Deep Sea Drilling Project Initial Reports Volume 60 · 2007-05-08 · A SYNTHESIS OF DRILLING RESULTS IN THE SOUTH PHILIPPINE SEA1 ... and interbedded boninites and arc tholeiites

53. PETROLOGIC EVOLUTION OF THE MARIANA ARC AND BACK-ARC BASIN SYSTEM—A SYNTHESIS OF DRILLING RESULTS IN THE SOUTH PHILIPPINE SEA1

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

John Tarney, Department of Geology, University of Leicester, LEI 7RH, United Kingdom

ABSTRACT

The results of Deep Sea Drilling Project Leg 60 pertaining to the petrologic evolution of the Mariana Trough andMariana arc are summarized in Part I. The rocks recovered at five principal sites are exceptionally diverse, includinggabbros and calc-alkalic andesites derived from the West Mariana Ridge; fresh and hydrothermally altered basalts inthe Mariana Trough; and interbedded boninites and arc tholeiites in the forearc region between the modern arc and thetrench. Part I outlines the stratigraphy, alteration, and petrology of these rock suites and summarizes hypotheses ontheir origin.

Part II integrates these results with those of previous DSDP legs in the region, and with island sampling, to present acomposite 40-m.y. history of the Mariana arc system. Arc volcanism began in the Eocene, probably as a result of achange to the west in the motion of the Pacific plate. A north-south-trending fracture zone was transformed into atrench-subduction complex, trapping a portion of the Pacific plate in the Philippine Sea. The crust of this basin, drilledin Hole 447 (Leg 59), has the composition of typical ocean floor basalt.

The earliest Eocene arc was built up dominantly of arc tholeiite and boninitic lavas, with lesser calc-alkalic lavas,based on the results of Leg 60 drilling at Sites 458 and 459 in the forearc region; Leg 59, Site 448, on the Palau-KyushuRidge; and exposures on the islands of Palau, Guam, and Saipan. Near Sites 458 and 459, the forearc crust is thin,formed entirely under water, and includes no known component of ocean crust. Nevertheless it has many of thefeatures of an ophiolite, produced in situ by earliest arc volcanism. In the Miocene, the Palau-Kyushu Ridge was splitfrom this ancestral arc by opening of the Parece Vela back-arc basin. A new arc formed on the east side of this basin,the West Mariana Ridge. Lavas recovered from this ridge at Sites 451 and 453, and dating from this time on Guam, havecalc-alkalic compositions. In the Pliocene, a second back-arc basin formed, the Mariana Trough, splitting off the WestMariana Ridge and causing volcamsm along it to cease. The present arc has been built again on the east side of the newback-arc basin. Based on the compositions of ash at Site 453 and island exposures, the new arc first erupted arctholeiites but has shifted to calc-alkalic compositions in the past 200,000 years. Both the Parece Vela Basin and MarianaTrough contain tholeiitic basalts with most of the features of mid-ocean-ridge tholeiites, but basalts in the Trough havesome of the trace element characteristics of island arc basalts.

Petrogenetic models for the evolution of the arc system therefore must provide for four distinctive magma types—boninites, arc tholeiites, calc-alkalic lavas, and back-arc-basin tholeiites—and explain their distribution in time andspace. The relative contributions to the production of these magma types of the subducted slab, the overlying mantlewedge composition, partial melting, and fractional crystallization need to be considered. One important conclusion isthat segregation of cumulus gabbros at elevated /?(H2O) such as those cored at Site 453 may be a major mechanism forproducing calc-alkalic andesites in the Mariana arc system. Immediately following back-arc rifting, when sources ofmagma are quite shallow and heat flow is high, the conditions necessary for such fractionation would be unlikely to oc-cur. Thus following each episode of back-arc basin rifting, arc volcanism should be tholeiitic. Only later will it becomecalc-alkalic. We also propose that progressive changes in the mantle beneath each arc resulting from voluminous extrac-tion of basalts may contribute to conditions necessary for new episodes of back-arc rifting.

GENERAL INTRODUCTION

Deep Sea Drilling Project Leg 60 completed the tran-sect of the Mariana arc-interarc basin system in thePhilippine Sea, begun on Leg 59. Between them the twocruises successfully recovered basement samples fromall the main physiographic features in the area (Fig. 1):the West Philippine Sea (Site 457), the Palau-KyushuRidge (Site 448), the Parece Vela Basin (Sites 449 and450), the West Mariana Ridge (Sites 451 and 453), theMariana Trough (Sites 454 and 456), the Mariana Ridge(Site 457), the Mariana forearc (Sites 458 and 459), andthe wall of the Mariana Trench (Sites 460 and 461). Sites453 through 461 were drilled during Leg 60. Addition-ally, Leg 58 drilled basement samples from the Shikoku

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

Basin (Sites 442, 443, and 444), which is a northwardcontinuation of the Parece Vela Basin and roughlyequivalent to it in age, and from the Daito Basin in thenorthern West Philippine Sea (Site 446). The petro-logical and geochemical information now available forthese samples, together with that assembled as a resultof field investigations and dredging operations, pro-vides a body of data more comprehensive than thatavailable for any other arc-marginal basin system.

We have divided this chapter into two parts. In PartI, we summarize the results of basement drilling duringLeg 60, drawing together principal results from all con-tributions to this volume dealing with basement sam-ples i These results concern not only primary petrogene-sis, but basement structure and alteration as well. InPart II, we review the primary geochemical data fromLegs 58, 59, and 60 in relation to possible models for thepetrological evolution of island arc-marginal basin sys-

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J. H. NATLAND, J. TARNEY

40°

120°

# Leg 60 Sites + Leg 58 SitesO Leg 59 Sites if Leg 31 Sites

130 140°

Ridges (all types) Trenches with related subductionjJProposed 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.

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terns and for the Mariana-Bonin arc system in particu-lar. A considerable body of geophysical and tectonicdata is already available for the Philippine Sea andforms a valuable background on which to construct pet-rological-geochemical models for the evolution of intra-oceanic island arcs. In addition, the sedimentary historyof the drill sites provides information about the timingand patterns of volcanism and interarc basin rifting,refining considerably the original interpretations ofKarig (1971a, 1971b, 1975). These will be the principalconcerns of this report.

PART I. SUMMARY OF PETROLOGICALINVESTIGATIONS BASED ON LEG 60 DRILLING

Leg 60 Basement Drilling Data

Site locations, water depth, depth of basement, andrecovery data for basement drilled during Leg 60 aregiven in Table 1. Altogether, basement drilling was verysuccessful. Over 600 meters total basement penetrationwas obtained at five principal sites, and a variety of rocktypes were recovered. The following sections summarizethe principal results pertaining to igneous basement foreach site. Where applicable, the various lithologic, chem-ical, and magnetic units are defined, and a summary ofthe principal petrological and geochemical features ofthe rocks, as well as the state of their alteration, is given.Relevant geophysical information from site and othersurveys and from shipboard programs (mainly heat flow)is considered. Additional interpretation of regional tec-tonic history in light of the drilling is in the synthesis byHussong and Uyeda (this volume).

Drilling in the Mariana Trough

Four sites (453-456) were drilled in the MarianaTrough (Fig. 1) in an effort to find a location where adeep penetration multiple reentry hole could be drilledand to establish the spreading history of this youngestMariana back-arc basin. Although no reentry site wasfound, igneous and metamorphic rocks were recoveredin four holes at three sites. Site 453 was located in asmall north-trending sediment pond on the western sideof the Trough. A complex gabbro-metabasalt polymictbreccia was cored for 146 meters beneath 450 meters ofturbiditic vitric tuffs ranging in age from late Plioceneto Recent. Pillow basalts underlying Pleistocene sedi-ments were cored nearer the center of the Trough in

PETROLOGIC EVOLUTION

Holes 454A, 456, and 456A. The latter two holes wereabout 200 meters apart. In each of these holes, base-ment penetration was low, and hole instabilities pre-cluded reentry. Detailed operational, lithologic, andpetrographic summaries of the rocks recovered in allthese holes can be found in the site chapters.

Polymict Breccias at Site 453

Three types of igneous and metamorphic rock se-quences were recovered in Hole 453. The upper two arebreccias containing coarse cobbles or boulders of ig-neous and metaigneous rocks (Fig. 2). The deepest is asheared and highly altered gabbro, recovered in twofragments totalling 150 cm in length. Gabbros andmetabasalts occur in the upper breccia (86 m thick) withrare minor lithologies, including both foliated and non-foliated quartz diorite (Sharaskin, this volume). Somegabbro clasts reach 40 cm in longest cored diameter, andwhere such large boulders are abundant, core recoveryis highest. Metabasalts are either aphyric or have largerecrystallized Plagioclase phenocrysts. There are no largemetabasalt boulders, although cores contain intervalswhere several adjacent metabasalt pieces evidently camefrom the same boulder.

The lower breccia contains aphyric greenstones, somequite pyritized, with the compositions of metamorphosedandesites and dacites (Wood et al., this volume). Therocks are small (3-7 cm), pale green, and are associatedwith a small amount of indurated pyroclastic breccia,also metamorphosed, and soft, dark green, friable brec-cia, perhaps originally a hyaloclastite. Some of the lithicfragments are veined with epidote.

The rocks have experienced a variety of low- andhigh-grade metamorphic events. The highest grade ofmetamorphism is represented by gabbros containing an-orthite, green aluminous amphibole, and green hercy-nitic spinel (see Natland, gabbros chapter, this volume).Some gabbros have been completely recrystallized toprehnite-clinozoisite-plagioclase assemblages. Othershave been partially recrystallized to chlorite and actino-lite, with almost complete transformation of mafic min-erals but only partial sericitization of Plagioclase. Stillother gabbros shows tectonized fabrics, including ex-tensive development of (100) twinning on clinopyrox-ene, and bent Plagioclase crystals. Greenschist faciesmetamorphic conditions evidently also affected the meta-basalts, which contain groundmass andesine-oligoclase,

Table 1. Basement drilling and recovery data, Leg 60.

Hole

453454A456456A458459B

Totals

171817171717

"40.17"00.78"54.68"54.71"51.84"51.75

Location

'N'N'N'N'N• N

143144145145146147

°4O.95'E°31.92'E° 10.77'E° 10.88'E°56.O6'E°18.O9'E

WaterDepth

(m)

469338193590.5359134494115

Depth to Basementfrom Seafloor

455.566.0

134.0118.5256.5558.5

BasementCored

(m)

149.50104.50a

35.0040.50

209.00133.00

671.50

BasementRecovered

(m)

33.3019.953.662.34

35.7924.12

119.16

BasementRecovered

(%)

22.319.110.55.8

17.118.1

17.7

Note: Basement in Hole 453 is gabbro-metabasalt polymict breccia. At all other sites it is pillow lavas. Igne-ous and metamorphic rocks were also recovered as talus in Holes 460 (17°40.14'N; 147°37.92'E), 460A(17°40.02'N; 147°35.16'E), 461 (17°46.05'N; 147°41.18'E), and 461A (17"46.01'N; 147°41.26'E), inwater depths between 6443 and 7034 m in the Mariana Trench.

a Includes sediments interbedded with basalts.

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CoreNumber of Clasts 3 cm Longest Diameter (%)

20 40 60

Recovered Interval (%)

40 60

Recovery (%)

20 40 60 80

Breccia Matrix(Cemented Pieceswith Clasts <3 cmLongest Diameter

Recovery Too Low Cores 59 to 61for Meaningful Statistics

137

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

amphibole, chlorite, epidote, clinozoisite, and sphene.They are not foliated but have granoblastic and por-phyroblastic textures. Some basalts of similar composi-tions experienced retrograde(?) prehnite-pumpellyite fa-des metamorphism (Sharaskin, this volume).

Despite the evidently high grades of metamorphismand the probability that the metabasalts were metamor-phosed prior to emplacement in the breccias, much ofthe metamorphism was hydrothermal and occurred af-ter the breccias were deposited. The alteration is concen-trated in two zones lower in the breccias (Fig. 2). Its ef-fect was to form chlorite-actinolite aggregates in somerocks and to riddle many of them with chlorite veins.The breccia matrix, which originally consisted of crushedangular fragments and chips from the larger clasts, hasbeen intensely altered, especially in the two zones justmentioned. Mafic minerals throughout most of the up-per breccia matrix (originally olivine, diopside, amphi-bole, and biotite) are now largely or entirely altered toserpentine, talc(?) chlorite, and abundant reddish ironhydroxides. In the zones of intense alteration, the ma-trix is green, not red, in color and is almost completelychloritized. One effect of chlorite formation has been toredistribute potassium, which does not enter the chloritecrystal lattice. It was originally fairly abundant in suchminerals as amphibole and biotite in the breccia matrix

and in the more evolved rock types (andesite, dacite) inthe lower breccias. Additional potassium probably wasprovided by seawater. The potassium now occurs in al-most pure potassium feldspar in the chloritized breccias,replacing calcic plagioclases around the edges of coarsegabbro cobbles and completely replacing many Plagio-clase chips in the matrix, giving them a bleached appear-ance. In the zone of intense alteration in the upper brec-cia (Cores 55 and 56, Fig. 2), this alteration has com-pletely demagnetized the gabbros (Bleil, this volume).Both the formation of chlorite and the exceeding ofCurie temperatures of the gabbros imply quite elevatedtemperatures of hydrothermal alteration. Oxygen iso-tope data obtained on drusy quartz in the topmost pieceof breccia recovered, and on potassium feldspars, indi-cate temperatures of formation of these minerals ofabout 100°C near the top of the breccias and 200 °C inthe highly chloritized zone (Lawrence and Natland, thisvolume). The association of chlorite with epidote sug-gests that temperatures locally exceeded 250°C, basedon analogy with marine geothermal systems (e.g., Hum-phris and Thompson, 1978; Mottl and Holland, 1978)and Iceland (Tomasson and Kristmannsdottir, 1972).The occurrence of minerals such as prehnite, actinolite,chlorite, and biotite suggests that temperatures mayeven have reached 35O°C, based on comparisons with

880


the Cerro Prieto and Salton Sea geothermal systems (El-ders et al., 1979; McDowell and Elders, 1980). Thesegreenschist facies assemblages did not necessarily format elevated pressures, as the Salton Sea and Cerro Prietooccurrences demonstrate. Those gabbros with aluminousamphibole and green spinel apparently indicate elevatedpressures (Natland, gabbros chapter, this volume), butthese minerals clearly formed before the breccias wereemplaced and hydrothermal alteration occurred.

Some alteration, but not at quite these temperatures,may be occurring today. A heat flow of 2.1 heat flowunits (HFU), or 88 mW/m2, was measured in Hole 453(Uyeda and Horai, this volume). This extrapolates totemperatures at the top of the breccias of about 50°.Moreover, sediment pore fluids show sharp reversals ofMg2+ and Ca2+ just above the breccias to near seawatercompositions. Gieskes and Johnson (this volume) inferfrom this that little-modified seawater must be flowingbeneath the sediments at the top of the still-permeablebreccias, driven by hydrothermal convection. These musthave flushed out pore fluids produced during extensivechloritization of the breccias, which should have hadcompositions considerably different from seawater (i.e.,lower Mg2+ and higher Ca2 + ).

These various types of metamorphism and alterationmodified the compositions of the volcanic rocks andgabbros in the breccias. The most important effects areremoval of CaO and probable addition of MgO duringhigher grades of metamorphism and addition of K, Rb,and Ba during chloritization of the breccia matrix andtransformation of plagioclases to K-feldspar. These ef-fects result in a higher proportion of normative albite,lower normative hypersthene (or more olivine), andhigher normative orthoclase, respectively, than in lit-tle altered basalts of similar composition recovered atDSDP Site 451, on the West Mariana Ridge, 44 kmaway (Natland, gabbros chapter, this volume). The hy-drothermally altered metaandesites and dacites of thelower breccia have been almost entirely albitized com-pared with fresh andesites and dacites from the Marianaarc system.

