Hawkins, J., Parson, L., Allan, J., et al., 1994Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 135

38. GEOCHEMISTRY AND ORIGIN OF IGNEOUS ROCKSFROM THE OUTER TONGA FOREARC (SITE 841)1

Sherman H. Bloomer,2 Anthony Ewart,3 Janet M. Hergt,4 and Wilfred B. Bryan5

ABSTRACT

Two igneous rock units were recovered at Site 841. More than 200 m of island-arc rhyolites, rhyolitic tuffs, lapilli tuffs, andpumice breccias, divided into five units, compose the basement at the site. These rhyolitic volcanics are late middle Eocene orolder and formed part of a subaerial rhyolitic volcano. These low-K rhyolites were produced by fractional crystallization of amore mafic arc-tholeiitic lava or by dehydration melting of lower crustal arc tholeiites. The Site 841 basement rocks are similarin composition to high-SiO2 lavas in the Eocene basement on 'Eua and crystallized from depleted island-arc-tholeiitic basalts likethose exposed on 'Eua. No evidence is present in the rhyolites, or in the clasts enclosed within them, for boninite series magmasat Site 841. The Site 841 rhyolitic complex bears no resemblance to Cretaceous rhyolites from the Lord Howe Rise, which areenriched in K and incompatible elements. The volcanic rocks at Site 841 are part of a widely distributed Eocene volcanic episodethat marked the earliest phases of subduction in the Tonga region; they are not part of an older crustal fragment.

The second igneous sequence is a series of basaltic dikes and sills that intruded Miocene sediments. These basalts have traceelement abundances and ratios identical to upper Miocene lavas from the Lau Ridge. The Site 841 basalts do not have anygeochemical characteristics that suggest they were generated by unusual thermal conditions in the shallow sub-forearc mantle.They are most reasonably interpreted as intrusions fed by basement dikes propagated from the associated active arc.

No evidence for local serpentinite exposures, like those that are common in the Mariana forearc, was found at Site 841. Theresults from Site 841 provide strong support for hypotheses of forearc evolution that have been advanced for the Izu-Bonin-Mariana system.

INTRODUCTION

Forearcs are an integral part of intraoceanic convergent margins.Their basement contains a record of the early evolution of the sub-duction system (e.g., Hussong and Uyeda, 1982; Bloomer, 1983;Taylor, 1992) and the volcaniclastic sediments covering them docu-ment the tectonic and geochemical evolution of the arc.

Marine geologic studies and Deep Sea Drilling Project (DSDP)and Ocean Drilling Program (ODP) transects of the intraoceanic Izu-Bonin-Mariana (IBM) forearcs have generated four important newideas about the origin and evolution of these terranes. The first, andmost striking, is the recognition that the initial phases of volcanism inthese subduction zones developed nearly synchronously in the middleto late Eocene over a zone up to 300 km wide (Taylor, 1992) andthousands of kilometers long (Stern and Bloomer, 1992; Taylor, 1992).This early, or "infant," arc volcanism was characterized by the eruptionof very depleted boninitic and arc-tholeiitic lava compositions (e.g.,Meijer, 1980; Ishii, 1985; Bloomer, 1987; Murton et al., 1992) anddeveloped in an extensional setting (as evidenced by dikes on Chichi-jima in the Bonin Islands; Umino, 1985), in places possibly to the pointof true seafloor spreading (Stern and Bloomer, 1992). This mode ofarc volcanism is unlike that developed during the "normal" or maturephases of arc activity and is a plausible mechanism for developingsupra-subduction zone ophiolites (what Pearce et al. [1984] termed"pre-arc" ophiolites; Robertson, 1990; Stern and Bloomer, 1992).

A second new observation is that the exposure of Eocene arcbasement immediately adjacent to the axis of the Mariana-Bonin

Hawkins, J., Parson, L., Allan, J., et al., 1994. Proc. ODP, Sci. Results, 135: CollegeStation, TX (Ocean Drilling Program).

2 Department of Earth Sciences, Boston University, Boston, MA 02215, U.S.A.' Department of Geology and Mineralogy, University of Queensland, St. Lucia,

Brisbane, Queensland. 4067, Australia.4 Department of Earth Sciences. The Open University, Walton Hall, Milton Keynes,

MK7 6AA, United Kingdom (present address: School of Earth Sciences, The Universityof Melbourne, Parkville, Victoria, 3052, Australia).

• Department of Geology and Geophysics, Woods Hole Oceanographic Institution,Woods Hole, MA 02543, U.S.A.

