthopyroxene in his Agrigan xenoliths.

Amphiboles in the olivine-bearing gabbros of Hole453 occur as blebs in clinopyroxenes and as narrow co-ronas surrounding olivines between grains of olivineand Plagioclase. They have the compositions of ratheraluminous magnesio-hornblendes and, in one case,tschermakitic hornblende (Table 5), based on the classi-fication of the Subcommittee on the Amphibole Groupof the International Mineralogical Association Commis-sion on New Minerals and Mineral Names (Leake, 1978).The aluminous compositions compared to ideal horn-blende are evident on Figure 5. In this respect, theycompare to amphiboles in the Soufrière plutonic blocksand in the Peninsular Ranges gabbros, although on thewhole they have lower Na + K (Fig. 5). The K contentsof the amphiboles are especially low (Table 5), suggest-ing a much lower K in the parent liquid of the Hole 453gabbros than in either the Lesser Antilles or PeninsularRanges gabbros (e.g., Helz, 1973). This accords with thegenerally low K2O in analyzed Mariana arc and WestMariana Ridge lavas (Dixon and Batiza, 1979; Stern,1979; Mattey et al., 1980) compared to Lesser Antilleslavas (e.g., Brown et al., 1977) or to the tonalites andgranodiorites of the Peninsular Ranges batholith (Lar-sen, 1948; Walawender and Smith, 1980).

Opaque minerals in the olivine-bearing gabbros ofHole 453 are magnetites with fairly high abundances of

A12O3, MgO, TiO2, and Cr2O3 (Table 6). In these re-spects they are similar to opaque minerals in the LesserAntilles ejected blocks (Lewis, 1973) but differ fromopaque minerals from the Peninsular Ranges gabbrosreported by Nishimori (1976). In these rocks, the mag-netites analyzed have lamellae of exsolved ilmenite, henceare low in TiO2. However, the abundances of MgO,Cr2O3, and A12O3 are comparable to those in magnetitesof the Hole 453 and Lesser Antilles gabbros. Optical ex-amination with reflected light of the opaques in theHole 453 olivine-bearing gabbros revealed no exsolutionlamellae, though most of them had undergone partialoxidative alteration.

Spinel-bearing Gabbro

Plagioclases in the recrystallized spinel gabbro havecompositions nearly identical to those in the olivine-bearing gabbros (Table 2). On this basis, the gabbro wasalmost certainly originally an olivine-bearing gabbro aswell. However, all olivine and clinopyroxene has beenconverted to amphibole and green spinel.

The amphiboles analyzed in this sample have compo-sitions almost identical to those forming coronas aroundolivines in the olivine-bearing gabbros (Table 5; Fig. 5)—that is, they are somewhat aluminous magnesio-horn-blendes, with not quite enough tetrahedrally coordinatedAl to be termed tschermakitic. Rather than being a minorphase, however, in this sample amphibole forms nearlyhalf the rock.

Spinels (Table 7) in this sample have compositionsintermediate between spinel (MgAl2O4) and hercynite(Fe2+A12O4), with very low Cr2O3. Spinels of this com-position have not been reported from samples of theLesser Antilles ejected blocks or from the xenoliths ofAgrigan, but they do occur in the Peninsular Rangesgabbros (Nishimori, 1976, Walawender and Smith,1980). However, in those samples they occur only assmall grains associated with amphibole coronas in sym-plectites surrounding olivine. Nishimori (1976) attri-butes their formation to a subsolidus reaction betweenolivine and Plagioclase in the presence of water, viz.:

01 + Plag + H2O — Amph + (Cpx and/or Opx) + Spinel.

This reaction has been determined experimentally tooccur at 7 kbar pressure and at approximately 900 °C(Yoder, 1966), although on the basis of field relations,Frost (1974, 1975) estimated pressures of 2-4 kbar fora similar assemblage found in the metamorphosed peri-dotites at contact boundaries of the Mt. Stuart batholithin Washington.

Gabbros

Plagioclases in the gabbro sample examined by elec-tron microprobe are more sodic and have a wider rangeof compositions (An53_69) than in the olivine-bearinggabbros (Table 2). The particular sample has no un-altered clinopyroxene but has abundant intergrowths ofbiotite and opaque minerals. The biotite is quite iron-rich (Table 8) and surrounds large irregular crystals ofilmenite (Table 6) and lesser magnetite. Arculus andWills (1980) report that biotite in the Lesser Antilles

587

J. H.

Table

NATLAND

2. Electron microprobe analyses of plagioclases.a

Sample 453-54-5, 133-135 cm

Analysis 1

SiO 2

A1 2O 3

FeOMgOCaONa 2 OK 2 O

Totals

SiAlFe

MgCaNa

K

43.4035.78

0.370.00

18.440.560.02

98.57

8.1351,, „ . ,7 .908/ 1 6 - 0 4 3

0.0580.0003.7040.204

3.971

0.005

An94 7Ab5 2 θ r Q j

Spinel Gabbro

2

43.5835.950.400.00

18.810.550.01

99.30

8.1181., m ,7 .897/ 1 6 - 0 1 5

0.0620.0003.7550.199

4.018

0.002

An 9 4 . 9 Ab 5 .oOro. i

3

43.8335.37

0.460.04

18.080.730.04

98.58

8.216),, m s

7.812f16 0 2 8

0.0720.0113.6290.265

3.987

0.010

An93_QAbggOro 2

4

43.6334.930.350.00

19.310.330.00

98.54

7 .738/ 1 5 " 9 3 3

0.0550.0003.8880.120

4.063

0.000

\t197.0Ab3.0Orc

Sample 453-55-3, 25-29 cmOlivine-bearing Gabbro

5

43.9234.92

0.410.00

18.840.590.02

98.70

^ } , 5 . 9 5 O

0.0640.0003.7850:214

4.068

0.005

.0 An 9 4 .5Ab5.3θr 0 .2 A

6

43.7635.060.390.01

19.130.450.04

98.84

iZY5•9Al

0.0610.0033.8400.163

4.077

0.010

t i 9 5. 7 Ab 4 . 1 Or 0 .2

Sample 453-53-3, 0-2 cmOlivine-bear ng Gabbro

7

43.6534.370.300.00

18.450.580.00

97.35

SJ15.9760.0480.0003.7510.2130.000

4.012

A n 9 4 . 6 A b 5 . 4 θ r o . o

Sample 453-49-1,33-35, Piece 6,

Gabbro

8

53.2127.96

0.410.01

10.804.710.84

97.94

9.8481,, 0496.101/0.063>0.0032.142 > 4.0961.6900.198''

A n 5 3 . 2 A b 4 1 . 9 O r 4 . 9

Structural formulae on the basis of 32(O).