Despite these alteration effects, the original gabbrosand volcanic rocks now in the breccias are undeniablycalc-alkalic (Wood et al., this volume). Alteration hasnot removed the signature of fairly low iron enrichmentthat characterizes calc-alkalic suites, and, more impor-tant, much of the calc-alkalic trace element chemistryof the rocks is intact. Elevated abundances of Sr andlight rare-earth elements show them to be kindred tolittle-altered calc-alkalic rocks from the Mariana arc(Dixon and Batiza, 1979) and the West Mariana Ridge(Mattey et al., 1980; Wood et al., 1980).

The gabbros have a remarkable and surprisingly re-stricted range of compositions expressed in terms of"cryptic variation" parameters such as Mg/Mg + Feand normative An/Ab + An. Of 31 gabbro samplesanalysed from this site, all but 4 have normative An/(An + Ab) > 0.75, and in many this ratio exceeds 0.90(Fig. 3A). Most conform petrographically to olivinegabbro, but with widely varying modal proportions ofPlagioclase and clinopyroxene. A few gabbro samples

are olivine-free and consist of clinopyroxene, Plagio-clase, biotite, magnetite, and ilmenite or of amphiboleand Plagioclase, with minor opaques. Most of the sam-ples have heteradcumulus textures, but others have beenrecrystallized and are now granoblastic. The Plagioclasein olivine gabbros is extremely calcic, and anorthite(An94_97) in composition. Olivines are fairly iron-rich(Fo72_76), clinopyroxenes are diopsidic to salitic, andamphiboles are hornblendic except in the metamor-phosed gabbros (Natland, gabbros chapter, this vol-ume). The metagabbros with green hercynitic spinel arefew and are compositionally similar to the olivine gab-bros. Their spinel and aluminous amphiboles appear torepresent a subsolidus reaction between olivine and pla-gioclase in the presence of water.

The calcic Plagioclase and high normative anorthitecontents of the whole-rock analyses distinguish thesegabbros from gabbros in fracture zones, in ophiolites,and in stratiform complexes such as the Bushveld orSkaergaard (Fig. 3A). They most resemble gabbros oc-curring as ejected blocks from Soufrière volcano, St.Vincent, West Indies (Lewis, 1973; Arculus and Wills,1980); as zoned fragments of plutons tectonically em-placed at high levels among granodioritic and quartz-dioritic plutons of the Peninsular Range batholith inSouthern California (Nishimori, 1976); and as xenolithsin lavas from Agrigan volcano in the Mariana Arc(Stern, 1979), all calc-alkalic associations. The averagecomposition of the gabbros at Site 453 is nearly identicalto the composition of a cumulus gabbro mass calculatedby Stern to underlie Agrigan and to be responsible forits calc-alkalic trend (Fig. 3B).

To explain the compositions and mineralogy of thesimilar Peninsular Range gabbros, Nishimori (1976) ar-gued that /?(H2O) of up to 2 kbar caused the Hquidus toshift to more calcic Plagioclase compositions (followingarguments of Yoder and Tilley, 1962; and Yoder, 1969).Fractionation of mineral assemblages with a high pro-portion of such calcic Plagioclase (which has only about43% SiO2) would cause rapid SiO2 enrichment in resid-ual liquids before there could be much iron enrichment.Such fractionation involving removal of material equiv-alent to the Site 453 gabbros could explain the calc-alkalic variation of West Mariana Ridge lavas cored atSite 451, 44 km to the west (Mattey et al., 1980).

The gabbros are proof of the sundering of an islandarc during back-arc spreading. They also indicate that aportion of this Miocene arc must occur on the easternside of the Mariana Trough—which is evident from co-eval volcanic exposures on Guam (Stark, 1963). If thehigh grades of greenschist facies metamorphism—par-ticularly the formation of aluminous amphibole andgreen spinel—manifest in the gabbros and metabasaltsindicate elevated pressures, then considerable uplift (per-haps 10 km or more) attended rifting. This seems exces-sive in the context of simple rifting. A hydrothermalorigin for the metamorphic minerals eases this problemconsiderably. Still, some uplift must have occurred tohave caused the exposure of gabbros which crystallizedat elevated /?(H2O). There is at present about 4.5 km ofrelief on the West Mariana Ridge, more than half the to-

881


• Site 453 gabbros

+ North Peak, Peninsular Range gabbrosSan Diego County(Nishimori, 1976)

S Soufriere volcano, St. Vincentejected gabbroic blocks(Lewis, 1973)

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

M Masirah ophiolite, Omangabbros, data in Woodet al. (this volume)

× Gabbros, Mid-Atlantic Ridge26°N (Tiezzi and Scott,1980)

< possible albitizationduring hydrothermaland higher-grade green-schist facies meta-morphism

80

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

Figure 3. 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,Mariana arc, compared with fields for Site 453 and Peninsular Range, southern California batholith (Nishimori, 1976), and the average cumulusmass beneath Agrigan calculated by Stern (1979) to be responsible for the Agrigan fractionation trend (solid line). The effect of albitization onHole 453 lower breccia metavolcanic samples is indicated by dashed arrows.

total amount of combined exhumation and uplift neededto expose gabbros which crystallized at /?(H2O) —p(total) of about 2 kbar.

The nearest sources for the breccias are the walls ofthe sediment pond in which Site 453 was cored. Theclosest and most likely source is a small promontory orspur to the northwest which appears to link directly withthe West Mariana Ridge (Packham and Williams, thisvolume). Based primarily on the flatness of the brecciasurface in the sediment pond as seen in airgun profiles(Site 453 chapter, this volume), Natland (gabbros chap-ter, this volume) and Sharaskin (this volume) believethat the breccias most closely resemble deposits of cata-strophic rock falls or "stürtzstroms" (Hsü, 1975). Al-ternatively, "fans" of mass-wasted volcanic and plu-tonic rocks with slopes as little as 2° have been seen inlarge North Atlantic transform-fault zones (P. J. Fox,personal communication). The process of breccia em-placement at Site 453, then, need not have been cata-strophic. The breccia formation process ceased com-pletely sometime prior to 5.0-5.2 Ma, the time of for-mation of the oldest paleontologically dated turbidite atSite 453 (Ellis, this volume).

In summary, the occurrence of polymict breccias atSite 453 is the result of two fundamentally differentthermal-tectonic events. The rocks represent both theextrusive and intrusive products of calc-alkalic crystalfractionation at elevated water pressure. The exposureof these rocks and their chaotic dismemberment intobreccias was caused by back-arc basin rifting prior to4.5 Ma. The rifting apparently reached the very heartof the arc, reaching solidified magma chambers deepwithin it. The hydrothermal alteration of portions of thebreccias is clearly related to geothermal activity in theMariana Trough. Whether this was caused by magmaticinjection beneath Site 453 cannot be confirmed by pres-ent data, although the high temperatures of hydrother-mal alteration probably indicate a magmatic source ofheat nearby.

Interarc Basin Basalts at Sites 454 and 456

The lithology, distribution of chemical types, and lo-cation of changes in magnetic inclinations downhole areshown in Figure 4 for the three holes penetrating basaltsat Sites 454 and 456, located on either side of, yet closeto the center of spreading in, the Mariana Trough. The

882


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)

*- possible albitizationand chloritizationduring hydrothermalalteration and green-schist facies meta-morphism. Arrowsare traced back tounaltered Agriganlavas with similarSiO2

Gabbro fields, Skaergaardtrend as on Figure 3.Solid line is least- squaresfit to Agrigan lavas usingdata of Stern (1979).r2= 0.9867

20 40 60 80

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

Figure 3. (Continued).

Hole 456A Hole 456 Hole 454

Unit

Depth(m) £

3700

11

12

13

14

15

Lithology

955,"Im

AlterationZones

Aphyric pillow basaltswith vesicles

Plagióclose - phyricbasalts

Vitric siltymudstones

MagneticUnit

NModerately

alteredto fresh \

~ 200 m apart

Magnetic InclinationsN

Normal polarity

f RI Reverse polarity

Y//////\ ^ ° data; sediments

? No oriented samples

Figure 4. Basement lithologic, chemical, and magnetic stratigraphy, Sites 454 and 456, Mariana Trough.

883


oldest sediments at Site 454 are about 1.6 m.y. old andat Site 456 are 1.8 m.y. old (Kling, this volume; Ellis,this volume). The basalts at Site 454 are interbeddedwith vitric mud. The two holes at Site 456 are about 200meters apart and there are no interbedded sediments.However, one basalt piece in Hole 456 has a recrystal-lized sedimentary inclusion several centimeters in diam-eter. The basalts of both sites are mostly sparsely phyricor microphyric vesicular pillows and flows, moderatelyto highly olivine-rich. Only one basalt type in Hole456A was strongly Plagioclase and olivine phyric (Fig.4). The basalts are quite vesicular, although great depthof eruption ( - 3500 m) has undoubtedly kept the size ofvesicles quite small. The upper massive flow in the mid-dle of the basement section at Site 454 is more vesicular,hence less dense, toward the top, a feature readily re-vealed by the density logs (Fig. 5). At Site 456, manyvesicles are filled with secondary minerals, includingquartz, calcite, and pyrite, giving the rocks higher thanusual thermal conductivities (Horai, this volume). The

greater abundance of vesicles in these basalts than innormal mid-ocean-ridge basalts (MORB) is suggestiveof a higher primary volatile content (cf. Garcia et al.,1979; Muenow et al., 1980), though this is not necessar-ily typical of all interarc basin basalts.

Several chemical types occur at Sites 454 and 456,identified by the letter B (for basalt) and an alphabeticalor numerical subscript on Figure 4. The various types ofbasalt can be distinguished by trace element variation,such as that of Ni versus Zr (Fig. 6). Those chemicaltypes present in Hole 456A but not Hole 456 are givenalphabetical subscripts. The basalts have the major ele-ment characteristics of little-fractionated or moderatelyfractionated mid-ocean-ridge basalts (i.e., TiO2 1.0%;K2O =0.3%; Mg/Mg + Fe = 0.6-0.7). Despite this,only a few of them have the geochemical characteristicsof depleted normal (N-type) MORB. Wood et al. (thisvolume) note in particular "higher Sr, Ba, Th, and lightREE contents relative to Zr, Ti, Y, and the heavy REEthan N-type MORB." They also are depleted in Ta and

fragments ofpillow basalt

(no sediments)

fragments ofvesicular

basalt

continuouslyrecoveredpieces ofmedium-grained

vesicularbasalt(MVB)

Figure 5. A portion of the three density logs (overlapped) obtained in Hole 454A (Mariana Trough) during Leg 60, showing the inferred relationshipto interbedded massive basalts, pillow basalts, and sediments. Core number, recovery, and lithologic units are also shown.Triangles with errorbars indicate locations of samples and shipboard density measurements obtained on them by T. Francis. Note decrease in density upward in up-per massive basalt, caused by vesicles.

884


120

100

80

3 60

40

20

456, 456A

100 200Ni(ppm)

300 400 500

Figure 6. Ni versus Zr for basalts of Holes 454A, 456, and 456A, Mariana Trough. ( = Hole 454A; o = Holes 456 and 456A. Chemical types andsubtypes of Figure 4 are indicated. Data from Wood et al., this volume.) In Hole 454A, olivine-controlled Type B2 was succeeded by olivine-controlled B; subtypes with systematically lower Zr. Subtypes B1E arrd B1F, which stratigraphically represent the earliest B! subtypes, have in-termediate Zr, suggesting that they may be hybrids. Ni concentrations in any given sample are probably controlled by local accumulations ofolivine in pillows and flows. Holes 456 and 456A samples are more fractionated than those from Hole 454A; Ni abundance probably was af-fected by alteration.

Nb relative to Th and La. This geochemical signature ischaracteristic of island arc basalts (Saunders et al.,1980). Such features seem to be gradational throughoutthe basalts of Hole 454A and occur in one unit (the pla-gioclase-olivine strongly phyric basalt type BA) of Hole456A. Wood et al. (this volume) argue that this couldrepresent mixing of depleted N-type MORB with islandarc magmas at Site 454 and interbedding of the twomagma types at Site 456. It could also reflect modifica-tion of the mantle source by fluids carrying large-ionlithophile (LIL) elements which were derived from de-hydration of the subducted slab or from sediments car-ried down the subduction zone. Alternatively, the anom-alous chemistry might be produced by assimilation ofthe intimately associated glass-rich sediments, derivedfrom the Mariana arc, and which would have the re-quired "arc" component. However, the fact that simi-lar geochemical features are apparent in other marginalbasin basalts, at least during the early stages of back-arcspreading (Tarney et al., 1981), lends support to the sug-gestion that the cause is more fundamental and relatedto modification of the mantle source composition priorto back-arc spreading.

There is good trace element evidence that the variouschemical types are not related by crystal fractionationbefore or after possible contamination. As an example,substantially higher Zr levels occur in basalt type B2 atSite 454 than in basalt type BA, despite similar ranges ofNi (Fig. 6). Ni abundance in these samples is controlledprimarily by olivine fractionation, which produces theparallel trends on Figure 6. The trends are an indication

that a range of parental compositions are producedeither by variable degrees of melting or by differences inthe abundances of low-partition-coefficient (hygromag-matophile, or HYG) elements in the mantle source ofthese basalts. Such variations resemble those along cer-tain portions of the Mid-Atlantic Ridge (e.g., DSDPSites 332 and 395; Natland, 1978; Sites 409-413, Tarneyet al., 1979).

Fryer et al. (this volume) argue that basaltic glassesfrom Sites 454 and 456, as well as those from dredgehauls from the central axis of the Mariana Trough, re-semble other interarc basin basalts, particularly those ofthe Scotia Sea (e.g., Saunders and Tarney, 1979), butdiffer significantly in major element composition fromnormal MORB. The features they single out are (1) gen-erally lower TiO2, (2) generally higher Na2O, (3) lowerFeO* (total iron as FeO), and (4) higher A12O3 whenspecified at a given MgO content. They also note someof the trace element differences cited by Wood et al.(this volume), somewhat higher 87Sr/86Sr (Hart et al.,1972) as well as more H2O-rich volatile inclusions (Gar-cia et al., 1979) than in MORB.

Alteration at Site 454 was at low temperatures andoxidative, producing brown or reddish alteration rindson some basalts but generally having little effect on ma-jor oxide chemistry. At Site 456, however, alterationwas both extensive and diverse, resulting in markedchanges in the bulk compositions of the basalts. In bothHoles 456 and 456A, at the top of basement, alterationwas nonoxidative and occurred at high temperature, re-sulting in the large-scale development of chlorite in the

885


basalts and of pyrite, opal, quartz, and chalcopyrite(Natland and Hekinian, this volume). Recrystallizedtuffs just above basalt contain quartz and wairakite,which typically forms under high temperature hydro-thermal conditions in silica-rich materials and has beenproduced experimentally only above 250°C (Liou, 1971).

Below the chloritic, pyritized basalts, over a verticaldistance of only a few meters, alteration becomes dis-tinctly oxidative in both holes, with extensive develop-ment of reddish alteration rinds and of soft, friable,clay-rich basalts. At the bottom of each hole, even thisalteration disappears and fresh glass occurs.

The chemical effects of the alteration contrast in thenonoxidative and oxidative zones. In the nonoxidativezone, there is considerable variation in CaO and MgOamong basalts within distinct chemical types (defined onthe basis of immobile oxides such as TiO2 and traceelements such as Zr). Yet there is virtually no variationin very low abundances of K2O. In the zones of ox-idative alteration, however, K2O, Rb, and Ba vary con-siderably from sample to sample, evidently reflectingthe formation of abundant K-Fe clay minerals in thesamples.