Trench requires some subduction erosion since the Eocene (Hussongand Uyeda, 1982; Bloomer, 1983). However, if the Eocene basementdid form by unusual, voluminous volcanism in an extensional envi-ronment, the required amount of erosion may only be 20-50 km(Johnson et al., 1991; Stern and Bloomer, 1992; Taylor, 1992). Clearevidence is also present of episodic, post-Eocene accretion of PacificPlate sediments and crustal fragments to the outer forearc (Karig andRanken, 1983; Bloomer, 1983; Johnson et al., 1991).

The large volume of serpentinite in the outer forearc has been asurprising discovery. The serpentinites occur as diapirs, with associ-ated serpentine mud flows (Fryer et al., 1985; Bloomer, 1983). Pore-water geochemical studies suggest that the fluids responsible for theserpentinization are derived in part by dehydration of the downgoingslab (Haggerty, 1987; Mottl et al., 1989).

Finally, the drilling transects documented episodes of magmaticrejuvenation in supposedly "cold" forearcs. Sills and dikes cuttingMiocene and younger sediments were found in two holes drilled inthe outer Mariana-Bonin forearc (Shipboard Scientific Party, 1990a,1990b). These intrusions were emplaced in the forearc at a time whenthe active arc was well to the west, and indicate either a reheating ofthe forearc or intrusion of long dikes and sills from the active arc. Leg126 also found evidence for rifting and volcanism within the Izu-Bonin forearc during the Oligocene, at a time when the main activearc farther to the west (Taylor, 1992).

These observations in the Izu-Mariana-Bonin system have painteda far different picture of intraoceanic forearcs than existed 11 yr agowhen the results of DSDP Leg 60 documented in-situ Eocene bonin-itic basement in the outer Mariana forearc (Hussong and Uyeda,1982). These forearcs are geologically complex, and are clearly directproducts of subduction, not just trapped pieces of oceanic crust. Theyhave geologic and geochemical similarities to supra-subduction zoneophiolites like those in Troodos and Oman and represent a previouslyunappreciated type of crustal construction.

Before Leg 135, it was not known whether the hypotheses offorearc evolution derived from the IBM studies were generally appli-cable to other intraoceanic forearcs. Seismic and dredging studies inthe Tonga forearc had suggested that arc basement did occur in the

S.H. BLOOMER. A. EWART, J.M. HERGT, W.B. BRYAN

outer forearc (cf. Fisher and Engel, 1969; Bloomer and Fisher, 1987),but there were not good constraints on the age and composition of thisbasement. Site 841 was drilled just west of the outer trench-slopebreak to determine the age and composition of the forearc basementand to examine the evolution of the Tonga forearc as a test of thehypotheses proposed for the Izu-Marian-Bonin system. A secondobjective at the site was to recover the volcaniclastic sediments over-lying the basement to constrain the variation of arc volcanic composi-tions through time and the uplift and subsidence history of the forearc.

Two igneous sequences were recovered from Hole 84IB. Theuppermost is a series of basaltic dikes and sills intruded into Miocenesediments and the lower one is a sequence of low-K rhyolitic tuffs,rhyolites, and pumice breccias that constitute the forearc basement atthis location. This silicic complex is older than late middle Eocene,as there are late middle Eocene calcareous volcanic sandstones, con-taining rhyolitic clasts, normally faulted against the volcanics.

The purpose of this paper is to present detailed geochemical analy-ses of the two major igneous groups from Hole 84IB and to examinethe tectonic implications of their compositions. We will also draw somecomparisons with the evolution of the Izu-Mariana-Bonin forearc.

BACKGROUND

Regional Geology

The Tonga forearc is a part of a complex array of arcs and backarcsthat have developed as a result of the subduction of the Pacific Platebelow the Indo-Australia Plate since at least middle Eocene time (e.g.,Herzer and Exon, 1985). This subduction is presently near perpen-dicular to the trench at a rate of about 16 cm/yr (including an 8 cm/yropening rate in the Lau Basin; Pelletier and Louat, 1989). There aretwo chains of islands parallel to the trench (Fig. 1), separated fromeach other by the deep, sedimented Tofua Trough (Raitt et al., 1955).The eastern chain, the Tonga platform, includes extinct, coralline-capped Eocene to Miocene volcanic islands and the western chain,the Tofua Arc, comprises active submarine and subaerial volcanoeserupting basalt, basaltic andesite, and minor dacite (e.g., Ewart et al.,1977). West of the Tofua Arc lies the actively spreading Lau Basin,which is bounded on the west by the inactive Lau Ridge, a remnantarc (Cole et al., 1990).