Table 3. Electron microprobe analyses of olivines in olivine-bearing gabbros.a

Analysis

S1O2A1 2O 3

FeOMgOCaONiOC r 2 O 3

MnO

Totals

SiAlMnNiCrFeMgC a <[Y]6

Sample 453-53-3,0-2

1

38.420.00

22.2439.450.020.040.000.38

100.55

0.9940.0000.0080.0010.0000.4811.5210.0012.012

F o 76.0

cm

2

38.350.00

21.9939.18

0.030.040.020.39

100.00

0.9970.0000.0090.001

<O.OOl0.4781.5180.0012.006

F°76.1

Sample 453-55-3,

3

38.010.03

21.7740.28

0.050.050.020.49

100.70

0.9820.0000.0110.001

<O.OOl0.4701.5510.0012.034

F076.7

25-29 cm

4

38.000.02

21.3540.36

0.040.010.000.44

100.22

0.9840.0010.010

<O.OOl0.0000.4621.5580.0012.032

F077.I

5

38.110.02

21.5340.44

0.020.010.040.40

100.57

0.9840.0010.009

<O.OOl0.0010.4651.5560.0012.032

Fo77.o

a Structural formulae computed on the basis of 4(O).

samples occurs only in orthocumulates and agglomera-tive clusters and has a wide range in composition but isassociated with relatively Fe-rich pyroxene and Plagio-clase of composition of approximately An^. They donot mention associated opaque minerals. Nishimori(1976) reports only minor abundances of biotite in Pen-insular Ranges norites, where it is associated with pla-gioclase of composition An76_79, salitic clinopyroxene,hypersthene, amphibole, ilmenite, and magnetite. Lar-son and Draison (1950) present analyses of biotites fromnorites of the Peninsular Ranges similar to those re-ported here. It is possible that the Hole 453 gabbrosoriginally contained hypersthene, which has since beenaltered, and hence were also noritic in composition.

The occurrence of late-formed aggregates of biotite,ilmenite, and magnetite, raises the question of whetherthey formed from trapped, hydrous, intergranular melts,

or from a subsolidus reaction. If from an intergranularmelt, it would have been exceptionally iron-rich and de-ficient in silica. There are no alkali feldspars or quartznear the biotites that suggest the presence of a silicicgranophyre. Probably, therefore, the biotite formed bymeans of a subsolidus reaction after deformation of thegabbros.

Amphibole Gabbro

The minerals analyzed in the amphibole gabbro arePlagioclase (An55_52; Table 2; An55_35 estimated opti-cally), hornblende (Table 5; Fig. 5), and ilmenite (Table6). Although plagioclases as sodic as those in the amphi-bole gabbro were analyzed in the gabbro, most plagio-clases in the gabbro are more calcic. Thus the amphibolegabbro is more evolved in terms of mineral composi-tions. This is also the case in the Peninsular Ranges gab-bros (Nishimori, 1976; Walawender and Smith, 1980).

In contrast to the Peninsular Ranges gabbros and theLesser Antilles ejected blocks, however, these are theonly gabbros in the Hole 453 suite in which amphibole isa major mineral. Both Nishimori (1976) and Arculusand Wills (1980) describe olivine-bearing gabbros inwhich amphibole forms up to 40% of the mode. Theirgabbros and norites also oftimes carry abundant amphi-bole. Probably, there is considerable sampling bias inthe gabbro breccias, so that only a few out of the spec-trum of possible gabbro types present beneath the WestMariana Ridge were incorporated into the breccias. Theabundance of amphibole gabbros may be dispropor-tionately low as a result.

Metabasalt

At this time, only a few minerals in a single meta-basalt have been analyzed. They are included here pri-marily to provide contrast to the gabbro suite and to in-dicate something, if only a little, about contrasting con-ditions of pre- and postbreccia metamorphic processes.

The minerals analyzed in the metabasalt sample arePlagioclase (Table 2) amphibole (Table 5), and chlorite(Table 8). The plagioclases include one clearly affectedby K-metasomatism. The unaltered Plagioclase has acomposition of about An50. Of the two amphiboles

588


Table 2. (Continued).

11

50.3130.150.230.00

12.843.890.15

97.57

Sample 453-49-1, 33-35,Piece 6, Gabbro

12

49.2229.810.260.00

13.713.310.16

96.47Q 77-3 Q ")QOIf I 15.996 I™6.623 6.6400.0360.000

0.0410.000

2.564 4.041 2.7761.4050.036

1.2130.039

15.938

4.069

1

52.2028.44

0.220.00

11.504.370.21

96.94

mi0.0340.0002.2981.5810.050

15.989

3.963

A n 6 4 . 0 A b 3 5 . 0 O r 1 . 0 A n 6 8 . 9 A b 3 0 . 1 O r 1 . 0 A n 5 8 . 5 A b 4 0 . 2 O r 1 . 3

Table 4

Analysis

SiO2

A12O3

FeOMgOCaONa 2OK 2OTiO 2

MnOC r 2 O 3

Totals

Si

A1 V I

TiCrFeMnMgCaNaK

Electron

1

50.184.336.57

14.4823.310.000.000.360.160.17

99.56

0.1290.06L0.0100.0050.2050.0050.8050.9310.0000.000J

microprobe analyses of clinopyroxenes.