The simple downhole sequence of nonoxidative andoxidative zones of alteration followed by fresh basaltssuggests that one episode of alteration was involved inwhich high-temperature nonoxidative hydrothermal flu-ids made their way along the permeable sediment/basaltcontact (Natland and Hekinian, this volume). The zoneof oxidative alteration was a zone of intense mixing withoxygenated pore fluids (essentially seawater). K, Rb, orBa leached from basement rocks and carried by thehigh-temperature fluids did not enter the chlorite crystallattices forming in the nonoxidative zone, but continuedflux and mixing of such fluids with seawater in the ox-idative zone favored formation of K- and Fe-rich clayminerals. Below the mixing zone, neither the tempera-ture of the fluids nor their diluted composition pro-duced significant alteration.

Fore-arc and Trench Drilling

The objective of drilling in the fore-arc region andthe trench was to determine if fragments of ocean crusthave been emplaced by faulting into the landward wallof the Mariana Trench. The principal result of this drill-ing was to establish quite the contrary. In two holes inthe fore-arc region, 458 and 459B, extrusive igneousbasement was reached beneath arc-produced Eocene-late Oligocene turbidites. In Hole 458, the lavas includean unusual rock type, boninite, known elsewhere onlyfrom island arc settings and certain ophiolites. These areinterbedded with island arc tholeiites, which also wererecovered in Hole 459B. Although there were drillingdifficulties in igneous and metamorphic talus in fourholes at two sites in the trench (460, 460A, 461, and461A), those rocks, too, have primarily the composi-tions of island arc tholeiites and in Hole 460 include aboninite. No ocean crust or marine pelagic sedimentswere cored in any of these holes.

The following sections provide an evaluation of lavastratigraphy in Holes 458 and 459B and a summary of

the principal hypotheses presented in this volume con-cerning the origin of the fore-arc lavas and structure.Operational summaries and detailed lithologic and pet-rographic descriptions are in the site chapters.

Igneous Rock Series in the Fore-arc and Trench Region

The boninitic rocks of Hole 458 are the first suchlavas ever recovered by drilling. Although boninites areunusual, they have chemical and petrographic charac-teristics that are of great importance to the under-standing of island arc magmatism. The boninites ofHole 458 are of particular importance because island arctholeiites occur in the same hole and because we canspecify that they erupted in late Eocene-early Oligocenetimes, early in the history of the Mariana arc. Outsidethis volume, the Hole 458 boninites have already beenrepeatedly cited with reference to the earliest stages ofarc magmatism (Cameron et al., 1979, 1980; Beccaluvaet al., 1980; Meijer, 1980) and because of their relevanceto ophiolites (Cameron et al., 1979, 1980). Here, wefocus on the stratigraphy of the boninites and arc tho-leiites of Hole 458 and 459B and consider possible geo-chemical relationships among the rock types. Otherchapters in this volume may be consulted for details ofpetrography and mineralogy (Meijer et al. and Natland,crystal morphologies chapter), geochemistry (Wood etal., Bougault et al., and Hickey and Frey), and experi-mental petrology (Kushiro).

The chemical and magnetic stratigraphy of volcanicbasement in Holes 458 and 459B is shown in Figure 7. Inboth holes, the rocks consist of alternating pillows andmore massive flows or possibly intrusives. In each hole,several distinctive chemical and magnetic units wereidentified. Chemical units are defined on the basis ofone or more essentially identical, or at least very similar,chemical analyses in stratigraphic order, based mainlyon the data of Wood et al. (this volume). In makingcomparisons, alteration, which affected mainly suchmobile elements as K, Rb, and Ba, was taken into ac-count by keying the stratigraphy to less mobile elementssuch as Ti, Zr, Y, and the like. Also discounted were theeffects of possible in situ magmatic differentiation,which has occurred in some of the more massive coolingunits. Using the data of Wood et al. (this volume),average analyses of the four principal chemical types inHole 458 (plus comparative individual and averageanalyses) and seven chemical subtypes in Hole 459B arelisted together with trace element averages and CIPWnorms in Table 2. There are two principal rock types,designated A and B, referring respectively to boniniticlavas and basalts. The distinct chemical types and sub-types are designated by numerical and alphabeticalsubscripts on Figure 7.

The boninitic rocks of Hole 458 have been variouslyreferred to in this volume as members of the boniniteseries (Meijer et al.; Sharaskin), as high-MgO bronziteandesites (Site 458 chapter), or more simply as boninitesand bronzite andesites (Natland, crystal morphologieschapter). This lack of consistent nomenclature is a con-sequence of petrographic dissimilarities between the Hole458 lavas and certain previously described boninites as


Hole 458

3700-

Hole 459B

3900 -

35=

14°

33°

21°

37°

-12°

-12°

—3°

-10°

4700

4800

Basalts(ChemicalTypes B)

High Mg BronziteAndesites

(Chemical Types A)

Pillows

Thin to massiveflows or sills

Inferred contact. _ _ . • N Possible contact

SedimentsMagnetic

InclinationN

Zones of IntenseFracturing

Muddyvitric tuff

Claystone iNomalpolarity

Reversepolarity

No orientedsamples

Vesicle trains

Figure 7. Basement lithologic, chemical, and magnetic stratigraphy, Holes 458 and 459B, Mariana fore-arc region, drilled during Leg 60.

well as of generally lower MgO, Ni, and Cr in Hole 458samples. For these reasons, Meijer (1980) defines nor-mative criteria to distinguish boninite series lavas frommore typical andesites and from basalts. He is criticalof early petrographic schemes. Natland (crystal mor-phologies chapter, this volume) noted that glassy bonin-ites, on which the original petrographic definitions arebased, represent only thin outer chilled rims of boninitepillows, and questioned the applicability of the designa-tion "high-MgO" to many of the Hole 458 boniniticlavas. He proposed extending the petrographic defini-tion of boninites to include pillow interiors and massiveflows in which Plagioclase can be an abundant mineraland, for the purposes of classification, distinguishedolivine boninites from simple boninites that lack olivine,

such as the Hole 458 lavas. The lower MgO Hole 458samples can thus be termed bronzite andesite and in-cluded in a boninite series (Bloomer et al., 1979). Forconvenience we shall use these conventions in the fol-lowing discussion.

The distinctive petrographic feature of all boninites iscrystallization of groundmass microlitic clinopyroxenebefore Plagioclase, even in fairly coarse-grained pil-low interiors. The Hole 458 lavas also contain complexbronzite-augite intergrowths in glassy and spheruliticsamples, plus separate phenocrysts of both minerals.Temperatures of crystallization of these minerals, calcu-lated using the formula of Ishii (1980), are in the rangeof 1250° ± 60°C, higher than two-pyroxene tholeiitic is-land arc basalts and calc-alkalic andesites from the

887

J. H. N ATL AND, J. TARNEY

Table 2. Averages of chemical types and subtypes of boninites and island arc tholeiites, DSDP Hole 458 and 459B.a

SiO2

TiO2

A12O3

Fe 2 θTMnOMgOCaONa 2OK2OP 2Os

Total

Mg

Mg + Fe

Ni

CrSrZrY

Nb

QOrAbAnNe

(WoDi { En

IFS

Hv 1 E n

H > \Fs

01 ( F o

1 FaMt11

Ap

Ave.Type A]Boninite

l(21) b

55.00.32

15.29.310.126.879.592.600.980.03

100.02

0.594

7621210533

32

4.315.78

21.9826.90

0.008.544.833.34

12.398.580.000.001.720.610.07

Sample458-29-2,

37 cmBoninite

2

52.00.36

14.410.230.129.718.702.630.880.01

99.01

0.653

7621610330

23

0.005.19

22.2424.87

0.007.594.652.51

15.19

8.19

3.101.841.900.690.02

Ave. ofBronzite

Andesite Flo\Cores 32-35

3(6)b

58.50.31

14.2

9.120.115.629.792.480.960.04

101.13

0.550

6118510332

85

10.865.67

20.9724.77

0.009.815.214.298.897.320.000.001.690.590.09

Hole 458

Sample458-40-2,

v 90 cmBoninite

4

52.80.35

14.910.240.098.977.982.471.030.01

98.86

0.634

9729010438

67

0.826.08

20.8826.51

0.005.423.241.90

19.1511.220.000.001.900.670.02

Type

BlBasalt

5b

53.21.13

15.59.660.063.736.125.701.520.20

96.82

0.433

1111

16910131

7

0.008.97

44.2412.212.157.023.193.770.000.004.305.611.792.150.46

Ave.Type A 2

Boninite6(6)b

53.90.52

14.310.540.136.218.332.650.660.03

97.27

0.558

Ave.Type B 2

Basalt7(6)b

52.01.10

14.013.630.116.696.553.310.750.09

98.23

0.493

Ave.Type B 1 A

Basalt8(3)b

54.30.69

14.69.950.145.78

10.822.870.480.07

99.70

0.535

Trace Elements (ppm)

7416711049154

2933

1396224

5

CIPW Normsc

6.253.90

22.4125.16

0.006.653.492.96

12.1010.260.000.001.950.990.07

0.914.43

27.9921.12

0.004.492.222.18

14.5214.220.000.002.532.100.21

5615311937154

4.492.83

24.2725.52

0.0011.546.095.108.457.070.000.001.841.310.16

Ave.Type Big

Basalt9(2)b

51.20.83

14.310.670.149.648.473.310.620.09

99.27

0.641

6062

1255329

6

0.003.66

27.9922.32

0.007.964.892.618.474.527.534.441.981.580.21

B2ABasalt

10

52.61.21

13.913.630.166.076.063.730.890.06

98.35

0.462

2413

1256827

6

0.525.25

31.5418.550.004.632.222.35

13.0713.860.000.002.532.300.14

Hole 459B

Ave.Type B2ß

BasaltU(3)b

56.70.97

13.111.890.155.097.253.180.980.08

99.39

0.459

2315

1085323

2

8.565.78

26.8918.570.007.033.293.669.55

10.620.000.002.201.850.19

Ave.Type B 2 c

Basalt12(2)b

57.70.74

13.310.820.125.527.633.151.290.08

100.35

0.502

2923

13049193

8.067.61

26.6318.330.007.923.963.789 909.450.000.002.011.410.19

Ave.Type B 2 D

Basalt13(2)b

55.70.91

14.311.010.115.408.253.710.910.10

100.40

0.493

2638

1355926

4

3.545.37

31.3719.670.008.594.264.159.299.060.000.002.041.730.23

Ave.Type B2E

Basalt14(4)b

55.11.13

13.911.910.115.197.404.021.020.12

99.90

0.463

1416

1346932

5

2.196.02

33.9916.860.007.953.774 079.269.990.000.002.202.150.28

a Compiled from data of Wood et al. (this volume).Number, of analyses in parentheses if more than one.

c Calculated assuming Fe^ + /(Fe^ + + Fe3 + ) = 0.86.

western Pacific (Natland, crystal morphologies chapter,this volume). Olivine boninites contain, in addition tothese minerals, phenocrysts of olivine and clinoensta-tite, and fairly abundant magnesiochromite. The highlymagnesian pyroxenes as well as the groundmass crystal-lization sequence allow these rocks to be clearly distin-guished from both basalts and andesites (in which pla-gioclase crystallizes before augite) whether or not theyare glassy (Natland, crystal morphologies chapter, thisvolume).

The two boninite sequences in Hole 458 are desig-nated Types A and A2 in Figure 7A and Table 2. Chem-ically, the rocks have the SiO2 range of typical maficandesites (52-59%) but high MgO (6-9%), Ni (100-200ppm), and Cr (200-400 ppm). They have very low TiO2(0.3-0.5%), Zr (25-50 ppm), and Y (3-15 ppm). Somechemical variation within Chemical Type Aj (columns2-4, Table 2) suggests the presence of subunits, but theirboundaries appear to have been obscured by alteration.Besides boninites, there are nine tholeiitic basalt typesand subtypes in the two holes (Bi and B2 in Hole 458;B1A, B1B, and B2A_E in Hole 459B). Apart from TypeB1A, which has some chemical and mineralogical fea-tures suggesting that it is transitional to boninite compo-sitions (cf. Natland, this volume, and discussion below),these various basalts have the geochemical features ofisland arc tholeiites (Wood et al., this volume; Meijer

et al., this volume; Sharaskin, this volume), includingTiO2 and Zr lower than in abyssal tholeiites (but higherthan in boninites; Table 2), and low Ni and Cr. Severalof them are distinctly iron-enriched. They also have gen-erally depleted rare earth element abundances, but mod-erate abundances of mobile elements such as K, Rb, Ba,and Th (Wood et al., this volume; Hickey and Frey, thisvolume; Bougault et al., this volume). The two majorchemical types, boninite and arc tholeiites, alternate inHole 458 (Fig. 7A).

Magnetic units shown in Figure 7 are defined on thebasis of one or more measurements of magnetic inclina-tion which prove to be nearly identical in stratigraphicorder, based on the data of Bleil (this volume). Sequen-tial measurements no different from those that might beexpected from the precision of the technique served todefine magnetically coherent intervals in the core. Theseare designated by the average inclinations shown on Fig-ure 7. In the case of both magnetic and chemical units,unit boundaries are assumed to be at the base or top of acore in which only one chemical and magnetic measure-ment apiece was made. This was the usual case for mag-netic measurements, since recovery of oriented speci-mens was poor for most cores. In some cores, however,unit boundaries are within them. In these cases, the ex-act boundaries may have been impossible to determine,owing to the difficulty of determining chemistry on ad-


jacent rock pieces (the sampling for shore-based X-rayfluorescence was done onboard ship) and the lack oforiented samples precisely across a magnetic transition.For these, unit boundaries are taken to be any obviouslithologic or petrographic transition in the core betweenthe measured samples.

Chemical units are presumed to represent individualeruptive events. In a number of instances, chemical andmagnetic boundaries coincide, implying that the lavasof those particular eruptive events "sampled" the earth'smagnetic field at specific points in its history of secularvariation and polarity reversals. Given the low latitudeof the sites, and a latitude lower still during the Eocene(Bleil, this volume), secular variation totaling about 30°could given rise to apparent reversals, although gener-ally low inclinations would prevail. This could explainthe numerous reversals in basalts of Hole 459B. Thereversal in Hole 458 between Cores 40 and 41, however,has an average difference in inclinations above andbelow (weighing each magnetic unit equally) of 37°, inexcess of the secular variation. It also occurs across atransition between major chemical types (A] and B inFig. 7). Therefore the eruptive pile could have built upacross the period of a reversal in the Earth's magneticfield. Assuming that the reversal itself took about 2000years to occur (Stacey, 1977), the total time over whichthe eruptions occurred might have been of the order of0.5-1 × lO^y.

Changes in inclination and apparent reversals couldalso have been produced by faulting. In both holes,there are magnetic unit boundaries within clearly homo-geneous chemical units. Some of these boundaries coin-cide with zones of intense fracturing in the rocks. Be-cause these are almost certainly fault zones, the mag-netic inclinations in part reflect rotation of fault blocks.However, the typical magnitude of such rotations is lessthan the average 37 Q difference between the intervals ofdifferent polarity in Hole 458 basement. Since that ma-jor transition coincides with a chemical boundary aswell, it seems likely that a reversal is indeed recorded inbasement in Hole 458.

This has important consequences for the magmatichistory of the fore-arc region. Both the major lava typesthat occur in Hole 458 (boninite and arc tholeiite) occurabove and below the major magnetic reversal. None ofthe chemical types is identical to any other (Table 2);hence faulting has not repeated any chemical unit. Con-sequently, it appears that both magma types eruptedalternately throughout a period sufficiently long for areversal in the Earth's magnetic field to occur. The twochemical types do not belong to any sort of evolutionarysequence (as for example alkalic basalt cappings ontholeiitic oceanic volcanic edifices) but rather representtwo distinct compositions supplied to the fore-arc re-gion at much the same time.