The Tonga forearc and platform have existed since Eocene time,when they formed as part of the ancient Melanesian arc, which includedfragments of what are now the Fiji, Lau, Tonga, and New Hebridesarcs. The development of the subduction zones associated with thesearcs was preceded by rifting of the eastern Australian margin in LateCretaceous time; the Lord Howe Rise and the Norfolk Ridge are blocksof this margin that were rafted to the east (Burns and Andrews, 1973).These ridges were separated from the margin by seafloor spreading inthe New Caledonia Basin in late Paleocene time and in the TasmanBasin and Coral Sea through early Eocene time (Bentz, 1974).

The west-dipping subduction that built the Melanesian arc wasinitiated sometime after the early Eocene (Herzer and Exon, 1985).The early growth of the arc was accompanied by spreading in theSouth Fiji Basin, which continued until about 26 Ma (Malahoff et al.,1982). The New Hebrides arc separated from the Lau-Fiji-Tonga arcduring the late Miocene, after the initiation of an east-dipping sub-duction zone beneath the New Hebrides arc (Herzer and Exon, 1985).Volcanism in the Tonga system continued through the Miocene, alongthe Lau Ridge. The Tonga platform accumulated a thick clastic sedi-mentary cover during the Miocene, as it was the forearc east of theactive Lau Ridge (Herzer and Exon, 1985). This forearc sedimentaryfill is cut locally by Miocene dikes and sills (21.3 and 13.9 Ma;Cunningham and Anscombe, 1985). The main phase of this period ofarc volcanic activity is represented by the Lau Group volcanics on theLau Ridge (Cole et al., 1985). A regional unconformity developedover the Tonga platform in the late Miocene or early Pliocene, as aconsequence of uplift associated with the initiation of spreading inthe Lau Basin (Herzer and Exon, 1985;Packham, 1985). This opening

15° S

2O°S

25"S

180°W 17O°W

Figure 1. Location of Site 841 (star). Filled circles show places where volcanicor plutonic rocks of island arc affinities have been dredged from the forearcand trench slope (Fisher and Engel, 1969; Bloomer and Fisher, 1987; Valuer,O'Connor,etal., 1985;Gnibidenkoetal., 1985; Sharaskinetal., 1983;Falloonet al., 1987). Bathymetry in meters.

of the Lau Basin about 6 Ma ago (Parson, Hawkins, Allan, et al., 1992)split the Lau Ridge from the Tonga Platform and led to the develop-ment of the presently active arc to the west of the Tonga platform (Fig.2). Volcanism continued on the Lau Ridge till about 2.4 Ma, as theLau Basin opened. This volcanism produced the Korobasaga Group,a large-ion-lithophile-enriched, slightly alkalic, arc series (Cole et al.,1985). The opening of the Lau Basin has rafted the Tonga platformto the east, and rotated the entire forearc clockwise (see Sager andMacLeod, this volume).

Forearc Geology

The forearc is flanked on the west by the Tonga platform, aninactive frontal arc now covered by a thick sedimentary sequence(Fig. 1). The basement of the Tonga platform is exposed only on theisland of 'Eua (Fig. 1). That basement comprises depleted island-arc -tholeiitic basalts, andesites, and dacites (Ewart and Bryan, 1972;Hawkins and Falvey, 1985; Cunningham and Anscombe, 1985). Thebasement rocks yield radiometric ages of 46 to 40 Ma (Ewart et al.,1977; Duncan et al., 1985) and, locally, are unconformably overlainby upper middle Eocene calcareous conglomerates and breccias (Cun-ningham and Anscombe, 1985). Oligocene lava flows (33-31 Ma)and Miocene dikes (19-17 Ma) indicate later episodes of arc activity(Duncan et al., 1985) that predate the opening of the Lau Basin whenthe Tonga platform was still part of the Lau Ridge.

Seismic reflection profiles suggest that the Eocene basement on'Eua can be traced across the forearc and crops out on the landward

-

GEOCHEMISTRY AND ORIGIN OF IGNEOUS ROCKS

Site 841

Arc crust

Oceanic crust

Vertical exaggeration = 1.5

Figure 2. Schematic cross section of the Tonga forearc showing the relativepositions of 'Ata, the active arc, 'Eua, on the eastern Tonga Ridge, and Site 841on the trench-slope break. Crustal thicknesses are roughly after Raitt et al. (1955);these are not well constrained. Depth to Benioff zone from Billington (1980).

trench slope, just outboard of the trench-slope break (Greene andWong, 1983; Gnibidenko et al, 1985). Fragments of arc-tholeiiticbasalts and andesites, boninites, highly depleted serpentinites, andgabbros have been dredged from a number of places on the landwardslope (Fig. 1) supporting this suggestion that the entire forearc isfloored by arc crust. The age of the outer forearc basement is notconstrained by these dredge samples. Boninite glasses from the north-ern trench have yielded ages of < 10 Ma (A. Crawford, pers. comm.,1992), but gabbroic rocks from the same suite have a 50 Ma age(Acland et al., 1992). Boninitic glasses commonly yield anomalouslyyoung ages (Tsunakawa, 1983); the boninites at the northern trenchslope are in a complicated region at the terminus of the active arc andLau Basin, along a trench-trench transform. They may indeed be young,but their tectonic significance is difficult to assess at present. Thisambiguity concerning the age of the forearc basement made one of theprincipal objectives of Site 841 to determine the basement age in aportion of the forearc that has undergone only normal convergence andis well-removed from the loci of Oligocene and younger arc volcanism.