Sample 453-53-3, 0-2 cπOlivine-bearing Gabbro

2.000

2.022

W o 48.0 E n 41.5 F s 10.6 w <

2

50.084.106.68

15.0222.620.150.000.350.130.00

99.13

o.mr 000

0.054,0.0100.0000.2090.0040.8370.9070.011

2.032

o.αxr>46.4En42.9Fs

l

10.7

3

51.982.805.36

16.3422.50

0.080.000.260.180.04

99.54

0.0810.041,0.0070.0010.1660.0060.8990.8900.0060.000J

Sample 453-49-3, 52-58 cm

V

53.4128.640.310.00

11.084.850.19

98.49

9.7826.1950.0480.0002.1781.7260.044

Amphibole Gabbro

15

54.5728.16

0.410.00

10.555.070.22

98.88

15 977 9 ' 9 3 5 15 9916.0560.0630.000

4.012 2.062 3.9691.7930.051

A n 5 5 . S ? A b 4 3 . 7 O r l . l A n 5 2 . 8 A b 4 5 . 9 O r l

2.000

2.105

W045.5En46.oFsg.5

4

49.734.685.80

14.7523.08

0.180.000.400.210.15

98.97

1 .8601 _ ç^r.

0.066,0.0110.0040.1810.0070.8220.9250.013

2.030

o.ooo'Wo48.oEri42.6Fs9.4 W

Sample 453-453-49-2, 68-69 cmMetabasalt

16

54.5628.210.230.00

10.125.600.08

98.67

lilt 1601°0.0350.0001.979 4.0161.9820.019

.3 A n 49.7 A b 49.8 O r 0.0!

Sample 453-55-3,

17

57.0824.450.640.076.692.247.64

98.81

10.604 - c QCQ

5.3560.0990.0191.332 4.0690.8071.811

An 33. 7Ab20.4θ r45.8

25-29 cmOlivine-bearing Gabbro

4

50.473.445.84

15.1123.820.100.000.370.210.00

99.38

OΛTS)2-000

0.043.0.0100.0000.1820.0070.8400.9560.007

2.036

o.oαr048.3En42.5Fs9.2 W

6

53.911.404.81

16.1325.00

0.030.010.210.260.02

101.78

0.049/2 °(X)

0.011,0.0070.0010.1460.0080.8700.9690.002

2.013

<o.oor

048.8Eri43.8Fs7.4 W

7

52.772.275.37

15.8624.30

0.040.010.250.190.00

101.06

0.0731 2 " 0 0 0

0.025,0.0070.0000.1640.0060.8630.9510.003

2.019

<o.oor048.lEn43.6Fs8.3

(Table 5), one is actinolitic and the other has the com-position of magnesiohornblende. The chlorites contrastwith two chlorites analyzed in the spinel gabbro (also inTable 8). Those in the metabasalt are somewhat less sili-ceous and aluminous but considerably more iron-richand less magnesian than those in the spinel gabbro. Thatis, they are ripidolites rather than penninites. The chlo-rites in the spinel gabbro clearly reflect extraction of Mgfrom pore fluids during hydrothermal alteration, where-as in the metabasalts, Fe and Mg are derived solely fromthe host rock. One other metamorphic mineral was ana-lyzed, an epidote group mineral occurring as an inclu-sion in a clinopyroxene crystal in one of the olivine gab-bros (Table 8). It, too, probably formed during the epi-sode of hydrothermal alteration, and has the composi-tion of clinozoisite.

Discussion

On the basis of the chemical and mineralogical com-parisons in the preceding sections, there can be little

doubt that the Hole 453 gabbros belong to a class ofgabbros that characteristically occurs in calc-alkalic is-land arcs. In the cases of the other gabbro suites, therehas been general recognition that they could play an im-portant role in calc-alkalic differentiation. Nishimori(1976), for example, pointed to the predominance inthe Peninsular Ranges gabbro cumulates of anorthite,which has such low SiO2 that its fractionation forcesresidual melts to high SiO2 compositions. Nishimori(1976), Walawender and Smith (1980), and Arculus andWills (1980) favor a parental liquid composition of ba-salt or basaltic andesite, not particularly rich in MgO,based on the relatively iron-rich compositions of oliv-ines. A similar conclusion could therefore be drawnfrom the Hole 453 olivine-bearing gabbros. Both Ar-culus and Wills (1980) and Stern (1979) present quanti-tative models for fractional crystallization; Stern (1979)comes closest to matching a calculated fractionationassemblage to an actual gabbro xenolith composition(Fig. 4B) that is very similar to Hole 453 olivine-bearing

589

J. H. NATLAND

Table 5. Electron microprobe analyses of opaque minerals.

Magnetites453-53-3,25-29 cm

OlivineGabbro

1 2

Ilmenites453-49-3,52-58 cm

AmphiboleGabbro

3

453-49-1,33-35 cmGabbro

4

Siθ2A1 2O 3

FeO*MgOCaOT1O2C r 2 O 3

MnO

0.002.78

84.910.970.033.331.190.35

0.002.27

84.180.370.073.981.510.33

0.020.00

47.260.030.14

47.630.012.95

Totals 93.56 91.20 98.04

0.000.01

44.780.030.03

47.630.043.17

95.69

Structural Formulae onthe Basis of 6(O)