This is not just a feature of Hole 458. Chemical datalisted in Table 2 and pyroxene compositional data sum-marized in Natland (crystal morphologies chapter, thisvolume) show that Chemical Subtype B1A of Hole 459B,although petrographically a basalt, resembles boniniteType A2 of Hole 458. Both boninitic and basaltic lava

fragments were obtained in talus deposits drilled be-neath Eocene sediments (Ellis, this volume) deep in theMariana Trench at Sites 460 and 461 (Sharaskin, thisvolume; Meijer et al., this volume). Lavas from theseholes have arc tholeiite compositions (Wood et al., thisvolume). Boninites and "marianites" (which are bonin-ites with abundant clinoenstatite) were also obtained ina dredge haul in the Mariana Trench near Guam (Die-trich et al., 1978; Sharaskin et al., 1980). These wereassociated with an "ophiolitic" assemblage of gabbros,norites, graywackes, and ultramafic rocks. A "low-TiO2 basalt" from this dredge (compositionally a bo-ninite) has a K/Ar age of 10.8 m.y. (Beccaluva et al.,1980). Another nearby dredge haul recovered island arctholeiites (Sharaskin et al., 1980). Boninites and is-land arc tholeiites have been recovered in several otherdredge hauls in the trench and form several fore-arcbasement highs similar to that near Site 458 (S. Bloomerand J. W. Hawkins, personal communication). Meijer(1980) has pointed out that boninites series lavas occurin the Miocene Umatak Formation on the island ofGuam. These overlie Eocene basalts and andesites andare associated with Miocene calc-alkalic lavas. Stark(1963) describes the boninitic lavas as basalts but notesthat they are "hypersthene"-bearing, on the basis ofparallel extinction, with hypersthene occurring in glom-erophyric clots with augite. Plagioclase microlites areabundant in groundmass. These are the petrographicfeatures of the more holocrystalline Hole 458 boninitepillows, although the latter contain bronzite pheno-crysts (Natland, crystal morphologies chapter, this vol-ume; Bougault et al., this volume; Meijer et al., this vol-ume; Sharaskin, this volume). For comparison, chem-ical analyses of the Guam boninites are provided inTable 3.

In summary, we conclude that the boninite and arctholeiite lava series were supplied to the Mariana arcand fore-arc region essentially simultaneously at most

Table 3. Compositions of boninites from Guam compared with a rep-resentative Hole 458 boninite.

SiO2

A12O3Fe2O3FeOMgOCaONa2OK2OTiO2

P2O5MnOH2O +

H 2 O "Total

iNiCrZrYSrBa

50.8513.55

543} 8•27C

10.019.551.580 .16

0.340.030.141.914.13

99.92

300900

1030

20020

Umatak Formation (Guam)Boninite-type Lavasa

51.6314.10

9.939.472.210.520.430.050.151.421.84

99.95

200700

1030

30070

54.214.6

6C 5 A ] 8 . 9 C

8.07.92.41.40.360.0s0.12

1 2.91

100.46

55.013.83.8) 0 2c4.01 '9.47.92.30.880.340.070.11

1 2 61

100.20

Sample458-39-2,

94-96 cmb

52.515.7

9.43C

9.168.392.630.860.360.01———

99.20

9921735

< l10220

a Data from Tracy and Stark (1963).Data from Wood et al. (this volume).

e Total iron as Fe 2 θ3

889


locations in the Eocene, and again on the island ofGuam during the Miocene. There is no indication in thedrill sites or in any of the dredge hauls that genuineocean crust exists either beneath the fore-arc sedimentsor at very great depths in the Mariana Trench. Suchcrust would have to be at least Early Cretaceous, andprobably Jurassic, in age (Hussong and Fryer, this vol-ume) and be overlain by some thickness of pelagic sedi-ments. The fore-arc region is therefore not buttressed bymajor fault slices derived from the converging Pacificlithospheric plate.

Alteration in Fore-arc Basement

Many of the lavas in Holes 458 and 459B are exten-sively altered, especially along zones of intense fractur-ing of the rocks. A variety of clays and zeolites formed,and—mainly in Hole 459B—an unusual low-Al, high-Feform of palygorskite (Natland and Mahoney, this vol-ume) occurring exclusively in veins. The alteration wasin two stages in both holes: (1) an early oxidative, pos-sibly hydrothermal stage during which dioctahedralsmectites, celadonite, palygorskite, calcite, and iron hy-droxides formed and (2) a later, less oxidative stage inwhich di- and/or trioctahedral smectites and phillipsiteformed. The latter occurred especially along zones of in-tense fracture in the lavas, which probably formed as aresult of faulting of the fore-arc region. Profiler recordsindicate extensive normal faulting of the fore-arc, es-pecially near Site 459 (Mrozowski and Hayes, 1980),where the sense of faulting is predominantly toward thetrench. The faulting appears to be occurring even today,since Recent sediments have been affected by it (Hus-song and Fryer, this volume). Takigami and Ozima (thisvolume) have obtained an apparently reliable age for thesecond-stage alteration in Hole 458 by ^Ar/ Ar in-cremental heating techniques. This age is about 10 m.y.younger than the age of the oldest (early Oligocene)sediments at the site.

The palygorskite in Hole 459B is its first reported oc-currence in igneous basement. Elsewhere in the oceans,it has been reported only in pelagic or volcaniclasticsediments. Natland and Mahoney (this volume) specu-late that it formed under oxidative hydrothermal condi-tions and that the second stage of alteration was causedby fluids circulating in the basement rocks whose com-positions may have been different from those of fluidsnormally found in ocean crust. An important compo-nent of the fluids may have been derived from subduc-ting ocean crust and sediments now only a few kilom-eters beneath the fore-arc region. The fluid composi-tions may have been additionally modified by having totravel along faults through a thick assemblage of maficand ultramafic rocks similar to those drilled and dredgedin the landward wall of the Mariana Trench.

Geochemical Relationships among Lavas ofHoles 458 and 459B

Contributors to this volume generally agree that thereis a spectrum of forearc parental types produced bythe combined effects of different degrees (and possiblydepths) of partial melting, and source heterogeneities.

There is also a general consensus that the sources differfrom those of abyssal tholeiites, principally by additionof mobile constituents derived from the subductedocean crust and sediments. Finally, it is apparent thatthe oxidation state of the magmas influenced fractiona-tion, producing different degrees of iron, TiO2, andSiO2 enrichment.

These conclusions can be demonstrated using simplevariation diagrams, such as those of Figures 8 and 9 onwhich the averaged data of Table 2 are plotted. Figure8A, TiO2 versus total iron as Fe2O3 (Fe2OJ), shows thatthe boninites of Hole 458, as well as basalt Type B1A ofHole 459B, have systematically lower TiO2 and Fe2OJthan the remaining basalts. Moreover, with only one ex-ception, both boninites and basalts have systematicallylower TiO2 at a given Fe2OJ than abyssal tholeiites fromthe East Pacific Rise and Galapagos-Costa Rica Riftsshowing comparable variation in Fe2OJ. This is one ofthe criteria by which Jakes and Gill (1970) distinguishedisland arc tholeiites from mid-ocean-ridge basalts. Thesingle exception on Figure 8A is basalt Type Bj fromHole 458 (Table 3), which has unusually high Na2O, lowMgO, and low SiO2. Its composition is based on a singleanalysis of rocks probably greatly modified by altera-tion. Even so, its TiO2 content compares only with thelowest TiO2 abundances found in abyssal tholeiites.

Three groups of rocks can be distinguished on a plotof Fe2OJ versus SiO2 (Fig. 8B): a group of four bonin-ites (here including Hole 459B Subtype B1A and theaverage bronzite andesite of Column 3, Table 3); agroup of four Iow-Siθ2 tholeiites, two of which havemarked iron enrichment (Type B2 of Hole 458 and Sub-type B2A of Hole 459B); and a group of four high-SiO2

tholeiites (Subtypes B2B_E of Hole 459B) which have lessiron enrichment. They are also lower in TiO2 than thetwo most iron-enriched Iow-Siθ2 tholeiites.

The variation diagrams of Figure 9 use Zr on the ab-scissa as an index of differentiation unaffected either byalteration or by oxidation state of the magmas. On sev-eral of these diagrams, the three major groups fall intodifferent fields, and on some they overlap. For refer-ence, the least fractionated arc tholeiite (Subtype B1B ofHole 459) and the most iron-enriched from the samehole (Subtype B2A) are connected by a solid line. Themost fractionated basalt type, with the highest Zr and Y,and the lowest normative An/An + Ab, is Type Bj ofHole 458, the altered sample mentioned earlier. Zr, Y,and TiO2 should not have been much affected by thealteration. Consequently, this basalt has surprisinglylower TiO2 and Fe2OJ than the two iron-enriched ba-salts, even though it appears to be more fractionated onthe basis of trace elements. Either iron enrichmentamong its precursors was not as extreme, there was amarked late-stage fractionation of titanomagnetite (con-trasting dashed arrows on Figs. 8 and 9), the source ofits parent in the mantle was less depleted than the sourceof the other basalts, or its parent was a smaller partialmelt.

The high-SiO2 basalts overlap the iron-rich tholeiitesin Y, Zr, and TiO2 on Figure 9 variation diagrams buthave lower Mg/Mg -I- Fe and normative An/An + Ab.

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56 58SiO2(%)

Figure 8. A. Fe2O3(T) versus TiO2 for basalts and boninites of Holes458 and 459B, using average data of Table 2. Trend lines for EastPacific Rise and Galapagos-Costa Rica rifts fractionation trendsare based on glass compositions in Natland and Melson (1980) andunpublished data. (X = boninites, <S> = Subtype B 1 A of Hole459B, = low SiO2 tholeiites, a n d ® = high SiO2 tholeiites.) B.SiO2 versus Fe2O3(T) for average data of Table 2. (Same symbolsas A. Solid lines on A and B connect least and most fractionatedHole 459B low SiO2 tholeiites. Dashed arrows are possible alter-native paths of fractionation to Hole 458 Type B, basalt.)

They have 2.5% -6.5% higher SiO2. Within the fourhigh-SiO2 basalts, however, SiO2 decreases with increas-ing Zr, Y, TiO2, and Fe2OJ, contrary to what might beexpected from a normal fractionation sequence. Thenormative ratio An/An + Ab, however, decreases in away consistent with fractionation.

These are complex variations, unlike the simple trendtoward iron enrichment shown by eastern Pacific ferro-basalt suites (cf. Clague and Bunch, 1976; Natland andMelson, 1980). If all the tholeiitic basalts are related toa single parent, then some of the variation in TiO2,Fe2OJ, and SiO2 might be related to variable fractiona-tion of titanomagnetite, together with Plagioclase andclinopyroxene. This would simultaneously suppress en-richment in Fe2OJ and TiO2 and promote SiO2 enrich-ment (Osborne, 1959, 1962, 1979). The parent, how-ever, would have to be far more magnesian and less sili-ceous than the least-fractionated basalt, Subtype B1B ofTable 2. Moreover, the extent of titanomagnetite frac-

tionation would not be the same for any of the high-SiO2 tholeiites, since the least evolved of those in termsof Y and Zr (Subtype B2C) has the highest SiO2.

There is little reason, however, to suppose that acommon parent was involved. Instead, the Iow-Siθ2

tholeiites have systematically higher Zr at a given Nicontents (Fig. 9H) than the high-SiO2 tholeiites; henceone might suppose that the parent to the latter repre-sents a greater degree of melting of a homogeneoussource than the parent of the Iow-Siθ2 tholeiites. This isbased on an argument by Bougault et al., (1978) that Niis insensitive, whereas elements such as Zr are quite sen-sitive, to variations in the degree of melting in an oliv-ine-rich peridotitic source. In terms of most of the ox-ides and trace elements considered in Figures 8 and 9,we can even say that such a parent would have ap-proached boninite compositions and that the high-SiO2

tholeiites would be iron-enriched with respect to such aparent.

We therefore propose that the mantle source of theMariana fore-arc region produced a spectrum of paren-tal compositions ranging from olivine boninite to oliv-ine tholeiite and that different oxidation states in thetholeiite magmas produced variable SiO2 enrichmentand TiO2 and iron depletions. There is some petrographicevidence for this. Titanomagnetite is particularly abun-dant in Hole 459 tholeiites (indeed, it is so abundant thatit produces intensities of magnetization 3-5 times higherthan in even more iron-enriched abyssal tholeiites; Bleil,this volume). Some of the basalts contain segregationvesicles in which titanomagnetite is the only well-crystal-lized phase, indicating high oxygen fugacities in thevesicles (Natland, crystal morphologies chapter, thisvolume). Exactly at what stages titanomagnetite enteredfractionation sequences is not clear. This may havedepended on concentrations of volatiles in the mantlesources. What is clear is that its impact was quite vari-able, possibly reflecting conditions in magma chambersand conduits that held these magmas. Some of thechemical contrasts among the basalt types could alsohave been caused by mixing variably fractionated andoxidized magma batches. Whatever the case, it nowseems that in consideration of a suite of rocks, variableiron, TiO2, and SiO2 enrichments may characterizemany island arc tholeiites and be useful as discriminantsbetween these and ocean floor basalts. A given arctholeiite may not be particularly iron-enriched, or in-deed much fractionated, but it still can be distinguishedfrom abyssal tholeiites by having lower TiO2 at a giventotal iron abundance (Fig. 8A).

Both the basalts and the boninites of Holes 458 and459B have fractionated compositions. No compositionspotentially representative of parental compositions wererecovered. This has been explicitly determined experi-mentally for the Hole 458 boninites by Kushiro (this vol-ume), who inferred that they did not melt in equilibriumwith a hydrous Plagioclase lherzolite mantle but insteadseem to be derived from an olivine boninite parentwhich did. This is also indicated by the absence of bothclinoenstatite and olivine in Hole 458 boninites and theirsomewhat low MgO contents.

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60Zr (ppm!

Figure 9. Zr variation diagrams for average chemical types and subtypes and subtypes of Holes 458 and 459B, from Table 2. (Symbols as in Figure8A). A. SiO2-Zr; B. Mg/(Mg + Fe)-Zr; C. TiO2-Zr; D. Fe2O3(T)-Zr; E. Normative An/(An + Ab)-Zr; F. Y-Zr with fractionation and partialmelting trends of MORB suites from Tarney et al. (1979); G. Cr-Zr; H. Ni-Zr.

Contrasts to MORB and the Compositions ofFore-arc Mantle Sources

Data contributed to this volume and to DSDP Vol-ume 59 (Kroenke, Scott, et al., 1980) add to the growingbody of information on the distinctive geochemistry ofisland arc lavas. As summarized from Wood et al. (thisvolume), Bougault et al. (this volume), and Hickey andFrey (this volume), the island arc tholeiites of Holes 458and 459B differ from MORB in the following ways: (1)They have lower Ti and Zr and higher Ti/Zr at a givenMgO content despite equivalent or greater iron enrich-ment; (2) Overall, they have lower rare-earth elementabundances despite being generally more fractionated.However, they have similar or even somewhat greaterlight rare-earth element depletion; (3) Although theyhave lower Zr, Ti, and Hf than MORB, the abundancesof these elements are not as low relative to the depletedrare-earth element abundances as in MORB; (4) Ta andNb are even more depleted than rare-earth elementswhen compared with MORB; (5) Highly incompatibleelements such as Cs, K, Rb, Ba, Pb, U, and Th are en-riched in the arc tholeiites compared with MORB andtheir own depleted rare-earth element abundances; (6)The arc tholeiites have higher 87Sr/86Sr (0.7036-0.7038)than MORB (Armstrong and Nixon, 1980), althoughsome of this may have resulted from alteration.