The exposure of arc volcanic rocks in the mid- and upper-land-ward trench slopes has been taken as evidence for subduction erosionof the forearc (Bloomer and Fisher, 1987; Valuer, O'Connor, et al,1985) since such exposures are unlikely to have been generated insitu. Lonsdale (1986) has postulated that subduction erosion has beenaccelerated by the passage of the Louisville Ridge southward alongthe forearc. There has been, however, minor accretion of seamountfragments and offshore pelagic sediments to the lower landward slopes(Bloomer and Fisher, 1987; Valuer, O'Connor, et al., 1985).

The forearc has been affected by a number of tectonic events.Drilling at Site 840 on the Tonga platform documented two periodsof rapid subsidence on the crest of the ridge, one after 9 Ma, coincidingwith the breakup of the Melanesian Arc, and one dating from 5.25Ma, related to arc rifting during the opening of the Lau Basin (Clift,this volume). The passage of the Louisville Seamount Chain south-ward down the forearc does not appear to have produced any docu-mentable uplift or subsidence in the forearc (Clift, this volume),though it does appear to have produced some faulting and fracturingof the forearc (Gnibidenko et al., 1985; Bloomer et al., 1988).

Site 841

Site 841 was located just west of the trench-slope break (Figs.1-2), 48 km inboard of the axis of the Tonga Trench, 140 km outboardof the active arc island of 'Ata, and about 70 km outboard of theeastern edge of the Tonga platform (the equivalent position of 'Eua).The major objective for the site was to determine the age and compo-sition of the forearc basement, to develop a model for the origin of

2 0 0 ^

6 0 0 -

I Plθist.-Pliocθnθclays, sands

II upper Miocene

clays, silts, sands

II upper Miocene

siltstoneconglomerate

IV lower mid. Miocene

volcanicsandstone

V. Iwr. Olig.-u. Eoc.calc. sandstones.

rhyolitesand

rhyolitictuffs

6 0 0 -

w§o&

<ri

8 0 0 -

2A breccias

2B rhyolitβ

2C pumice breccia

3Atuff3B iapillituff3Ciapilli tuff

3D rhyolite

breccias

4 Iapilli tuff

5 sheared breccias

6 welded

Iapilli

tuffs

8 0 0 -

Figure 3. Schematic stratigraphy of the principal sedimentary and igneous unitsfound at Site 841 (from Shipboard Scientific Party, 1992).

the forearc, and to compare the evolution of the Tonga forearc tohypotheses of forearc evolution derived in the IBM region. A second-ary objective was to recover the overlying sedimentary section, whichcould be used to understand the tectonic history of the forearc and thegeochemical evolution of the adjacent island arc.

Hole 841B penetrated 834.2 mbsf in a water depth of 4810 m. The210.55 m of drilled igneous basement (with about 14% recovery) atthe site is composed of low-K rhyolites, rhyolitic tuffs, breccias,welded tuffs, and Iapilli tuffs (Fig. 3). The welded fragments, theabsence of interbedded sediments and marine fauna, and the lack ofdensity sorting within the pumice breccias indicate a subaerial originfor the rhyolitic complex. Two possibilities were considered for theorigin of this complex (Parson, Hawkins, Allan, et al., 1992). First,this could be an Eocene construct equivalent to the rocks that areexposed on 'Eua. Alternatively, the Site 841 basement could be afragment of a rifted block like the Cretaceous (93.7 ± 1.2 Ma; van derLingen, 1973) Lord Howe Rise, which also comprises high-silicaIapilli tuffs and rhyolites. Preliminary K-Ar dating of one of therhyolites yielded ages of 44 and 45 Ma, and a suspect date of 67 Ma(see McDougall, this volume). Illites from the rhyolitic tuffs yield

SH. BLOOMER. A. EWART. J.M. HERGT, W.B. BRYAN

K-Ar ages of 52.8,51.4, and 31.2 Ma (D. Schöps, pers. comm., 1993).These ages cluster in the Eocene, supporting the former hypothesis(see McDougall, this volume).