SiTiAlCrFe 3 +

MgFe2 +MnCa

0.0011.8930.000 2.000

<O.OOl0.1050.0021.984 2.1070.1130.008

0.0001.9270.001 2.0000.0020.0700.0021.945 2.0720.1230.002

PENINSULAR ^RANGES ^ / *

TSCH

+

TREM

: ;'+

HBL

\PARG

EDEN

0.5

• olivine-bearing gabbros

x spinel-bearing gabbros

Al 1.0

o amphibole gabbros-f metabasalts

1.5

Figure 5. Tetrahedrally coordinated aluminum (A1IV) versus Na + Kfor amphiboles of Table 5, compared with fields for the amphi-boles of the Peninsular Ranges gabbros (Nishimori, 1976) and Les-ser Antilles gabbroic blocks (Arculus and Wills, 1980). HBL =hornblende, TREM = tremolite, EDEN = edenite, PARG = par-gasite, TSCH = tschermakitic amphibole.

gabbros. The problem for the Lesser Antilles is morecomplex than for the Mariana arc because there is agreater range of both gabbro and lava compositionswhich need to be related. Arculus and Wills (1980) em-phasize the role of amphibole and magnetite in enrich-ing SiO2 in the course of calc-alkalic fractionation, eventhough amphibole does not occur as a phenocryst inmost Lesser Antilles lavas. Lavas from the Mariana arc

(Stern, 1979; Dixon and Batiza, 1979) also lack am-phibole phenocrysts, but their rather limited composi-tional range compared with the Lesser Antilles may bereflected in a more restricted range of gabbro types,with olivine-bearing gabbros being most abundant.Whatever the details of fractionation models, I concurwith the sentiment of Arculus and Wills (1980) that "thevery existence of cumulus assemblages is strong supportfor the hypothesis of fractional crystallization in thearc, and this process must be a significant factor in thedevelopment of the chemical characteristics of the vari-ous suites of rocks" (pp. 790-791).

The conditions of crystallization can be deduced quan-titatively from consideration of their mineralogies (e.g.,Arculus and Wills, 1980). At this point, however, stud-ies of mineral compositions in the Hole 453 gabbros areincomplete, and it would be premature to do this. Com-parison to some of the other gabbro suites is instruc-tive, however. On the basis of field evidence (gradeof metamorphic aureoles, association with epizonalgranodiorite and tonalite plutons, etc.), Nishimori(1976) inferred that the Peninsular Ranges gabbros crys-tallized at pressures of 5 ± 2 kbar. Arculus and Wills(1980) inferred a wider range of pressures (4-10 kbar)for the Lesser Antilles blocks. Both studies concur inconcluding that the highly calcic plagioclases resultfrom elevated P(H2O). Arculus and Wills (1980) arguethat something other than simple suppression of the pla-gioclase liquidus (Yoder, 1969) is involved. Instead, theshapes of the liquidus and solidus curves in the plagio-clase-H2O system change (Johannes, 1978), causingmelts to be in equilibrium with more calcic Plagioclase.Both the Peninsular Ranges and Hole 453 gabbros areassociated with metamorphic rocks that restrict infer-ences about depth of crystallization. In the PeninsularRanges, gabbros have silimanite-bearing pyroxene horn-fels contact aureoles (Nishimori, 1976). No such highgrade metamorphic rocks occur among the Hole 453breccias, but the abundant greenschist-facies metaba-salts indicate that the gabbros were covered with at leastseveral kilometers of volcanic ejecta. The sparse recrys-tallized tonalites in the breccias suggest that an associa-tion of gabbros and small silicic plutons with composi-tions similar to plutons in the Peninsular Ranges batho-lith exists beneath the West Mariana Ridge, althoughtheir scale can hardly have been comparable. A particu-larly intriguing indication of elevated water pressures inboth the Peninsular Ranges and Hole 453 gabbros is theoccurrence of compositionally similar green spinels andaluminous amphiboles in several of the rocks. If theexperimental indications that the reaction that producedthere minerals requires 7 kbar water pressure (Yoder,1966) are correct, these rocks must have experiencedconsiderable uplift, up to 20 km, to become exposed inthe Mariana Trough. Even if the experiments overesti-mate the pressure for this reaction by a factor of two,uplift must still have occurred. It is important to notethat such a pressure is consistent with the other esti-mates of depth of gabbro crystallization inferred byNishimori (1976) for the Peninsular Ranges gabbros andwould be consistent with the range of pressures inferred

590


by Arculus and Wills (1980) for the Lesser Antillesblocks, if those minerals were to be discovered in them.Clearly, further study of the Hole 453 gabbro mineral-ogies is important, since it may bear considerably on themechanism of initial rifting of the Mariana Trough.

There is no question that the gabbros are now exposedbecause of this rifting. Consolidated magma chambersdeep within the West Mariana Ridge have been exposedby faulting. Probably the gabbros and metamorphicrocks were derived from surrounding ridges. The brec-cias do not have the typical high angle of repose of typi-cal talus slopes, however. Instead, they are very nearlyflat lying (see Site 453 chapter, this volume). Steep anglesof repose are not necessarily the rule for material derivedby mass wasting from steep submarine escarpments. Gab-broic material derived by simple mass wasting does occurin nearly flat-lying deposits in transform faults on theMid-Atlantic Ridge (P. J. Fox, personal communication).Alternatively, nearly flat-lying breccias of the type coredin Hole 453 could be derived from catastrophic rock fallscomparable to "stürtzstroms" (Hsü, 1975), which areknown to drop from cliffs and sweep across valley floorsto form nearly flat-lying deposits. They are variouslyinterpreted as having ridden a cushion of air, retaininginternal coherence and attaining high velocities (Shreve,1968), or to represent a type of particulate fluid flowthat does not require an air or other fluid lubricant toreduce the effective stress between the moving mass andthe ground (Hsü, 1975). Since there are two differentbreccias in Hole 453, containing different lithologiesand clasts of different size ranges, there must be twodiscrete sources, favoring the interpretation that cata-strophic rockfall occurred.