Despite having erupted apparently contemporane-ously with the arc tholeiites, the Hole 458 boninites can-not have been derived from a similar mantle source,

although they, too, differ significantly from MORB.Their very low abundances of Ti, Zr, rare-earth ele-ments, and other highly incompatible elements and theirhigh MgO, Ni, and Cr indicate that their presumedolivine boninite parent was a high partial melt of themantle. Wood et al. (this volume), Meijer et al. (thisvolume), and Kushiro (this volume) argue from variousstandpoints that melting occurred under hydrous condi-tions and that the source was refractory (had experi-enced a previous melting episode of basalt extraction).

Normally, one would expect that combining highpartial melting and a refractory source, would resultin considerable depletions in highly incompatible ele-ments; not so with these lavas. They are even more en-riched, relative to their rare-earth element abundances,in large ion lithophile elements such as K, Rb, U, andTh than the associated arc tholeiites, or than MORB. Inthis respect, they display the characteristics of calc-alkalic suites. They show an even more marked enrich-ment of Zr relative to Ti than the arc tholeiites (al-though overall abundances of both are quite low). How-ever, they do not show the depletion in Ta and Nb rela-tive to rare-earth elements of the arc tholeiites.

When the spectrum of magmatic processes that haveacted on these volcanic rocks is put in perspective, it isevident that although fractionation and even meltingprocesses have been important, the distinctive featuresof these rocks relate to their source compositions. Pe-trogenetic models proposed by contributors to this vol-ume quite naturally have this as their major concern.

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There is, for example, agreement among Wood et al.(this volume), Hickey and Frey (this volume), and Bou-gault et al. (this volume) that the mantle sources havebeen enriched in large ion lithophile elements, probablyby fluids derived from the subducting ocean crust andits sediments (e.g., Hawkesworth et al., 1979; Saunderset al., 1980, Kay, 1980). Wood et al. (this volume) ex-plain the refractory behavior of Ti, Ta, and Nb by argu-ing that a Ti-rich phase such as rutile or sphene is stableunder hydrous conditions and retains these elementsduring the partial melting of a hydrous mantle source(Hellman and Green, 1979). The lack of Ta and Nb de-pletions in the high-MgO bronzite andesites, they sug-gest, reflects the exhaustion of such a phase or phasesduring extreme partial melting. The unusual enrichmentin Zr and Hf relative to Ti, Y, and the heavy rare-earthelements is more difficult to explain by crystal-melt rela-tionships and would require retention of Ti and theheavy rare earths in garnet during melting (Wood et al.,this volume) or replenishment of the mantle source withincompatible elements such as Zr and Hf in addition toK, Rb, U, Th, etc.

Models for Fore-arc Magmatism

Reconciling these geochemical features with plausibletectonic-magmatic sequences in the Mariana arc systemhas not produced unanimity of opinion. Most petro-logic contributors to this volume have seized on theEocene-Oligocene age of the boninite series lavas in theBonin Islands and at Site 458 as an indication that theseunusual lavas are somehow restricted to eruption duringthe earliest stages of arc volcanism only. Meijer et al.(this volume) and Meijer (1980), for example, suggestthat at the onset of subduction, cold hydrous oceancrust is suddenly plunged into hot mantle. The conse-quent release of water from the subducted crust into thehot mantle suppresses the liquidus sufficiently to initiatemelting, and a spectrum of magma types is produced asthe zone of melting rises from the vicinity of the subduc-tion zone to shallower levels in the mantle wedge aboveit. Boninites are produced when melting reaches as shal-low as 40 to 60 km below the seafloor. As subductioncontinues, the temperature of the mantle around thesubducted lithosphere is lowered, and because the zoneof melting cannot rise to such shallow depths, boninitescease to erupt. This hypothesis, however, does not takeinto account either the early Miocene boninites onGuam or late Miocene (10.8 Ma) boninite lavas dredgednear Guam. For the latter, Beccaluva et al. (1980) pro-pose that the thermal conditions necessary to produceboninites were reestablished by a rise in isotherms at theinitiation of Mariana Trough back-arc basin spreading.This could have caused additional melting of shallow,but nevertheless refractory, subarc mantle. Hickey andFrey (this volume) and Wood et al. (this volume) havediscussed models in which the mantle above the oceancrust becomes progressively enriched in the highly in-compatible elements as subduction proceeds. Hickeyand Frey relate this specifically to the sequence of vol-canism recorded in Hole 458, in which the relativelymore enriched boninites rest stratigraphically above the

arc tholeiites. Meijer (1980) made the similar point thatboninite series lavas seem to occur later in volcanic suc-cessions on Guam and the Bonin Islands as well as inHole 458. In our view, the evidence for later eruption ofboninites is circumstantial and based on extremely re-stricted outcroppings or drill holes. In Hole 458, the twomagma series clearly alternate. Mattey et al. (1980) andWood et al. (this volume) make the more general pointthat the Mariana arc as a whole has evolved to morecalc-alkalic compositions, especially on the West Mari-ana Ridge and the currently active arc, and that this en-tails steady enrichment in the mantle of the mobile in-compatible elements. They mention the similarities be-tween boninite series and calc-alkalic series lavas inthese elements. Hickey and Frey (this volume) demon-strate that boninites show quite variable enrichments inlight rare-earth element abundances. This suggests tothem that reenrichment of depleted mantle sources bythe highly mobile incompatible elements can result in acontinuum of depleted to enriched boninite series lavas.

Meijer et al. (this volume), Wood et al. (this volume),and Hickey and Frey (this volume) all favor an originfor arc tholeiites in the mantle above the subductingoceanic lithosphere. Of these authors, only Wood et al.(this volume) propose that the depleted source of theboninites was originally the uppermost mantle of thesubducting lithosphere from which abyssal tholeiiteshad been extracted. Upon deep descent into the mantleduring subduction, this refractory peridotite would be-come lighter than surrounding mantle from which ba-saltic melts had not been extracted, probably because itwould contain comparatively less of the dense iron-richphase, garnet. It would then ascend diapirically into themantle above the subducting lithosphere, there to be-come variably reenriched in mobile elements also de-rived from the subducting crust.

This proposal is similar to that of Kay (1980), whofavored derivation of the entire suite of island arc mag-mas from rising depleted peridotite diapirs originatingfrom within the subducting lithosphere, based on thelow abundances of Ti, Nb, Zr, and heavy rare-earth ele-ments in all of them (Green, 1976; Sun and Nesbitt,1978). Variable contamination with subducted sedi-ments and seawater again would cause the enrichmentsin highly mobile incompatible elements. The magmascould also be modified to a greater or lesser extent bycrystal fractionation prior to eruption.

On geochemical grounds, however, it is still impossi-ble to distinguish between models favoring subductionzone melting (e.g., Ewart et al., 1977), shallower melt-ing of depleted diapirs rising from the subducting litho-sphere (Kay, 1980), or melting of the mantle above thesubduction zone after it has been hydrated and variablyenriched in highly mobile incompatible elements (Ring-wood, 1974; Thorpe et al., 1976; Hawkesworth et al.,1979; Saunders et al., 1980). All of these models requireways of simultaneously depleting melts in Ti, Zr, Hf,Ta, Nb, and, sometimes, P, and enriching in K, Rb, Ba,Th, U, Cs, and radiogenic strontium. The depletions areaccomplished either by a two-stage melting process, inwhich a previous extraction of basalt has occurred, or

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by stabilization under hydrous conditions of minorphases such as rutile, sphene, and apatite, which retainthe relatively depleted elements during melting. Saun-ders et al. (1980) argue that the latter could occur any-where near the hydrous part of the subducting crust,leaving only the mobile incompatible elements to escapeduring dehydration. Once this has occurred, however, itwould seem that melting could occur either in the sub-duction zone or in rising diapirs and produce the samegeochemical anomalies, provided adequate enrichmentor reenrichment in the mobile elements also occurs.

Experimental evidence for the actual depth of segre-gation of island arc parental materials from the mantlefavors shallow depths rather than the great depths oftypical subduction zones. Experimental data indicatethat boninites could have melted in equilibrium with hy-drous peridotite at depths corresponding to about 10-20kbar (e.g., Kushiro, 1972, 1974, and this volume; Green,1973, 1976; Nicholls and Ringwood, 1973; Nicholls,1974; Mysen et al., 1974; Mysen and Boettcher, 1975a,1975b). In their review of the chemical characteristics ofisland arc basalts, Perfit et al. (1980) conclude that themajor oxide differences between arc basalts and MORBare not a consequence of differences at the source butappear to reflect different crystallization histories: No-tably, arc basalts have less initial Plagioclase fractiona-tion and more clinopyroxene fractionation. This, theseauthors argue, is a consequence either of higher /?(H2O)or greater depths of crystallization. We have argued thatvariable oxidation states of arc basaltic magmas causedifferent enrichments in total iron, TiO2 and SiO2. Thismight be related to a more hydrous mantle source thanthat of MORB, but this does not imply a greater depthof melting. There is no important compositional featureof these basalts to indicate depths of melting signifi-cantly greater than those of ocean floor basalts. The lat-ter have been interpreted to originate at depths of about30 km (9-10 kbar; e.g., Kushiro, 1973; Fujii et al., 1978;Bender et al., 1978; Presnall et al., 1979) or perhapstwice that much (e.g., Duncan and Green, 1980; Stol-per, 1980; Jaques and Green, 1980). Similar depths oforigin seem preferred for boninites, as already discussed.These conclusions are consistent either with melting ofperidotite diapirs rising from the subducting lithosphereor with the direct melting of mantle above the subduc-tion complex itself, following its hydration and enrich-ment in mobile incompatible elements.

The two magma types, arc tholeiites and boninites,can be viewed as complements in a direct melting model,with boninites derived from the refractory mantle pro-duced by extraction of the former. The situation wouldbe analogous to that of the FAMOUS area on the Mid-Atlantic Ridge, where continued melting of a mantlefrom which basalts have been extracted has been in-voked to explain the interbedding or close spatial prox-imity of sparsely Plagioclase phyric and strongly olivinephyric basalts (Langmuir et al., 1977; Natland, 1978;Duncan and Green, 1980). The differences would be (1)that the mantle beneath the Mariana fore-arc was more

hydrous and (2) that variable external enrichment inmobile elements was occurring. Based on Holes 458 and459B, it is our belief that olivine boninites and olivine-bearing parents to arc tholeiites are end components ofa continuum of parental compositions supplied to theMariana fore-arc region during late Eocene-early Oligo-cene times. Whether intermediate magma types mayhave been produced by hybridization between these endcomponents is uncertain. But there is no reason whymany of them could not have been derived directly fromvariably refractory and hydrated sources in the mantle.The existence of such a continuum implies that the endcomponent magma types were simultaneously available,as argued earlier on stratigraphic grounds, and probablymeans that their sources were not too greatly separatedspatially in the mantle. In other words, a deep sourcefor arc tholeiites, and a shallow source for boninites(e.g., Meijer, 1980), seems unlikely.

Boninite Constraints on Arc-Trench Geometry

The eruptive sources of lavas to Sites 458 and 459were probably not far away, certainly not as far away asthe present Mariana arc or even the ancestral Marianaarc which may have been on the northern extension ofthe ancestral Guam-Saipan arc (Fig. 1). This wouldhave been over 100 km to the west. If we assume thatmelting occurred more or less directly beneath the twosites, then great changes in the geometry of subductionmust have occurred since the Eocene. The present depthto the top of the subducted oceanic lithosphere beneathSite 459 is no more than 20-30 km (Hussong and Fryer,this volume), shallower than the experimentally esti-mated depth of segregation of boninites. If, as arguedby Wood et al. (this volume), boninites and possiblyisland arc tholeiites arose from diapirs of hydrous peri-dotite emanating from the subducted lithosphere thatascended through a denser garnet-bearing mantle, thenthe subduction zone was originally 100 km or morebeneath Site 458 and 459. Even if melting was confinedto shallower mantle never invaded by such diapirs, thedepth to the subduction zone would still have had to beconsiderably more than it is now. We believe that this isconsistent with tectonic removal, since the Eocene, ofportions of the eastern edge of the fore-arc region byprocesses related to subduction. There is evidence fromseismic profiler records for present-day downfaultingof the edge of the fore-arc into the Mariana Trench(Mrozowski and Hays, 1980; Hussong and Fryer, thisvolume), and it is evident that arc-related igneous andmetamorphic rocks crop out in the trench wall (e.g.,Dietrich et al., 1978; Beccaluva et al., 1980; Sharaskin etal., 1980; Meijer et al., this volume; Sharaskin, thisvolume; Wood et al., this volume; Sites 460 and 461chapters, this volume; S. Bloomer, personal communi-cation). The result of this tectonic process has been toshift the zone of subduction westward relative to Sites458 and 459, and probably even farther to the west atthe northern and southern ends of the Mariana arc,where the fore-arc region is narrower than near 18°N.

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Mariana Fore-arc Ophiolite

The Mariana fore-arc is a plateau-like region betweenthe high subaerial volcanoes of the Mariana arc andthe abrupt chasm of the Mariana Trench. Beneath thesediment drape which thickens toward the arc, fore-arcbasement averages between 4 km and 5 km below sealevel, with local elevations as shallow as 1.5 km. On thebasis of Leg 60 drilling, the top of this basement appearseverywhere to have been produced by Eocene-late Oli-gocene arc volcanism. The drilling also demonstratedthat this region has always been in fairly deep waterand, except possibly locally, has never been uplifted.Within and near the Trench, in fact, subsidence hasclearly occurred, as evidenced by the Trench Sites 460and 461, which have subsided below the carbonate com-pensation depth since the late Oligocene (see Sites 460and 461 chapters, this volume). Normal faults are abun-dant near the Trench (Mrozowski and Hayes, 1980;Mrozowski et al., this volume); some even affect Recentsediments (Hussong and Fryer, this volume).

Seismic refraction studies show that the crust of thefore-arc is somewhat thicker and has lower velocitiesthan either normal ocean crust or back-arc basin crust.However, it is not grossly different from either. Ve-locities approach 5.6-6.1 km/s uniformly at depths of4.5 km below the seafloor throughout the fore-arc re-gion (Hussong and Fryer, this volume). But "on thebasis of seismic velocities only, it is impossible to deter-mine whether the uppermost layer 3 material beneaththe fore-arc is composed of arc volcanics or is old back-arc basin or oceanic crust that has been buried by, andnow serves as the foundation for, more recent arc vol-canics" (LaTraille and Hussong, 1980, p. 215).

The total thickness of the crust increases to the westbeneath the Mariana arc (Hussong, this volume, Plate2, back pocket). The crust of the Palau-Kyushu Ridge isof comparable thickness (Murauchi et al., 1968). Thereis now, and there was in the Eocene, a thicker crustalroot beneath shallow portions of the arc system, wherethere are, or were, lines of active shallow or subaerialvolcanoes.

The structural relationship between the fore-arc lavasand those of the higher Eocene volcanoes of Guam, Sai-pan, and the Palau-Kyushu Ridge is not clear. But therecovery of gabbros and ultramafic rocks in the wall ofthe Trench (Dietrich et al., 1978; Beccaluva et al., 1980;Sites 460 and 461 chapters, this volume; S. Bloomer andJ. Hawkins, personal communication), in many casesassociated with or close to evident outcrops of arc tho-leiites and boninites, is strong evidence that fore-arccrust is quite thin and lacks any component of faulted-inocean crust. The arc volcanics appear directly to overliegabbros and ultramafic rocks, and it is logical to pre-sume that the gabbros and cumulus ultramafic rocksconsolidated in fore-arc rather than in ocean crustmagma chambers. Put another way, for the gabbrosand ultramafic rocks to have originated in the oceancrust, one would have to suppose the existence of a fault

or faults which removed all vestige of several hundredmeters of Jurassic and younger marine sediments and ofseveral kilometers of Layer 2 pillow basalts from the topof the ocean crust section and subducted these withoutsubducting the deeper and denser gabbros and ultra-mafic rocks. This would probably also have caused up-lift of the outer part of the fore-arc region, whereas sub-sidence has occurred. The arc lavas would now be infault or depositional contact with the plutonic rocks.