Fifty-five m of Eocene and lower Oligocene calcareous volcanicsandstones, with basal ages of about 41.3 Ma (foraminifer Zone PI4;Parson, Hawkins, Allan, et al, 1992), are faulted against the silicicbasement. This fault strikes north to northeast and dips east at about10°-30° (see MacLeod, this volume). Parasitic structures are consis-tent with normal motion on this fault. The fauna in the Eocene sedi-ments indicate very shallow-water (photic zone) depositional condi-tions (see Chaproniere, this volume). In combination with the evi-dence for subaerial eruption of the rhyolitic basement complex, it isclear that the basement at Site 841 has subsided over 5000 m sincethe late Eocene.

The Eocene-Oligocene sediments are overlain by 91 m of lowermiddle Miocene turbiditic sandstones and siltstones; these are uncon-formably overlain by 125 m of upper Miocene siltstones, sandstones,and volcanic conglomerates, some with reworked Eocene fauna, and277 m of late Miocene vitric silts and sandstones. This Miocenesection is intruded by several orthopyroxene-clinopyroxene-plagio-clase-phyric basaltic dikes and sills. Fifty-six m of Pleistocene toPliocene clays, sands, and ashes cap the section. The entire sedimen-tary sequence is cut by microfaults, most of which are normal faults,although minor reverse faults also occur (see MacLeod, this volume).Two major faults cut the sections at 449-458 and 605 mbsf. The upperfaults dips about 60°, has a normal displacement, and may haveaccommodated over 1 km of motion (see MacLeod, this volume). Thelower fault separates the Eocene calcareous sediments from the rhy-olitic basement.

The two igneous groups at the site—the intrusives into the Mio-cene sediments and the pre-late Eocene rhyolitic basement—recordtwo different magmatic pulses in the history of the Tonga forearc.We will examine the geochemistry and significance of each igneousgroup separately.

METHODS

Major and trace element data reported here for igneous rocks fromSite 841 are from the JOIDES Resolution shipboard analyses, BostonUniversity, the Woods Hole Oceanographic Institution, the Universityof Durham, the University of Queensland, Monash University, andthe Australian National University. Shipboard analyses were by X-ray fluorescence (XRF); methods are discussed in Parson, Hawkins,Allan, et al. (1992). Samples were ignited at 1100°C before beinganalyzed. A discussion of shipboard precision and sample contami-nation during grinding are presented in Hergt and Sims (this volume).

Analyses at the Woods Hole Oceanographic Institution are ofwhole-rock powders by XRF. Techniques are discussed in Bryan etal. (this volume). Samples were ignited before being analyzed. Analy-ses from the University of Queensland for major elements and for Rb,Zr, Zn, Y, Ni, Cr, V, Se, Co, Cu, and Zn were also by XRF. Be and Naon the same samples were determined by atomic absorption spec-trometry; some of these samples were analyzed for rare earths byinstrumental neutron activation analysis (INAA) at the AustralianNational University (B.W. Chappell, analyst) and for rare-earth ele-ments (REEs), Cs, Ba, Nb, U, Th, Pb, Ta, Hf, W, TI, and Ga byinductively coupled plasma source mass spectrometry (ICP-MS) atMonash University (W.W. Ahlers, analyst). Samples were ignitedbefore being weighed and analyzed. All analyses were of bulk-rockpowders, except for the analysis of Sample 135-841B-50R-1,5-9 cm,which was of glass separated from a plagioclase-quartz phyric pitch-stone. Details of the analytical methods and errors are included inEwart et al. (this volume).

Analyses from the University of Durham were of glass separatesand were done by ICP-MS. The methodology is given in Hergt andNilsson (this volume). Samples were not ignited before being ana-lyzed and major element analyses were normalized to 100%.

Analyses from Boston University were by inductively coupledplasma emission (ICP) spectrometry. Samples were dried at 110°Cbefore being weighed, but they were not ignited. Loss on ignition(LOI) was not determined for the samples because of the small sizeof many of them. Analyses of samples from Sections 135-841B-57R-I and -59R-1 were of whole-rock samples crushed in diamonitemortars. Glass was separated from a pitchstone from Section 135-841B-47R-2; analyses from Sections 135-841B-23R-4, -25R-4, and-26R-2 are of hand-picked separates from crushed and sieved bulkrock. This separation was done to try to remove the ubiquitous sili-ceous and carbonate veining in the dike and sill samples. The cleanedsamples were powdered in a diamonite mortar, dried at 110°C, andmixed 1:3 with a lithium metaborate flux. The mixtures were fusedat 1000°C for 20 min and the glasses then dissolved in 50 mL of 10%HNO3. These solutions were used for trace element determinations;major elements were determined on 1-mL aliquots from the traceelement solutions, which were diluted with 25 mL of 10% HNO3 andspiked with Li2CO3 as a matrix modifier. Standard curves were cal-culated using aqueous standards and USGS reference rock powders.