Following formation of the breccias, they were sub-jected to hydrothermal alteration to temperatures of atleast 200°C (Lawrence and Natland, this volume). For-mation of minerals such as chlorite, epidote, actinolite,and prehnite, however, occurs at temperatures consider-ably higher in a number of geothermal localities. Basedon comparisons to Iceland (Tomasson and Kristmanns-dottir, 1972) and the Salton Sea and Cerro Prietto geo-thermal wells (Elders et al., 1979; McDowell and Elders,1980), temperatures of alteration in Hole 453 may ac-tually have reached 350°C in some portions of the brec-cias. This is comparable to temperatures that attendmass flux of water and precipitation of sulfides at oceanspreading centers (e.g., RISE Project Group, 1980).Theoretical considerations (Lister, 1972, 1974) indi-cate that such high temperatures probably cannot beachieved without proximity of magma, hence it is prob-able that magmas were extruded or intruded near Hole453 during the initial stages of rifting of the MarianaTrough. The rocks produced by such magmatism werenot cored, however.

ACKNOWLEDGMENTS

Analytical costs for this study were defrayed by postcruise activ-ities funds allocated to Leg 60. I thank Don Hussong and YvesLancelot for approving the use of those funds. The assistance of RoyFujita was instrumental. Stan Kling helped me obtain the scanningelectron micrographs. I am also indebted to John Tarney and his col-leagues for providing the bulk of the whole-rock chemical analyses

used in this chapter, as reported in the chapter by Wood and others inthis volume. I am grateful to John Tarney, Jim Hawkins, and Sher-man Bloomer for many discussions relating to the problems of thegabbros and to Sherman Bloomer for going over the manuscript. Iowe a special debt of gratitude to Dick Nishimori, whose dissertationon the Peninsular Ranges gabbros, undertaken while we were bothstudents at Scripps Institution, provided so many parallels to thesamples described here.

REFERENCES

Arculus, R. J., and Wills, K. J. A., 1980. The petrology of plutonicblocks and inclusions from the Lesser Antilles Island arc. /.Petrol., 21:743-799.

Bonatti, E., Honnorez, J., and Ferrara, G., 1971. Peridotite-gabbro-basalt complex from the equatorial Mid-Atlantic Ridge. Phil.Trans. Roy. Soc. London, A, 268:385-402.

Bowes, D. R., Skinner, W. R., and Skinner, D. L., 1973. Petrochem-istry of the Stillwater Igneous Complex. Trans. Geol. Soc. S. Afr.,76:153-163.

Brown, G. M., 1956. The layered ultrabasic rocks of Rhum, InnerHebrides. Phil. Trans. Roy. Soc. London, A, 240:1-53.

Brown, G. M., Holland, J. G., Sigurdsson, H., et al., 1977. Geo-chemistry of the Lesser Antilles volcanic island arc. Geochim. Cos-mochim. Acta, 41:785-801.

Buchanan, D. L., 1975. The petrography of the Bushveld complex in-tersected by boreholes in the Bethal area. Trans. Geol. Soc. S.Afr., 78:335-348.

Coleman, R. G., 1981. Tectonic setting for ophiolite obduction inOman. J. Geophys. Res., 86:2497-2508.

Dixon, T. H., and Batiza, R., 1979. Petrology and chemistry of Re-cent lavas in the northern Marianas: Implications for the origin ofisland arc basalts. Contrib. Mineral. Petrol., 70:167-181.

Elders, W. A., Hoagland, J. R., McDowell, S. D., et al., 1979. Hy-drothermal mineral zones in the geothermal reservoir of CerroPrieto. In Elders, W. A. (Ed.), Geology and Geothermics of theSalton Trough, GSA Guidebook, U.C. Riverside Campus MuseumContrib. 5:36-43.

Engle, C. G., and Fisher, R. L., 1975. Granitic to ultramafic rockcomplexes of the Indian Ocean Ridge system, western IndianOcean. Geol. Soc. Am. Bull., 86:1553-1578.

Frost, R. B., 1974. Formation of Plagioclase peridotite by contactmetamorphism, Icicle Creek, central Cascades, Washington. Geol.Soc. Am. Abs. with Prog., 6:179-180. (Abstract)

, 1975. Contact metamorphism of serpentinite, chloriticblackwall, and rodingite at Paddy Go-easy Pass, Central Cas-cades, Washington. J. Petrol., 16:272-313.

Helz, R. T., 1973. Phase relations of basalts in their melting rangeat P H 2 θ = 5 kb as a function of oxygen fugacity. J. Petrol., 14:249-302.

Hopson, C. A., Coleman, R. G., Gregory, R. T., et al., 1981. Geo-logic section through the Samail ophiolite and associated rocksalong a Muscat-Ibra transect, southeastern Oman Mountains. /.Geophys. Res., 86:2527-2544.

Hsü, K. J., 1975. Catastrophic debris streams (stürtzstroms) generatedby rockfalls. Geol. Soc. Am. Bull., 86:129-140.

Humphris, S. E., and Thompson, G., 1978. Hydrothermal alterationof oceanic basalts by seawater. Geochim. Cosmochim. Acta, 42:127-136.

Johannes, W., 1978. Melting of Plagioclase in the systems Ab-An-H2O and Qz-Ab-An-H2O at PH2O = 5 kbars, an equilibriumproblem. Contrib. Mineral. Petrol., 66:295-303.

Kroenke, L., Scott, R. B., et al., 1980. Init. Repts. DSDP, 59: Wash-ington (U.S. Govt. Printing Office).

Larsen, E. S., 1948. Batholith and associated rocks of the Corona, El-sinore, and San Luis Rey quadrangles. Geol. Soc. Am. Mem.,29:1-182.

Larsen, E. S., and Draisin, W. M., 1950. Composition of the miner-als in the southern California batholith. Int. Geol. Congr., Rept.of 18th Sess., Great Britain, Part III: pp. 66-79.

Leake, B. J., 1978. Nomenclature of amphiboles. Can. Mineral., 16:501-520.