We have summarized evidence that the arc tholeiitesin particular, but also the boninites, of Holes 458 and459B are quite fractionated lavas. A complementarysuite of cumulus gabbros must exist somewhere in thefore-arc crust. Earlier we noted that calc-alkalic gab-bros were dredged from the Mariana Trench near Guam(Dietrich et al., 1978). It is possible that Seismic Layer 3of the fore-arc is made up almost entirely of cumulusgabbros involved in the fractionation of arc tholeiites,boninites, and even calc-alkalic lavas.

In short, we propose that the Mariana fore-arc regionis an in situ ophiolite succession with appropriately thincrust, a reasonably typical velocity structure (probablymodified by serpentinization and the extensive faultingthat has occurred), and the necessary upper pillowed ex-trusive sequence overlying appropriate plutonic rocks.It was produced during the early stages of arc volcanismand is not a fragment of ocean crust.

Exactly how this crust was produced must remain amatter for speculation. Cameron et al. (1979, 1980) havepointed to the occurrence of boninites in some ophio-lites including Troodos and, because of the boninitesdredged and drilled from the Mariana Trench and fore-arc, proposed that they formed in fore-arc settings.Troodos is one ophiolite that has a particularly well-developed sheeted dike complex (Gass, 1963; Mooresand Vine, 1971; Kidd, 1977; Coleman, 1977), whichis normally taken to indicate an origin by seafloorspreading. Miyashiro (1973) argued for an island arcrather than ocean crust origin for Troodos on the basisof geochemistry, a suggestion that was not well receivedprimarily because Troodos simply does not look struc-turally like an island arc but instead resembles oceancrust, or at least some type of back-arc basin. The dis-covery of boninites in the Troodos complex pillow lavasimplies that Miyashiro (1973) was right, as Cameron etal. (1980) have argued, but what now should be made ofTroodos's ocean crust type of structure?

We suggest that an appropriate equivalent of Troodosin an island arc setting is the Mariana fore-arc regionand that at the earliest stages of arc volcanism, a type ofseafloor spreading may have occurred before volcanismfocused along the line of the arc. Such an early spread-ing episode is not unreasonable in a tectonic setting thathas twice subsequently experienced arc rifting and for-mation of back-arc spreading basins. Whether or nottrue sea-floor spreading produced the Mariana fore-arcregion remains to be seen in the light of more detailedgeophysical experiments, careful dredging in the Trench,and perhaps drilling. Regardless, the occurrence of bo-

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ninites in certain ophiolites is good evidence that spread-ing has occurred in several ancient arc systems, prob-ably early in their history.

PART II. STRUCTURAL AND MAGMATICEVOLUTION OF THE MARIANA ARC SYSTEM

Trapped Ocean Crust, Inactive Arcs, and Back-arcBasins in the Philippine Sea

Here we review some of the more important results ofLegs 58 and 59 and integrate them with the Leg 60 datasummarized in Part I to provide an overall view of theevolution of the Mariana arc system. The two earlierlegs provided valuable information on the West Philip-pine Sea, the Palau-Kyushu Ridge, and the Parece VelaBasin, information which is important in understandingthe early stages of arc development. The location ofsites drilled during Legs 58 and 59 are shown on Figure1. Details of the petrology of igneous rocks recoveredduring these legs are in numerous chapters in Klein,Kobayashi, et al. (1980) and in Kroenke, Scott, et al.(1980).

West Philippine Basin

Hilde et al. (1977) have proposed that the West Phil-ippine Basin (Fig. 1), lying between the Ryukyu Arc atthe Asian plate boundary to the west and the Palau-Kyushu Ridge to the east, is "trapped" in origin, beingpart of normal Pacific ocean floor which became sepa-rated from the main ocean as a result of initiation ofsubduction along the line of the present Palau-KyushuRidge. They suggested that the eastern boundary of theWest Philippine Sea may initially have been an old N-Stransform fault which changed to a convergent plateboundary consequent upon a change in direction of Pa-cific plate motion from northerly to westerly some 45Ma (cf. Jurdy, 1979). Scott et al. (1980), although rec-ognizing this possibility, have constructed argumentswhich consider the West Philippine Sea as having formedby back-arc spreading but related to the Oki-DaitoRidge as a potential early Tertiary island arc (Karig,1975) rather than to the Palau-Kyushu Ridge.

Magnetic lineation patterns in the West PhilippineBasin have been studied by Louden (1976), Watts et al.(1977), and Shih (1980) and, together with earlier evi-dence from deep sea drilling (Sites 290-295; Karig, In-gle, et al., 1975), suggest that active spreading occurredin the early Tertiary (-62-39 Ma), with the NW-SW-trending Central Basin "Fault" representing the extinctspreading center (Ben-Avraham et al., 1972). The youngermagnetic anomalies (18-20) are particularly well defined(Watts et al., 1977) and suggest a (half-)spreading rateof 4.4 cm/y.; the older anomalies, though less well de-fined, indicate a much faster spreading rate. Andrews(1980) has suggested that the spreading direction mayhave changed 54 Ma from northeasterly to northerly.

Andesite, basalt, and granodiorite have been recov-ered from the Oki-Daito Ridge, and andesite, diorite,hornblende schists, and serpentine have been dredgedfrom the Daito Ridge; justifiably it has been suggestedthat these two ridges represent old island arcs (Murauchi

et al., 1978; Karig, 1975; Mizuno et al., 1975, 1979).However, there is no ridge feature south of the CentralBasin Ridge ("Fault") which could represent a remnantarc or rifted counterpart of the Oki-Daito Ridge andwhich would allow for the West Philippine Basin to haveformed by back-arc spreading behind the Oki-DaitoRidge during the early Tertiary (Karig, 1975). Scott etal. (1980) have also pointed out that there is no positiveevidence from DSDP Holes 294 and 295 that the Oki-Daito Ridge was an active arc during the early Tertiary.Nor indeed do DSDP Sites 445 and 446, close to theOki-Daito Ridge, record any evidence for arc activity aslong ago as the early Eocene (Klein, Kobayashi, et al.,1980). Scott et al. (1980) note that the 6000-meter waterdepth in the West Philippine Sea is deeper than that fornormal ocean crust of early Tertiary age as predicted bythe depth-time curve but does appear to fit the depth-time relationship of back-arc basin crust (Sclater et al.,1976). Evidence for the nature of the West PhilippineSea is therefore conflicting.

Site 447, in the eastern part of the basin, lies betweenthe Central Basin Ridge and the Palau-Kyushu Ridge.Paleontological evidence from the basal breccias sug-gests an age of 32-34 m.y. (Kroenke, Scott, et al., 1980),although the magnetic lineation patterns indicate an ageat least 4 m.y. older; there may thus be an appreciableerosion surface between the breccias and the basalticbasement. These breccias are overlain by over 60 metersof middle to upper Oligocene tuffs and polymict brec-cias representing a volcaniclastic apron derived from thePalau-Kyushu Ridge. The mineralogy, petrology, andgeochemistry of Site 447 basalts have been described byMattey et al. (1980) and Wood, Mattey et al. (1980).They recognize two distinct magma types with differentmajor element glass compositions and with differenttrace element ratios (e.g., Ti/Zr, Y/Zr) which appear tohave evolved in separate magma chambers but never-theless must have existed simultaneously in order to ac-count for the observed downhole chemical variations.Each magma chamber was periodically replenished witha porphyritic primitive magma and underwent mixingfollowed by variable degrees of fractional crystallizationprior to eruption. However, the overall geochemicalcharacteristics of these basalts—low K, Rb, Ba, Sr, lowNb and Ta, variable light REE depletion, and low 87Sr/86Sr ratios (-0.7026; Armstrong and Nixon, 1980)—areessentially similar to normal MORB. Abundances ofiron as FeO* and A12O3 resemble those of MORB ratherthan back-arc basin basalts, using criteria established byFryer et al. (this volume). There are thus no obvious arc-related geochemical characteristics in Site 447 basaltssuch as have been found in some other marginal basins(cf. Tarney et al., 1981; Saunders et al., 1980). There istherefore no a priori reason to invoke back-arc spread-ing for the formation of the West Philippine Sea, al-though such an origin is by no means excluded.

Basalts recovered from Site 446 in the Daito Basin,between the Daito and Oki-Daito ridges (Fig. 1), arevery different from those at Site 447. They occur as aseries of sills of post-early Eocene age (Marsh et al.,1980). There are two distinct types of basalt, one rich in

896


kaersutitic hornblende, the other hornblende-free. Bothtypes have strikingly high contents of titanium (upto 4.9% TiO2) and incompatible elements as well asrelatively strongly fractionated REE patterns (CeN/YbN-3 -6) . Zr/Nb ratios are low (<IO) but K/Rb ratioshigh. Many of the basalts are fairly iron-rich, but someare also Mg-rich (-15% MgO); although the latter mayhave high MgO contents as a result of intrasill fractiona-tion, they are still markedly enriched in incompatibletrace elements. The basalts are tholeiitic or mildly alka-line. Their geochemical features are not unlike "en-riched" MORB from Iceland or from 45 °N on the Mid-Atlantic Ridge (Wood et al., 1979; Tarney et al., 1979)although the latter are not Ti-rich. They are an unusualsuite of rocks. Their high Nb and Ta contents (Marsh etal., 1980; Wood, Mattey et al., 1980) contrast with thelow Nb and Ta contents of almost all arc lavas and in-deed with all marginal basin basalts so far described.Even the mildly alkaline lavas of Penguin volcano inBransfield Strait marginal basin, next to the AntarcticPeninsula, for instance, have very low Nb contents andhigh Zr/Nb ratios (Weaver et al., 1979).

It is clear that in their high concentration of Ti, Nb,and Ta these Site 446 lavas have no arc-related geo-chemical characteristics. The basement age at this site(53 m.y., Klein, Kobayashi et al., 1980) of course pre-dates the development of the Palau-Kyushu Arc. Where-as the petrogenesis of the Site 446 lavas must remainenigmatic, it is evident that there is no basis for linkingthem in any way to back-arc spreading. It seems morelikely, all evidence considered, that the West PhilippineSea is a "trapped" marginal basin unconnected withback-arc spreading in the strict sense.

If, as has been suggested, the Palau-Kyushu Ridgemarks the site of an earlier N-S transform which be-came a convergent plate boundary coincident with thechange in direction of Pacific plate motion approx-imately 45 Ma, the presence of the spreading center withits buoyant hot mantle on the western side of the trans-form would certainly have facilitated subduction of thecooler Pacific lithosphere on the eastern side of thetransform.

The Palau-Kyushu Ridge

The Palau-Kyushu Ridge is over 2000 km long (Fig.1) and rises to over 2000 meters above the adjacent basinfloors. At Site 448, over 600 meters of volcanic base-ment consisting of lava flows, dykes, and sills interbed-ded with volcaniclastic breccias were penetrated belowmiddle Oligocene oozes. Like the dredged samples fromthis locality (Anonymous, 1977), the basalts are fre-quently highly vesicular, suggesting eruption at rela-tively shallow depths or even subaerially. These perme-able rocks have been rather readily altered (Mattey etal., 1980), with the clay mineral alteration products be-ing kaolin-rich in the upper part of the core but withmixed-layer smectite-vermiculite-chlorite clays deeperin the section (Aldrich et al., 1980). Nonetheless, the es-sential geochemical characteristics have been preserved.Nannofossil assemblages and 40Ar/39Ar dates indicatethat the main period of volcanism began to fade about

32 Ma and had ceased altogether by 29 Ma (Scott etal., 1980). Assuming contemporaneity with the fore-arclavas on Saipan and Guam, which have yielded ages ofapproximately 42 m.y. (cf. Ingle, 1975) it would appearthat volcanism on the Palau-Kyushu Ridge spanned aperiod of about 10 m.y. and began within a few millionyears after the start of subduction.

The geochemical characteristics of the Site 448 lavashas been described by Mattey et al. (1980) and Wood,Mattey et al. (1980). Almost all the lavas are basalticand conform to the island arc tholeiite series of Jakesand Gill (1970). They are readily distinguished from thebasalts of the adjacent marginal basins by their low Cr(20-50 ppm) and Ni (0-20 ppm) contents.

The Palau-Kyushu lavas are quartz-normative tholei-ites and basaltic andesites with 48-56% SiO2, 13-17%A12O3, and 10-15% iron as total FeO. FeO/MgO ratiosrange from 1.5 to over 4.0, much higher than in the ba-sin basalts, implying that they may have undergone con-siderable crystal fractionation. Currently popular mod-els for the island arc tholeiite series invoke hydrousmelting of the mantle wedge above the subducting slabwith considerable crystal fractionation during ascentto explain the wide range of Fe/Mg ratios and low Crand Ni contents. Arc tholeiites from the Palau-KyushuRidge have similar geochemical characteristics to thosefrom the Mariana fore-arc (Site 458 and 459) but in-clude more iron-rich compositions. Mattey et al. (1980)noted, however, that the concentrations of many incom-patible elements in the Palau-Kyushu lavas, especiallythe REE, ZR, Nb, and Ta, were much lower than inMORB with equivalent Fe/Mg ratios, and that these didnot increase systematically with Fe/Mg ratio. More-over, they distinguished several petrographic types (ol-plag, opx-cpx-plag, and cpx-plag phyric) that couldnot be related to each other by fractional crystallization.Thus either several distinct magma chambers were feed-ing lavas to the Palau-Kyushu Arc or the lavas werecloser to "primary" melt compositions than has hither-to been admitted. The only source able to produce suchiron-rich compositions would be the subducted oceancrust itself. Although thermal models for steady-statesubduction zones (Anderson et al., 1976) tend not tofavor melting of the subducted ocean crust, it is to beremembered that the Palau-Kyushu Arc developed rap-idly following the start of subduction beneath the WestPhilippine Sea. Under such conditions high degrees ofmelting of ocean crust may have been possible. Thiswould account for the consistently high Fe/Mg ratiosand low Cr and Ni contents of the arc tholeiites. Inmany respects the arc tholeiites resemble normal MORBin their trace element characteristics but tend to haverather high LIL element contents, as is typical of mostsubduction zone magmas.

West Mariana Ridge

The West Mariana Ridge (Fig. 1) is shallower than thePalau-Kyushu Ridge, usually less than 2000 meters be-low sea level. Drilling at Site 451 penetrated 930 metersof volcaniclastic material, the breccia fragments beingcomposed mainly of basalts and basaltic andesites with

897


rare andesites and having phenocrysts of Plagioclase ±clinopyroxene ± orthopyroxene ± olivine ± magnetite(Mattey et al., 1980; Ishii, 1980).

Chemically these lavas have low contents of Cr, Ni,Zr, Nb, and Ta, like the Palau-Kyushu arc tholeiites,but have low Fe/Mg ratios (1.0-2.0), are lower in bothFeO and MgO, have much higher A12O3 (18-20%) andCaO (9-13%), and a wider range of SiO2 (45-58%).There are larger trace element differences. La/Zr andCe/Zr ratios are higher and REE patterns are more frac-tionated, with CeN/YbN ~ 2. Concentrations of Ba (110-450 ppm) and Sr (450-600 ppm) and other LIL elementsare notably higher than in arc tholeiites or in normalMORB. These characteristics are those of calc-alkalicmagmas, even though most of the lavas are still basalticin terms of major elements. Essentially similar, al-though metamorphosed, rock types were cored at Site453 during Leg 60 but include cumulus gabbros as well.