A number of small volcanic rock inclusions from the sedimentarysection and from the igneous basement were analyzed at Boston Uni-versity. These inclusions range from large clasts (over 1 g) to small,individual grains picked from crushed sediment or basement samples.The latter were as small as 1 mg. These samples were crushed in adiamonite mortar, or if less than 0.1 g, simply mixed directly withlithium metaborate flux, after being washed and dried. A flux:rockratio of 3:1 was used for samples larger than 0.02 g; these mixtureswere fused and dissolved at 50:1 in dilute nitric acid. Smaller sampleswere mixed with 0.05 g flux and dissolved in 13 mL of solution. Someof the solutions were too dilute to obtain reliable minor and traceelement analyses. Weighing errors for the very small samples pro-duced sums ranging from 90% to 117%. Sums outside this range werediscarded, and sums less than 95% and greater than 105% were thennormalized to 100%.

Analyses of interlaboratory reference powders, prepared aboardship in an agate mill, are listed in Table 1. The analyses from thedifferent laboratories and different techniques generally agree verywell. Also listed are analyses of USGS reference rocks W-2 andBHVO, and an estimate of precision for the Boston University lab,based on replicate analyses of the interlaboratory reference powders.

The Pb-, Nd-, and Sr-isotopic analyses were performed at theOpen University. Techniques are described in Hergt and Hawkes-worth (this volume).

Analyses of rocks from Site 841 are plotted normalized to normalmid-ocean-ridge basalt (N-MORB) compositions in several figures.The normalizing values used are from Sun and McDonough (1989).

POST-LATE MIOCENE DIKES AND SILLS (UNIT 1)

Description

Nine intervals of basalt, ranging in thickness from 7 cm to 18 m,were drilled within Miocene sediments between 324.76 and 497.68mbsf. These basalts are petrographically similar and were denoted asSubunits 1A through IL The basalt-sediment contacts, when recov-ered, are near horizontal to steeply dipping. Contacts include glassymargins on basalts, brecciated hyaloclastite zones, and baked sedi-ments, suggesting that the units are dikes and sills rather than flows.Subunits 1A-1G are intruded into upper Miocene sediments. Subunit1H occurs as broken fragments at the base of a major fault that juxta-poses upper Miocene and lower middle Miocene sediments. SubunitII is within lower mid-Miocene sediments.

Formation MicroScanner (FMS) records show that the distribu-tion of the basaltic sills and dikes is more complex than shown by therecovered core. For example, Subunit 1D was defined as a 17-m-thicksill between 383 and 400 mbsf. The FMS images show that the unitactually occurs between 380 and 396 mbsf as 11 individual intrusionswith thicknesses from 5 to 134 cm. The contacts of the dikes strike

GEOCHEMISTRY AND ORIGIN OF IGNEOUS ROCKS

1 0 0 -

fflerO

I1 0 -

1 -

0.1

A

+O×Δπ•

•—i 1 1 1 1 1 —

Eua, Eocene

Lau, Miocene

Korobasaga, Pliocene

Tofua Arc, Recent

Ata Island, Recent

Hole 841B, Unit 1

×

Rb Ba Nb La Ce Sr Zr TiO

Figure 4. Comparison of compositions of average Unit 1 basalt with arcvolcanic units in the Tonga region with 52%-56% SiO2, normalized to N-MORB (values from Sun and McDonough, 1989). Analyses from 'Eua fromCunningham and Anscombe (1985) and Hawkins and Falvey (1985); Lau andKorobasaga analyses from Cole et al. (1985); Tofua Arc (active arc) averagecompositions from Ewart and Hawkesworth (1987), and average for 'AtaIsland (active arc) from Valuer, Stevenson, Scholl et al. (1985).

211° to 31° and, with one exception, dip toward the trench at 2° to59° (see MacLeod, this volume; dips are calculated after rotation ofsedimentary layering to horizontal).

The sills and dikes are orthopyroxene-clinopyroxene-plagioclasephyric basalts. Plagioclase is the dominant phenocryst, constituting4-12 modal %. Clinopyroxene ranges from trace amounts to 5 modal%and up to 2 modal% orthopyroxene was found, though it is absent insome samples. The basalts have 3%-5% vesicles, which tend to be con-centrated toward the contacts of the intrusions. A detailed petrographyof each subunit can be found in Parson, Hawkins, Allan, et al. (1992).