LeBas, M. J., 1962. The role of aluminum in igneous clinopyroxeneswith relation to their parentage. Am. J. Sci., 260:267-288.

Lewis, J. F., 1973. Petrology of the ejected plutonic blocks of theSoufrière volcano, St. Vincent, West Indies. /. Petrol., 14:81-112.

591

J. H. NATLAND

Table 6. Electron microprobe analyses of amphiboles.'

Analysis

SiO2

A12O3

FeOMgOCaONaOK 2OTiO 2

MnO

Totals

Si

<vA 1 V I

TiMgFeFeMnCaNg

K

Description*'Amphibole

Typec

Sample 453-55-4, 133-135 cmSpinel-bearing Gabbro

45.3912.479.98

15.4311.38

1.360.130.050.27

96.49

6-602 8 fl-y,1.398>8OO°0.7410.0063.3450.908

5.000

0.306}0.033)2.1131.774)

° 3 8 4l0.4O80.024)

IC

MH

2

46.2011.529.21

15.7911.22

1.460.170.040.17

95.80

6;™}8.ooo

0.7150.0043.4310.850

5.000

0.273}0.021 µ.O471.753)

°• 4 1 >.4450.032'

IC

MH

3

44.7912.459.26

15.7411.20

1.350.140.070.16

95.17

" $ 8 , 0 0 0

0.7430.0083.4490.8000.3390.0201.765

5.000

2.124

0.385l 0 4 ] 1

0.026/IC

MH

4

49.336.407.11

16.9116.410.680.010.520.14

97.56

7.0730.9270.1150.0563.6140.8530.0170.3052.2170.819)

0.002


5

50.147.336.70

19.0812.150.980.080.190.07

96.72

8.000 J;J£}8.000

0.364'0.0204.046

5.000 0.5700.227)0.0081.853.

>5.000

2.088

0.191 ° "°lθ.2850.015/

RCPX RCPX

MH MH

6

46.2911.488.21

16.7612.43

1.410.340.290.13

97.34

6.647 lR ~ ^1.35318-0 0 0

0.5910.0313.5870.791

5.000

0.195}0.015 b.1231.913)

n ^ [0.4550.062/

RR

MH


7

43.9712.169.89

17.1810.84

1.350.250.250.14

96.03

6.4461 o ΛΛJΛ

1.554/ 8 0 0 0

0.548)0.0281, ΛΛΛ3 . 7 5 4 5 - 0 0 0

O.67θl0.54310.015 [2.2611.703/

0.047/

RR

TH

8

48.7110.887.11

18.2412.64

1.300.170.120.15

99.32

s •*»0.587

0-013 5 000

3 7 8 9 p.ooo0.61l)0.218}0.017[2.1231.888)

° ";}0.3810.0301

RR

MH

Structural formulae on the basis of 23(O).RCPX = replaces clinopyroxene, RR = reaction rim on olivine, IC = individual crystal in groundmass, ICM = intercumulus mineral.MH = magnesio-hornblende, TH = tschermakitic hornblende, H = hornblende, AH = actinolitic hornblende.

Lister, C. R. B., 1972. On the thermal balance of a mid-ocean ridge.Geophys. J. Roy. Astr. Soc, 26:515-535.

, 1974. On the penetration of water into hot rock. Geophys.J. Roy. Astr. Soc, 39:465-509.

McCallum, I. S., Raedeke, L., and Mathez, E. A., 1980. Investiga-tions of the Stillwater Complex: Part I. Stratigraphy and structureof the banded zone. Am. J. Sci., 280-A (Jackson Volume):59-87.

McDowell, S. D., and Elders, W. A., 1980. Authigenic layer silicateminerals in Borehole Elmore 1, Salton Sea Geothermal Field, Cali-fornia, USA. Contrib. Mineral. Petrol., 80:293-310.

Mattey, D. P., Marsh, N. G., and Tarney, J., 1980. The geochemistry,mineralogy and petrology of basalts from the West Philippine andParece Vela Basins and from the Palau-Kyushu and West MarianaRidges, Deep Sea Drilling Project Leg 59. In Kroenke, L., Scott,R. B., et al., Init. Repts. DSDP, 59: Washington (U.S. Govt.Printing Office), 753-800.

Miyashiro, A., Shido, F., and Ewing, M., 1970. Crystallization anddifferentiation in abyssal tholeiites and gabbros from mid-oceanicridges. Earth Planet. Sci. Lett., 7:361-365.

Nishimori, R. K., 1976. The petrology and geochemistry of gabbrosfrom the Peninsular Ranges Batholith, California, and a model fortheir origin [Ph.D. dissert.]. University of California, San Diego.

Pallister, J. S., and Hopson, C. A., 1981. Samail ophiolite plutonicsuite: Field relations, phase variation, cryptic variation and layer-ing, and a model of a spreading ridge magma chamber. J. Geo-phys. Res., 86:2593-2644.

Rayleigh, C. B., 1965. Glide mechanism in experimentally deformedminerals. Science, 150:739-741.

RISE Project Group, 1980. East Pacific Rise: Hot springs and geo-physical experiments. Science, 207:1421-1433.

Shreve, R. L., 1968. Leakage and fluidization in air-layer lubricatedavalanches. Geol. Soc. Am. Bull., 79:653-658.

Stern, R. J., 1979. On the origin of andesite in the northern MarianaIsland arc: Implications from Agrigan. Contrib. Mineral. Petrol.,68:207-219.

Tiezzi, L. J., and Scott, R. B., 1980. Crystal fractionation in a cumu-late gabbro, Mid-Atlantic Ridge, 26°N. /. Geophys. Res., 85:5438-5454.

Tomasson, J., and Kristmannsdottir, H., 1972. High temperature al-teration minerals and thermal brines., Reykjanes, Iceland. Con-trib. Mineral. Petrol., 36:123-134.

von Gruenewaldt, G., 1973. The main and upper zones of the Bush-veld Complex in the Roossenekal area, eastern Transvaal. Trans.Geol. Soc. S. Afr., 76:207-227.