Distribution of Mariana Arc Magma Series inTime and Space

In order to summarize the petrologic history of theMariana arc, it is important to establish as carefully aspossible the various igneous rock series present on eachstructural component of the arc system, including theislands which we have not yet discussed. Each of thethree arc magma series—boninites, arc tholeiites, andcalc-alkalic lavas—can be identified on the basis of ma-jor oxide chemistry, trace elements, and isotopic geo-chemistry and mineralogy. Boninite series lavas are par-ticularly distinctive chemically and petrographically,and there is consequently little uncertain about theiridentification. The distinction between island arc tholei-ites and calc-alkalic lavas is based increasingly on traceelements, although historically the contrasting degree ofiron enrichment between the two has served as the basisfor classification and been the starting pont for muchspeculation and experimental study. The original defi-nition for calc-alkalic lavas was based on Peacock's(1931) alkali-lime index (the value of SiO2 in a differen-tiation series in which total Na2O + K2O = CaO). Butthis is not strictly appropriate for most Mariana arcsuites containing andesites (they turn out to be calcicrather than calc-alkalic), and has been largely super-seded in contemporary petrological thought by the gen-eral idea that island arc andesites which show consider-able iron enrichment belong to the island arc tholeiiteseries (e.g., Kuno, 1968; Jakes and Gill, 1970; Miya-shiro, 1974).

Mattey et al. (1980) and Wood et al. (this volume)have coupled this generalized definition with trace ele-ment data, noting that calc-alkalic lavas have consider-ably higher abundances of the mobile hygromagmato-phile (HYG) elements than island arc tholeiites. Theyemploy a ternary plot of Th-Ta-Hf/3 as a discriminant.On such a plot (Fig. 10) the calc-alkalic lavas of theMariana arc system are considerably closer to the Thapex than arc tholeiites, and both suites are consider-ably depleted in Ta relative to MORB. On the basis ofthis and other discriminant diagrams, Wood et al. (thisvolume) propose that the earliest stages of Mariana arc

volcanism are represented by arc tholeiites, whereasyounger lavas in the system are calc-alkalic in composi-tion. That is, considering only data for drill sites, theancestral Mariana arc, which included the present fore-arc region and the Palau-Kyushu Ridge, had arc tholei-ite compositions. The younger West Mariana Ridge andthe modern arc, each formed after episodes of back-arcbasin spreading, have calc-alkalic compositions. Sinceeach of the successive arcs occupied the same structuralposition with respect to the fore-arc when they were ac-tive, Wood et al. (this volume) infer that much the samezone of mantle was tapped in each case. The transitionfrom arc tholeiite to calc-alkalic compositions thereforereflects a fundamental change in mantle source com-positions with time—specifically, steady addition ofmobile HYG elements to it, derived from the subductinglithosphere.

In terms of iron enrichment, however, this transitionwas not so straightforward. On Figure 11 A, the averagecompositions for the fore-arc lava types of Holes 458and 459B (from Table 2) are plotted as well as data fieldsfor samples from Hole 448 on the Palau-Kyushu Ridgeand Hole 451 on the West Mariana Ridge (Mattey et al.,1980). A separate, more restricted field for Hole 448glasses indicates that many of the lavas there have lostiron and gained magnesium as a result of alteration(Scott, 1980), thus suppressing the original iron enrich-ment and distorting the data field shown on Figure 11 A.Similar effects may have occurred in fore-arc Holes 458and 459B, but our earlier consideration of Figures 8 and9 suggests that petrological processes (fractionation oftitanomagnetite, different source compositions) ratherthan alteration prevented significant iron enrichment inthose basalts. Still, iron enrichment is evident in some ofthem, and it is probably fair to describe them as arctholeiites, even though they are not as iron-enriched asthe Hole 448 glasses.

Figure 1 IB compares lavas of the Eocene Alutom andMiocene Umatak formations of Guam (Stark, 1963)and the Eocene Aimeliik formation of Palau (Mason etal., 1956). The data fields for Holes 448 and 451 areagain shown. As discussed earlier, some of the UmatakFormation lavas are boninites. The suite from Palaushows so little iron enrichment that it resembles theclassical calc-alkalic Cascades suite (Daly, 1933; Smithand Carmichael, 1968). There is somewhat more ironenrichment in the Umatak Formation, but it is not as ex-treme as in Hole 448 glasses and appears to straddle thefield of calc-alkalic Hole 451 lavas. Figure HC showsdata for Eocene lavas of Saipan, which also have mini-mal iron enrichment.

Data for the present Mariana arc are depicted onFigure 11D. As noted by Dixon and Batiza (1979) andStern (1979), the extent of iron enrichment is somewhatvariable in the different volcanoes of the modern arc,with some islands, such as Sarigan, which have lavasclosely resembling the classical calc-alkalic trend andothers, such as Agrigan, having somewhat more ironenrichment.

Finally, all these trends can be compared with thecompositions of volcanic glasses derived as ash from the

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Hf/3

π=DSDP Site 458+ =DSDP Hole459Bv=DSDP Site 460•=DSDP Site 461

Th Ta

Figure 10. Ternary Ta-Th-Hf/3 diagram for island arc tholeiites and boninites from Holes 458, 459B, 460, and 461 (from Wood et al., this volume).Areas: A = N-type MORB; B = E-type MORB; C = within-plate lavas; D = active plate-margin lavas.

modern arc and deposited as turbidites at Site 453 (Fig-ure HE, from Packham and Williams, this volume).Many of these glasses show considerable enrichmentcompared with lavas of the modern arc. Since they alsohave low TiO2, Packham and Williams describe them asarc tholeiites and note that they erupted from at least5 Ma to about 200,000 y. ago. The present-day calc-alkalic suites on the modern arc, which show so littleiron enrichment, represent a distinct shift in the compo-sitions of lavas erupted on these volcanoes.

The plots of Figure 11 show that eruption of calc-alkalic lavas (those having little or no iron enrichment)and arc tholeiite sequences (with considerable iron en-richment) have varied in time and space in the Marianaarc. This is summarized in Table 4. The ancestral Eo-cene arc, which included the fore-arc region, Guam, and

Saipan structurally united with Palau and the Palau-Kyushu Ridge, consists of all three major arc rocksuites—boninites, arc tholeiites, and calc-alkalic lavas.The Miocene arc, formed after the opening of theParece-Vela Basin, consists of boninites and calc-alkaliclavas. On Guam these evidently represent a portion ofthe West Mariana Ridge left behind following openingof the Mariana Trough. The modern arc, which wasformed after the opening of the Trough, evidentlyshifted from arc tholeiite to calc-alkalic compositions inthe past 200,000 y.

The question presents itself whether the Eocene calc-alkalic exposures on Guam, Saipan, and Palau have thesame trace element characteristics as their younger coun-terparts on the West Mariana Ridge. Unfortunately,there are no data for the critical trace element discrimi-

899

•ooo

x Boninites458 459B-B1 Abasalt

© high Siθ2tholeiites

low SiOo tholeiites

Figure 11. AFM (Na2O + K2O-FeO*-MgO) ternary diagrams for tholeiites and calc-alkalic lavas of the Mariana arc system. All diagrams show Skaergaard iron enrichment and Cascadescalc-alkalic fractionation trends. A. Holes 458 and 459B, with symbols and groupings as in Figure 8A. (Closed field is Hole 448 glass compositions [Scott, 1980]; dashed field is Hole 448 freshand altered basalt compositions [Mattey et al., 1980].) B. Eocene and Miocene lavas from Guam (Stark, 1963) and Palau (Mason et al., 1956) compared with fields for Eocene arc tholeiiteglasses, Hole 448, Palau-Kyushu Ridge, and Miocene calc-alkalic basalts and andesites, Hole 451, West Mariana Ridge (Mattey et al., 1980). C. Eocene lavas from Saipan (Schmidt, 1957)compared with data fields for Hole 448 glasses and Hole 451 basalts and andesites. D. Modern Mariana arc lavas compared with data fields for Holes 448 and 451 samples. Data for Agriganare from Stern (1979), for Sarigan, from Dixon and Batiza (1979), and for 21 °N dredge haul from Wood et al. (this volume). E. Glass compositions from Hole 453 vitric tuffs derived from theMariana arc prior to 200,000 y. ago, after Packham and Williams (this volume). Symbols are as in their fig. 5.


Table 4. Known distribution of igneous rock series in the Mariana arc system.

ancestralVlarianaarc

West MarianaRidge

vlariana Arc

Palau-Kyushu RidgePalauGuamSaipanFore-arcTrenchWest Mariana RidgeGuam

SariganAgriganPaganAlamagan21.4°N

Sites

448

458, 459460, 461451

453

457

Arc Calc-AlkalicAge Boninites Tholeiites Lavas

late ""Eocene-earlyOligocene

Miocene

Plio- vPleistocene

> Recent *

Note: Contrasts between arc tholeiites and calc-alkalic lavas here are based only on the degree of iron enrich-ment in differentiation series, not to differences in trace element abundances or to parental magma com-positions. See text and Figure 11 for discussion.

nants (Ta, Th, Hf, Nb, etc.) in lavas from these islands.There is some suggestion in the major oxide data, how-ever, that they would not be as enriched in these ele-ments as the younger Mariana arc calc-alkalic suites.We mentioned earlier the high alkali-lime indices of Mar-iana arc lavas in general, which technically classify mostof them as calcic rather than calc-alkalic in composition.They are especially high for these older lavas (Table 4)because of their low total alkalis and, especially, theirlow K2O. There are semiquantitative spectrographictrace element data for some Guam lavas (Tracey andStark, 1963) which show low Zr and consistently lowerBa in Eocene Alutom lavas than in Miocene UmatakFormation lavas. Along the presently active arc, thereare considerable variations in K2O and related trace ele-ments, contributing to differences in the alkali-lime in-dex, even though all the modern arc volcanoes are con-sidered to be calc-alkalic in their general characteristics(Dixon and Batiza, 1979). We therefore conclude thatthere are variable enrichments in HYG element abun-dances in Eocene and modern Mariana arc calc-alkalicrock suites, with the younger lavas more enriched thanthe older. The development of calc-alkalic differentia-tion sequences does not seem to be entirely dependenton this aspect of their chemistry. The conclusion ofWood et al. (this volume) regarding steady enrichmentof mantle sources in the mobile HYG elements may stillbe valid, although we need the relevant trace elementdata on samples from Guam, Saipan, and Palau to con-firm this.

Origin of Calc-Alkalic Magmas in theMariana Arc System

The origin of andesites and the calc-alkalic magmaseries remains a major petrogenetic problem. Two as-pects of this problem stand out in the context of theMariana arc: the role of fractional crystallization andthe question of whether calc-alkalic lavas representsome sort of evolutionary sequence following island arctholeiites in time and space.

Andesites and other siliceous lavas are subordinate involume to basalts in much of the Mariana arc (Larson etal., 1974). Of the islands discussed earlier, only Saipanhas a dearth of basalts. Where basalts are abundant,

fractional crystallization tends to become an attractivehypothesis, especially where quantitative modeling sup-ports it. This is the case for the Recent Mariana vol-canoes (Stern, 1979; Dixon and Batiza, 1979). Stark(1963) also favored a fractional crystallization mecha-nism for each of the two sequences (Alutom and Uma-tak) of Guam. Only Schmidt (1957) expressed reserva-tions about the efficacy of fractional crystallization inproducing the dacites of Saipan.

Stern (1979) has provided more than the usual rangeof evidence in support of calc-alkalic fractional crys-tallization as the origin of the andesites of Agrigan vol-cano. His study included detailed examination of cumu-lus gabbros found as xenoliths in the lavas. The xeno-liths matched the compositions of a gabbroic mass hecalculated to underlie the volcano at depths of less than5 km. Similar gabbros were recovered in great abun-dance in association with calc-alkalic metavolcanicrocks derived from the West Mariana Ridge in Hole 453during Leg 60. In Part I we summarized evidence thatthe Agrigan xenoliths and Hole 453 gabbros are amonga class of gabbros uniquely suited to be the cumulusresidua of calc-alkalic fractionation. They have ex-tremely calcic Plagioclase, with resulting low whole-rockSiO2, and crystallized at high P(H2O), as indicated bytheir mineralogy. Fractionation of such mineral assem-blages in the proportions seen in the gabbros would leadto significant enrichment in SiO2 with little change inFeOVMgO (the calc-alkalic trend).

From the occurrence of the gabbros in Hole 453, onAgrigan, and even, as argued earlier, south of Guam inthe Mariana Trench (Dietrich et al., 1978), it seems evi-dent that they are a major part of the deep crustal struc-ture of the Mariana arc system. Such gabbros have beenfound in all three of the age-structural domains of theMariana arc (Eocene, Miocene, and Recent), all ofwhich have calc-alkalic volcanics (Table 4). This sug-gests that a similar type of fractional crystallization hasbeen a major factor in the production of calc-alkalicandesites throughout the history of the arc.

Two contributors to this volume suggest that calc-alkalic lavas follow extensive arc tholeiite volcanism inthe Mariana arc system. Wood et al. view this as ageneral shift affecting the two younger arc ridges, as we

901


have discussed, but it seems now that calc-alkalic lavasand their apparently complementary cumulus gabbrosoccurred much earlier in the history of the arc than sug-gested by DSDP data alone. Packham and Williamsconcern themselves only with the presently active arcand propose that the Recent calc-alkalic high volcanoesof the arc have followed arc tholeiite volcanoes whicherupted over a much longer time span. Although thereare inadequate age data to support it, it is possible thatthe high volcanoes of each of the older arcs also evolvedfrom an arc tholeiite to a calc-alkalic stage. In the caseof the Eocene arc complex, this is at least a spatial, ifnot a temporal, phenomenon. The subaerial and shal-low submarine Eocene lavas of Guam and Saipan clearlyerupted on structures higher than the pillow lavas atforearc Sites 458 and 459 and contrast with the latter inhaving calc-alkalic rather than arc tholeiite composi-tions. The data for the West Mariana Ridge are the leastcomprehensive, since calc-alkalic lavas only have beenrecovered from two closely spaced sites (451 and 453).

In terms of the fractional crystallization mechanismoutlined here for producing Mariana arc calc-alkalicandesites (segregation of cumulus assemblages compa-parable to Hole 453 gabbros and Agrigan xenoliths), aplausible evolutionary sequence can be proposed. Themost important condition for such fractionation to oc-cur is development of magma chambers in the deepcrust at high water pressures. Gravity and seismic datademonstrate that the crust beneath the fore-arc regionnear Sites 458 and 459 is thin (LaTraille and Hussong,1980; Hussong and Fryer, this volume). This is verifiedby the persistent recovery of ultramafic rocks at fairlyshallow levels at many places in the Mariana Trench(Dietrich et al., 1978; S. Bloomer and J. Hawkins, per-sonal communication). Fore-arc magma chambers thuswere shallow, and the conditions of high/?(H2O) requiredfor calc-alkalic fractionation could not be achieved. Be-neath lines of subaerial arc volcanoes, however, thecrust is considerably thicker (see Plate 2, back pocket,this volume; Hussong, this volume). The crust beneatheach of the arcs must also have thickened with time. Thecoarse and highly porous pyroclastic ejecta of even ba-saltic island arc volcanoes guaranteed that considerableamounts of water were trapped within thickening lavapiles. Even if the same magma chambers continued toexist, they had to become more deeply buried as the vol-canoes built. There should thus have been a tendencyfor thòleiitic island arcs to develop conditions suitableto calc-alkalic magmatism. This need not have* hap-pened simultaneously along the length of the arcs, butultimately it should have happened generally along allof them.