Most of the samples have been affected by a low-temperaturealteration. Siliceous, carbonate, K-feldspar, and thaumasite veins arecommon. Minor amounts of sulfides, principally pyrite, are devel-oped in some of the veins. The groundmass is typically 5%-50%replaced by clays, chlorite, and zeolites. The basalt in Subunit II isunusual in its lack of alteration.

Geochemistry

Eleven different sill or dike samples were analyzed; all are fromthe intrusions in upper Miocene sediments. SiO2 ranges from 50% to54%; the shipboard analyses appear to be high in comparison withthe Queensland and Boston University analyses, even with correc-tions for LOI (Table 2). The average composition for the intrusions is51.7% SiO2, including the shipboard analyses, and the samples shouldbe referred to as basalts (Le Bas et al., 1986).

All of the samples have essentially identical compositions, withininterlaboratory errors (Tables 2-3). The samples are relatively enrichedin Ba, K, and Sr; they are depleted in Nb and Ta; and they have slightlight-rare-earth-element (LREE) depletions (Fig. 4 and Table 2). Theirvery similar compositions support the shipboard conclusion, based onpetrography, that the individual basalts are all part of a single intrusiveevent. There are no analyses from Subunits IC, 1G, 1H, or II, but theirigneous mineralogy and textures are identical to the analyzed basalts;we interpret them to be part of the same intrusive episode.

Three of the basalt samples have Ba concentrations greater than250 ppm, and Ba in general varies more than other elements (Table 2).The three high-Ba samples are all noted as being extensively veined;

38.9

Q_

sPJ

21

37.7

• Site 841-dikes* Site 841-rhyolitθπ Ataθ'Eua

Tafahi andNiuatoputapu

18.1 18.9

15.65

Q_

15.45

18.1 206Pb/^Pb18.9

0.51315

T32

T3»

14

3f

0.51285

-\Sitθ 834

\Λ \ éPacific^V/

' MORB X ,Lau Group

--

Modern Lau Basin

\<^ o

IndianMORB^ • ^ ^ ,

1

. I t Korobasaga Group

‰r~^ Tofua \ ~y V Arc X

Tafahi and " " / V öNiuatoputapu

i i • i i

0.7025 0.7029 _7 . . 0.70378 7 Sr / 8 6 Sr

0.7041

Figure 5. Isotopic compositions of Site 841 igneous rocks compared to fieldsfor Pacific and Indian MORB, modern Lau Basin Basalts, the southern TongaArc (Tofua Arc), Tafahi Island (northern Tonga Arc), and Pacific and TongaTrench sediments. For a discussion of the analytical methods and sources ofreference fields, see Hergt and Nilsson (this volume). Dark-shaded Sr-Nd field= Lau Group volcanics, and light-shaded field = Korobasaga Group (both fromCole et al., 1990).

the high Ba concentrations are likely a consequence of alteration,perhaps from the development of authigenic alkali feldspar veins.

The compositions of the basalt intrusives are typical for arc vol-canics of all ages from the Tonga system (Fig. 4 and Table 3). Theyhave the same element enrichments and depletions and fall well withinthe range of arc compositions of similar SiO2. Their isotopic ratiosare within the range of modern Tonga Arc and Lau Basin basalts(Table 4 and Fig. 5) and do not show a pronounced sediment contami-nation signature (see Hergt and Nilsson, this volume). Their elementaland isotopic compositions indicate that the Unit 1 basalts, like mostarc lavas in the Tonga region, are fractionates of mantle-derived melts.

The Unit 1 basalts are not as depleted in large-ion-lithophile ele-ments (LILEs) and high-field-strength elements (HFSEs) as are theEocene lavas on 'Eua (Table 3). Neither are the basalts as depleted as

629

S.H. BLOOMER, A. EWART, J.M. HERGT, W.B. BRYAN

Table 1. Comparison of major and trace element analyses of shipboard powders (ground in agate) used for interlaboratory comparison.

Sample

LaboratoryMethod

BUICP

Major elements (wt%):SiO2TiO2A12O3Fe2O3yFeOMnOMgOCaONa2O

P 2 O 5SumLOI

52.841.04

15.4910.54*

0.1626.15

10.602.260.630.15

99.85—

Trace elements (ppm):ZnNiCrVCuZrSeYSrBaRbNbCsUThPbCoTaHfWGaTILaCePrNdSmEuGdTbDyHoErTinYbLu