Wager, L. R., and Brown, G. M., 1967. Layered Igneous Rocks: SanFrancisco (Freeman).

Wager, L. R., Brown, G. M., and Wadsworth, W. J., 1960. Types ofigneous cumulates. J. Petrol., 1:73-85.

Walawender, M. J., 1976. Petrology and emplacement of the LosPinos pluton, southern California. Can. J. Earth Sci., 13:1288-1300.

Walawender, M. J., and Smith, T. E., 1980. Geochemical evolutionof the basic plutons of the Peninsular Ranges batholith, southernCalifornia. J. Geol., 88:233-242.

Weedon, D. S., 1965. The layered ultrabasic rocks of Sgurr Dubh, Isleof Skye. Scot. J. Geol., 1:42-68.

Yoder, H. S., 1966. Spilites and serpentinites. Ann. Rept. Dir. Geo-phys. Lab. Carnegie Inst. Washington Yearbook, 65:269-279.

, 1969. Calcalkalic andesites: experimental data bearing onthe origin of their assumed characteristics. Oregon Dept. Mineral.Ind. Bull., 65:77-89.

592


Table 6. (Continued).

9

47.2810.818.96

17.9711.62

1.220.260.220.16

98.50

6.703)fi

1.297*8

0.5100.0233.7980.669

5

0.394)0.019[21.766^


10

46.2911.488.21 .

16.7611.89

1.410.340.290.13

97.34

ΛΛΛ 6.6710 0 0 1.329

0.622

ooo ° 031

0 0 0 3.6000.747

8.000

5.000

0.243]179 0.015 )2.094

1.836)ò ISM n ^Q4i„ „ " 0.382 „ „ : : 0.4570.047 J 0.063)

RR

MH

RR

MH

11

46.8311.088.18

17.2612.401.730.310.280.14

98.22

6.668>8 (yys1.332!8-000

0.5280.0313.6630.778

5.000

0.196)0.016)2.1041.892 J0.4781Q 5 3 40.056/

RR

MH

Sample 453-49-3, 52-58 cmAmphibole Gabbro

12

46.536.62

17.4912.3610.701.220.221.220.60

96.95

fjjglβ.0000.1710.1382.7701.921

5.000

0.199)0.076)1.9991.724 JO.356U , Q o0.042/° 3 9 8

ICM

H

13

46.506.39

17.7912.3010.441.380.251.360.69

97.11

6 998\

0.1310.1542.7591.956

5.000

0.283]0.088 2.0551.684J

αS>• 4 5 1

ICM

H

Sample 453-49-2, 68-69 cmMetabasalt

14

47.885.39

20.2010.3111.560.630.620.500.46

97.25

7.250{8 fyy>O.75O'8-0 0 0

0.2120.0572.3272.404

5.000

0.154)0.059^2.0891.876 J0 . 1 8 5 | 0 2 4 2

IC

AH

15

43.808.38

21.998.13

10.861.180.611.090.59

96.62

6.795 > 8 m n

1.25O58-000

0.328)

° 1 2 7 5 000j 8 g op.ouu2.665J0.189)0.078)2.0731.806J

OΛ"IC

MH

Table 7. Electron microprobe analyses ofspinels.

Analysis

SiO 2

A12O3

FeOMgOC r 2 O 3

NiOMnO

Sample 453-554,

Spinel-bearing

1

0.17

62.5125.3412.44

0.050.04

0.28

2

0.1163.7424.34

11.050.060.000.18

133-135 cmGabbro

3

0.0063.94

24.8913.13

0.020.020.22

Totals

SiAlFeMgCrNiMn

100.83 99.52 102.22

Structural Formulae on theBasis of 32(O)

0.0415.624.493.930.010.010.05

0.0216.034.343.510.010.000.03

0.0015.704.334.07

<O.Ol<O.Ol

0.04

24.15 23.94 24.15

593

J. H. NATLAND

Table 8. Electron microprobe analyses of biotites, chlorites, and an epidote-group mineral.

BiotitesSample 453-49-1, 33-35 cm

Siθ2 34.43AI2O3 14.00FeO 24.94MgO 8.13CaO 0.03Na 2 O 0.14K2O 8.18TiO 2 3.17MnO 0.15

Totals 93.16

Gabbro

35.7013.9622.98

8.850.020.158.603.830.13

93.64

Structural FormulaeCalculated on the

Si 5.534A1 I V 2.466A1 V I 0.187Ti 0.383Fe 3.353Mn 0.020Mg 1.948Ca 0.0051

Basis of 22(O)

8.000 ^ • 6 ^ } 8 - 0 0 0

0.1950.453

5.890 3.0200.0172.073

5.757

0.003^Na 0.044)1.727 0.046^1.773K 1.678 1.724J

ChloritesSample 453-49-2, 68-69 cm

26.3416.7827.7712.470.070.310.720.350.32

85.12

Metabasalt

28.7015.1926.4410.260.050.233.021.460.19

85.51


5.8622.1382.2660.0595.1700.0604.1370.0170.1340.204

Basis of 28(O)

8.000 ^ ^ ] 8.000

2.3390.2443.392

n r 0.03612.046 3 392

0.0120.0990.855

11.881

ChloritesSample 453-55-4, 133-135 cm

Spinel-bearing Gabbro

31.59 30.1918.39 19.398.24 9.43

24.32 25.921.75 0.140.06 0.000.01 0.000.01 0.000.84 0.87

85.29 85.98


on the Basis of 28(O)

6 2 5 0 \ s nnn 5 9 4 9 l s mn1.750/ "• 0 0 0 2 . 0 5 1 1 8 ' 0 0 0

2.5400.0011.3640.1417.1720.3710.0230.003

2.455'0.0001.5540.145

11.616 7 6 1 4

0.0300.0000.000

11.798

ClinozoisiteSample 453-53-3, 25-29 cm

Olivine-bearing Gabbro

37.8630.06

5.471.77

21.000.030.030.020.23

97.07

Structural Formulaeon the Basis

of 13(O)

3.0982.900

0.0010.3740.0160.2161.8410.0030.001

594


1.0mm

0.1mm

Plate 1. Petrographic features of olivine-bearing gabbros.