Back-arc Basin Opening and Arc Volcanism

Recovery of gabbros at Site 453 demonstrated thatopening of the Mariana Trough split the West MarianaRidge deep into its crustal roots. Solidified magmachambers were torn in two. There is no question thatthis must have profoundly affected arc volcanism. Scottet al. (1980) and Scott and Kroenke (1980) argue, on thebasis of available radiometric and paleontologic data

and on the occurrence of ash beds in sediments cored inthe Parece Vela Basin, that arc volcanism waned con-siderably, or ceased altogether, with each opening of aback-arc basin in the Philippine Sea. Clearly, however,following opening of the Mariana Trough, arc volcan-ism soon resumed. Vitric turbidites derived from air-fallvolcanic ash produced at the presently active Marianaarc have been deposited at Site 453 nearly continuouslyfor the past 5 m.y. (Packham and Williams, this vol-ume). Upper Miocene-lower Pliocene hiatuses in fore-arc Holes 458 and 459B (Kling, this volume; Ellis, thisvolume) suggest that the earlier stages of opening of theMariana Trough were unaccompanied by arc volcanism.However, there is no corresponding upper Oligoeene-lower Miocene hiatus in Hole 459B for the opening ofthe Parece Vela Basin. Some ash was deposited in everycore in this time interval, but since the sediments ofHole 459B are primarily turbidites, it is possible thaterosion and sediment transport processes simply per-sisted throughout any interruption in volcanism on thearc.

Rodolfo and Warner (1980) suggest that the earlieststages of back-arc rifting consist of mixed basalt-ande-site volcanism because emplacement of basaltic magmasin contact with water-saturated pyroclastic debris shedinto the newly opened narrow rift. Once this happened,however, what were the compositions of the earliestlavas to erupt along the new arcs? Packham and Wil-liams (this volume) think that the compositions of theMariana arc and Mariana Trough lavas have diverged inthe past 5.0 m.y. from a single basaltic compositionresembling Trough basalts. This makes sense in that themagma sources for the present Trough and arc are stillquite close together (approx. 120 km) and probablywere only a few tens of kilometers apart when the line ofthe arc first became well established. Wood et al. (thisvolume) and Fryer et al. (this volume), however, notethat basalts in the Trough have some of the geochemicalcharacteristics of arc lavas, notably enrichments in someof the mobile HYG elements. In the earliest stages ofrifting, such enrichments might have been caused eitherby contamination (e.g., Rodolfo and Warner, 1980) orby HYG-element enrichments in the mantle source.Probably, therefore, the initial arc and Trough lavaswere similar but had compositions in between what werecognize as modern arc and Trough basalts. The earlyproximity of arc and Trough magma sources almost cer-tainly means that arc lavas were then even more domi-nated by basalts than now. This is borne out for theWest Mariana Ridge by an increase in the abundance ofandesitic ash and a decrease in hyaloclastic basalt atParece Vela Basin Site 450 as the basin widened in theMiocene (Rodolfo and Warner, 1980).

From these considerations, it would seem that forcalc-alkalic magmatism to have occurred on either theWest Mariana Ridge or the Mariana arc, it must havefollowed greater or lesser outpourings of basalts. Withthe Trough opening so close to the arc, geothermal gra-dients beneath the arc, too, would have been steeper andthe depth of origin of arc basalts correspondingly shal-lower. Conditions were favorable to the development of

902


shallow magma chambers in which iron enrichmentfractionation would have occurred. This would havebeen essentially arc tholeiite magmatism. As the thermalregimes responsible for volcanism along the arc and inthe center of the Trough separated, the sources of arcmagmas should have deepened, eruption rates dimin-ished, the crust of the arc thickened, and the conditionsfor calc-alkalic gabbro fractionation approached. Asimilar sequence should also have occurred on the WestMariana Ridge, following opening of the Parece VelaBasin.

Formation of Back-arc Basins

Plate tectonic theory has had considerable success inaccounting for the principal physiographic features ofthe ocean floor and the ages of different portions of theocean crust. One of the great tasks remaining before acomprehensive global tectonic synthesis is achieved is toaccount for the tremendous quantity and variety of ig-neous activity and lava types at both accretionary andconvergent plate boundaries. Back-arc basins are a par-ticularly complex problem because they form in themidst of • the principal physical manifestations of vol-canism at convergent plate boundaries, the island arcs.Since we are far from understanding the laws whichgovern the placement and perpetuation of spreadingcenters in the ocean basins, these interarc basins wouldseemingly be even more difficult to explain.

The principal hypotheses to date that account forback-arc basins have their considerable parallels in .theoriginal hypotheses of seafloor spreading and plate tec-tonics—viz., the two-limbed convection model of Hess(1962) and, alternatively, the recognition that spreadingcenters are not directly linked to such a simple systembut respond more or less passively either to larger pat-terns of convection in the mantle or perhaps to the mo-tions of the larger plates (e.g., McKenzie, 1967, 1969).Thus Karig's (1971b) original back-arc basin hypothesiswas that a "diapir" of mantle material rose from thesubducting lithosphėric plate to sunder the arcs and in-itiate spreading basins by a type of shallow two-limbedconvection. Karig supposed that frictional heating at thesubduction zone (Oxburgh and Turcotte, 1970) causedthe diapir, but more recent models propose simply thatsubduction drags mantle material downward, forcingconvective turnover behind the arc and producing ten-sion and back-arc rifting (Sleep and Toksöz, 1971; An-drews and Sleep, 1973; Toksöz and Bird, 1977; Toksözand Hsui, 1978). Bibee et al. (1980) have extended Ka-rig^ hypothesis to explain apparently asymmetric spread-ing and ridge crest jumps in the Mariana Trough. Thesekeep the center of spreading close to the Mariana arcabove the subduction-triggered diapir. Uyeda and Ka-namori (1979), however, argue that western Pacificback-arc spreading occurs in response to retreat of theEurasian plate from the Pacific plate, which establishesa dominantly terisional stress regime in such regions asthe Philippine Sea, allowing back-arc basins to formwithout the formation of a mantle diapir. There aresimilar proposals by Chase (1978) and Jurdy (1979).

Any one of these hypotheses can explain how theMariana arc has twice been split by back-arc basins. Thediapiric model does not allow the locus of back-arcspreading to shift too far to the west of the arc and awayfrom the subduction zone. Episodically, a major ridge-crest jump to a position within the arc itself must occurfor the zone of rifting to remain centered on the mantlediapir. Toksöz and Hsui (1978) argue that back-arcspreading actually forces the arc against the subductingplate, decreasing its dip and ultimately making overturnof the mantle caused by subduction impossible. Back-arc spreading ceases, the steep dip of the subductionzone is gradually restored, and a new cycle begins. Ac-cording to the passive spreading model, periods of re-treat of the Eurasian plate from the Pacific plate mayalternate with periods of relative convergence in whichback-arc spreading ceases, so that the two halves ofback-arc lithosphere combine into a single "welded"plate. Renewed tensional stress will initiate a new riftalong the weakest line of the combined plate. In theabsence of an active back-arc spreading center, this isbelieved by Uyeda (personal communication) to be theline of the arc.

Petrological data presently do little to constrain thesehypotheses inasmuch as our understanding of the originof island arc lavas is sufficiently ambiguous to allowvirtually any combination of petrological and tectonicspeculation. The subduction zone drag model probablyhas the greatest difficulties from a petrological view-point since it places arc volcanism directly above thedown welling limb of the proposed convection cell, a dif-ficulty recognized by Andrews and Sleep (1973) andToksöz and Hsui (1978). The direct opposite is impliedby petrological models requiring components from thesubduction zone either to infiltrate or rise diapiricallyinto the overlying mantle to become sources for arclavas. Subduction zone drag offers no explanation forthe source of heat to produce these lavas. Possibly, nospecific addition of heat, produced for example by fric-tion, is necessary. Dick (1980) argues that the highvesicularity of arc and back-arc basalts reflects additionof water to the mantle above the subducting slab. Thislowers the solidus, produces melting (e.g., Wyllie, 1971;McBirnėy, 1965), and reduces mantle density (O'Hara,1975), causing diapiric upwelling. Melting continues inthe rising diapirs until the water-saturated solidus isreached at low pressures.

We have argued here generally in favor of segregationof island-arc-tholeiite, calc-alkalic, and olivine-boniniteparental magams from the relatively shallow mantlebeneath island arcs—i.e., not from the subducted slab—in a manner consistent with Dick's hypothesis. Thereappears to be no way to distinguish on geochemicalgrounds whether the immediate mantle sources of thevarious magmas were originally directly beneath the arcor arose by some diapiric mechanism from the slab tothe shallow levels where melt segregation took place. Atmost, we can infer only that the mobile HYG elementswere derived from the subducting materials and in-filtrated the shallow mantle beneath the arc, causing the

903


particular enrichments in these elements observed to agreater or lesser degree in all the arc lavas. But whetheror not the peridotite component of the magma sourceswas originally in place beneath the arcs, or rose from thedepths, extraction of magmas and the development of athick island arc lava pile would entail considerable ac-cumulation beneath it of refractory peridotite depletedin basalt or basaltic-andesite fractions parental to arclavas. Boninites themselves are evidence that just suchdepleted peridotite exists in reasonable abundance be-neath the Mariana fore-arc region and the island ofGuam.

We suggest that after sufficient accumulation of suchshallow refractory peridotite beneath an island arc, itwould become increasingly difficult to extract a meltfraction from it. With the building of high volcanoes, itwould also become hydraulically more difficult for thatmelt fraction to reach the surface. Only the more vola-tile and explosive differentiated lavas would erupt. Wecan therefore surmise that conditions in the mantle itselfwould add to those previously described and favor ashift toward calc-alkalic compositions during the his-tory of an island arc.

We now carry this one step further, to ask whetheraccumulation of such a refractory mass could also leadto the development of conditions favorable to the initia-tion of a new back-arc basin. Either some source of heatexists, related to subduction, that produces arc magmas,or, as Dick suggests, the mantle solidus is suppressed byaddition of water, resulting in diapirism and melting. Ineither case, the effects should have become more or lesscontinuous and involved the same zone of mantle tobuild three successive island arcs in the Marianas sys-tem. Volcanism itself represents release of energy, boththermal and gravitational (buoyant). One can readilysuppose that as the mantle beneath each arc becamesteadily more refractory and less susceptible to havingenergy released from it by volcanism, the energy wouldmanifest itself in other forms.

We suggest, then, that there may have been a type ofpetrological feedback mechanism in the Mariana arcsystem related to the long-continued build-up of arcmagmas in restricted geographical areas. In appropriateconditions of tensional stress in the region, this feed-back was manifested by major reorganizations of themantle beneath the arcs, wherein the thick, refractorymantle that formed beneath them was split open andreplaced by new depleted mantle. This became thesource for both back-arc basin basalts and lavas of thenew island arc (Fig. 12A). We see this simply as a con-sequence of buildup of heat and/or buoyant forces be-neath thickening zones of mantle that were an increas-ing obstacle to release of either form of energy to theearth's surface by means of volcanism. With such amechanism we would expect separate cycles of mantleenrichment in the mobile HYG elements correspondingto the building of each separate arc (Fig. 12B). Forreasons given in this and earlier sections, these cycleswould be superimposed on the evolution of each arcfrom arc tholeiite to calc-alkalic compositions.

With passive back-arc spreading models such as thatof Uyeda and Kanamori (1979), there would be no

necessary connection between the petrological state ofdevelopment of an arc (or the mantle beneath it) and thecessation of back-arc spreading in one basin and itsrenewal in another. Any apparent arc petrologic se-quence would be fortuitous. Conversely, lack of a con-sistent sequence might support such a model. There ispotential difficulty, however, in providing a mechanismby which successive arcs are split precisely down theirlengths in response to the motions of distant plates. Arcrifting involves stretching, thinning, and ultimatelyseparating the arc crust along one of its thickest parts,tearing into deep, solidified crustal magma chambers. Itis not obvious that this should be the weakest part of thecrust, even granting a "welded" back-arc lithosphere.The mechanism proposed here does not depend so muchon a "weak" zone of lithosphere as on a concentrationof forces beneath the arcs to produce rifting. We believethat the calc-alkalic gabbros and metamorphic rocks ofHole 453 in the Mariana Trough, which crystallized andwere metamorphosed at fairly substantial depths (5-10km or more), indicating uplift, imply the action ofbuoyant forces during this mantle reorganization at theoutset of rifting.

Uyeda and Kanamori (1979) attempt to account forvoluminous calc-alkalic lavas in eastern Pacific arcssuch as the Andes by proposing that a regional com-pressive stress pattern keeps back-arc basins from de-veloping, preventing eruption of magmas derived atshallow levels in the mantle (i.e., basalts). If this is in-deed the case, then one could as well say that the com-pressive regional stress regimes simply prevent basinsfrom opening, whether or not there are diapirs. The arcswould be forced to stay near or above any zones of con-centration of heat or buoyant forces. Since they wouldhave to feed upon themselves for release of energy,voluminous calc-alkalic magmatism might be expected.A tensional regional stress pattern in the western Pacificmay therefore contribute to the formation of back-arcbasins but does not necessarily preclude convective dis-turbances in the mantle such as the one depicted inFigure 12. We point out that once the type of convectivedisturbance envisaged has occurred, new and fertilemantle sources are available for generation of both arcand back-arc basalts for some time to come, withoutany additional major overturn or counterflow of themantle.

Future petrologic contributions to hypotheses of Ma-riana arc evolution and back-arc basin origin must focuson evaluation of the mantle sources of arc lavas, par-ticularly those of the readily avalaible calc-alkalic suitesof the older islands and of the earliest stages of vol-canism on the presently active arc. The latter may bepossible only with deep drilling on one of the modernvolcanoes.

ACKNOWLEDGMENTS

We are grateful to the shipboard participants on Leg 60, par-ticularly to Patricia Fryer, Arend Meijer, and Anatoly Sharaskin,whose contributions to this volume and conversations with us madethis synthesis possible. We are also grateful to H. Dick, T. Ishii, N.Marsh, D. Mattey, R. Scott, and G. Zakariadze, whose cruise par-ticipation and data for Initial Reports Volumes 58 and 59 provided thepetrologic context necessary for understanding Leg 60 results. The

904


Palau-KyushuRidge Parece Vela Basin

WestMariana

Ridge

Mariana

Ridgeπ•uge Parece Vela Basm " W — Fore-arc Mariana

VV _-#* * ^ \ JT*^ i I .' i . Trench

K X X X X X X X X X X XI X X X X X X X X X X X vX X X X X X X X X ×X X X X X X X X ×jrX X X X X X X X

xx y x xX X X X X

x x ×

X ×X^

iθ*β

X x x xI x x x|X X X XI X X X

WestMariana

Ridge

MarianaRidge

X X .X X X/

/ i× X X X X•– x x x xV x x X x

>X x X *>\X X X' X XIX X1 X X

\×

crust

y zones ofjf refractory

subarcmantle

generalizeddirectionsof mantleflow

x x x x x x x x x x x

3i x x x x x x x x x x x xr x x x x x x x x x xy

y

/K, Rb,I e tc .I i A / x x x x x xI 1 Ad xx x x xx' I | I X X X X X

/ I I \ X X X X

f A ^

Figure 12. Suggested model for back-arc basin rifting using the Mariana Trough as an example. A. Disruption of refractory subarc mantle andintroduction of new fertile mantle in its place. B. Continued back-arc spreading coupled with simultaneous modification of subarc mantle by (1)extraction of magmas at top and (2) addition of mobile HYG elements from below. Eventually (1) outstrips (2), resulting in refractory, infertilesubarc mantle from which melts are difficult to extract, necessitating return to A.

manuscript benefited from comments by S. Uyeda, S. Bloomer, T.Dixon, and J. Melchior and from discussions with J. W. Hawkins, Jr.,and D. M. Hussong.

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Deep Sea Drilling Project Initial Reports Volume 60 · 2007-05-08 · A SYNTHESIS OF DRILLING RESULTS IN THE SOUTH PHILIPPINE SEA1 ... and interbedded boninites and arc tholeiites