Figure 1. Sample 453-55-3, 25-29 cm. Interlocking Plagioclase crys-tals in heteradcumulus gabbro, crossed nichols.

Figure 2. Sample 453-54-4, 133-135 cm. Relict interlocking plagio-clase crystals in formerly olivine-bearing (now spinel-bearing) gab-bro, crossed nichols.

0.1mm

Figure 3. Sample 453-55-3, 25-29 cm. Partially serpendnized olivinewith magnetites in serpentine veinlets, crossed nichols.

Figure 4. Sample 453-53-3, 0-2 cm. Amphibole (low relief) betweenaltered olivine (dark, at left) and fresh Plagioclase (clear, high re-lief, at right), plane light.

Figure 5. Sample 453-53-3, 0-2 cm. Amphibole blebs within clinopy-roxene, crossed nichols.

595

J. H. NATLAND

1.0mm

0.1mm

Plate 2. Petrographic features of gabbros.

Figure 1. Sample 453-49-1, 33-35 cm. Bent Plagioclase crystals,crossed nichols.

Figure 2. Same sample as Fig. 1. Secondary gridwork of iron hydrox-ides following original cleavage and parting of a clinopyroxenecrystal. Plane light.

1.0mm

Figure 3. Same sample as Fig. 1. Biotite-ilmenite intergrowth. Planelight.

Figure 4. Sample 453-63-1, 80-83 cm. Biotite and opaque minerals inpredominantly clinopyroxene groundmass.

Figure 5. Same sample as Fig. 4, showing diallage on clinopyroxenes.

596


0.1 mm

Plate 3. Amphibole gabbro and metabasalts.

Figure 1. Sample 453-49-3, 52-58 cm. A zoned, partly altered Plagio-clase primocryst in amphibole gabbro, crossed nichols.

Figure 2. Same sample as Fig. 1. Plagioclase primocrysts and inter-cumulus amphibole, plus minor opaque minerals, crossed nichols.

1.0mm 1.0mm

Figure 3. Sample 453-54-3, 55-58 cm. Metabasalt. Granoblastic pla-gioclase, chlorite, and amphibole. Plane light.

Figure 4. Sample 453-52-1, 99-102 cm. Metabasalt. Plagioclase por-phyroblast, a recrystallized phenocryst in chlorite-plagioclase-stip-nomelane-epidote groundmass, crossed nichols.

Figure 5. Same sample as Fig. 4. Center of large recrystallized Plagio-clase glomerocryst.

597

J. H. NATLAND

1.0mm

Plate 4. Spinel- and prehnite-bearing gabbros.

Figure 1. Sample 453-54-4, 133-135 cm. Green spinel partly disaggre-gated by and transformed to chlorite. Light, high relief grains arerelict plagioclases; darker gray, low relief mineral is amphibole,plane light.

Figure 2. Sample sample as Fig. 1. Dark green spinels are surroundedby pale green amphibole (here darker gray) and scattered relict pla-gioclase (lighter gray). Plane light.

0.1mm

Figure 3-7. Sample 453-56-2, 85-87 cm. All crossed nichols. The sam-ple consists of two principal zones, partially recrystallized and en-tirely recrystallized. The partially recrystallized zones retain pri-mary Plagioclase but have mafic minerals replaced by actinolite(veinlet in Fig. 3) and chlorite (dark areas near veinlet Fig. 3). Theentirely recrystallized zones are either fine-grained and grano-blastic intergrowths of epidote and Plagioclase (Fig. 4) or coarser-grained patches of a zeolite (laumontite?) ± K-feldspar + epidote(Figs. 5 and 6) with sprays of prehnite (Fig. 7). Veinlets of this min-eral assemblage cross relict plagioclases and chlorite-actinolitepatches and therefore represent a retrograde episode of meta-morphism.

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Plate 5. Scanning electron micrographs of cementing minerals in theupper breccia matrix. The first three micrographs are of the oxi-dized red matrix; the fourth, of the green matrix in the hydrother-mally altered zone of Cores 55-57.

Figure 1. Sample 453-51-1, 69-72 cm, Piece #10. Clay-cemented feld-spars. Energy-dispersive X-ray (EDAX) qualitative analysis of thecrystal at upper right, central clump, gave principal peaks for K,Al, and Si, indicating that the grain is almost pure K-feldspar. Thesmall scale bar on the micrograph = 0.1 mm.

Figure 2. Same sample as Fig. 1. Close-up of clays coating surface ofclump at upper right. ED AX qualitative analysis gave principalpeaks for K, Fe, Si, and Al, with lesser Mg, indicating K-Fe smec-

tites or mixed-layer compositions. Scale bar on right side of micro-graph = 0.1 mm.

Figure 3. Sample 453-49-2, 137-138 cm, Piece #19. Red iron-richclays coating Plagioclase and K-feldspar next to metabasalt frag-ment. EDAX qualitative analysis gave principal peaks for K, Fe,Si, and Al on the clay minerals. Small scale bar on micrograph =0.1 mm.

Figure 4. Sample 453-57-2, 64-67 cm, Piece #7. Compactly arrangedchlorite identified by XRD on separate chip from same sample.EDAX quantitative analysis gave principal peaks for Al, Si, Fe,and Mg (the last only in particular orientations) but not K. Thesmall scale bar below the micrograph = 0.1 mm.

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drilling project hole 453 on the western side of the … · 31. petrography and mineral...

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