876907

26S1111003623

204172NDND

—

—————

11

—

———

—

W-2

Reported

52.811.06

15.4910.86*

0.1676.39

10.892.210.630.14

807092

2601051003623

190175NDND

10.5

BHVO

BUICP

49.162.90

13.4712.29*

0.1677.21

11.312.250.510.29

99.30—

Ill1282983151381753127

398121NDND

15

—

Reported

49.902.69

13.8512.23*

0.1707.31

11.332.290.540.28

1051203003201401803127

420135NDND

17

Precision ofreplicates

BUICP

0.320.010.060.03

0.0010.070.090.040.010.003

—

5.50.11.11.40.90.70.30.11.60.9

NDND

0.3

135-834B-33R-2, 105-110 cm

QueenslandXRF, AA

49.321.27

17.773.804.530.138.17

11.803.190.070.10

100.15f3.16

6488

31318756953123.7

156100.41.6

—

34

—

—

—

—

WHOIXRF

49.561.23

18.018.47*

0.148.08

12.272.770.080.13

99.97t—

57100264208

6597

—26

156142

46

—

BUICP

48.641.24

17.708.57*

0.1317.78

11.642.940.0750.13

98.85—

7089

326210

74923426

16813

———

—————4

——

——

—

DurhamICP-MS

—1.18——

0.142—————

—

65100301211

738135.924.8

18140.41.30.010.060.070.5

39.60.121.740.02

170.043.149.821.618.992.811.063.710.653.840.862.570.372.10.43

135-836B-5R-2, 65-74 cm

QueenslandXRF,AA

49.450.83

15.953.585.280.159.59

13.512.170.050.04

100.60t3.21

6995

25424810839.53016.6

1361910.95

—

0.9441—————

———

————

—

WHOI BUXRF ICP

49.30 48.210.78 0.83

15.82 15.289.14* 9.49*

0.14 0.1409.87 9.44

14.06 13.251.82 2.100.06 0.0450.06 0.06

100.22t 98.33— —

62 70107 98213 314239 266110 10841 37

47.519 19

137 1362 17.52 —— —— —

———

49 —__ —— —— —— —— —— 2

—_ —— —— —

——

— _— —— —— —

——

— —

DurhamICP-MS

—0.81

——

0.139—————

—

64106317271104344919

14370.60.390.010.050.050.65

45.10.050.850.02

140.081.314.260.724.181.70.632.230.452.80.591.850.271.660.37

the lavas of the presently active arc (Table 3). The Site 841 intrusivesbear the closest resemblance to lavas of the Lau Group, a sequenceof 14- to 5.4-Ma basalts to dacites from the Lau Ridge remnant arc(Cole et al., 1985; Table 3). The least altered basalts have Y/Zr andBa/Zr (the most diagnostic difference between arc units of differentages in the Tonga region) very close to those of the Lau Group lavas(Fig. 6). The Ba/La ratios in the least altered samples range from 24to 43; the Lau Group has Ba/La ratios of 10 to 37, whereas the activeTofua Arc has Ba/La ratios of 36 to 160 (Cole et al., 1990). Unit 1Sr/Nd values are 21 to 30, overlapping the range of the Lau Group(23 to 25), but they are much lower than lavas of the active arc (34 to116; Cole et al., 1990). The La/Zr of Unit 1 lavas are like those of theLau Group, although their La and Zr abundances are at the low endof the range for those lavas (Fig. 6). The Unit 1 basalts have Sr andNd isotopic compositions within the range for lavas of the Koro-basaga Group, a lava series that developed late in the evolution of theLau Ridge (4.4-2.4 Ma) as the Lau Basin opened (Cole et al., 1985;Fig. 5). However, the Unit 1 lavas are not anywhere near as enrichedin LILEs as the Korobasaga Group (Fig. 3 and Table 3).

The element abundances and ratios show a striking similaritybetween the Unit 1 basalt intrusives and the late Miocene Lau Group.

This similarity is consistent with the apparent age of the intrusives. Ifall of the dikes and sills are indeed part of a single intrusive event,that event must be late Miocene (foraminifer Zone NI6 to Subzone17a; 12-6 Ma; Parson, Hawkins, Allan, et al., 1992) or younger.

Discussion

The discovery of upper Miocene or younger sills and dikes in theouter forearc is surprising. Site 841 is now over 140 km from thepresently active Tofua arc, and at least 75 km from what may havebeen the axis of late Miocene volcanism (near 'Eua; Fig. 2). The outerparts of the forearc are generally assumed to be relatively cold, andfar removed from arc volcanic activity since the late Eocene.

However, this is not the first report of such intrusives in forearcbasin sediments. A diabase sill of basaltic andesite composition,intruded into mid-Miocene sediments, was cored at Site 793 in theBonin forearc, 75 km east of the presently active arc (ShipboardScientific Party, 1990b). The diabase has baked sediment along itsmargin, clearly showing that it was intruded into the sediments, andtherefore is younger than mid-Miocene. A basalt interval was recov-ered within late Pliocene sediments at Site 781 in the Mariana forearc;

GEOCHEMISTRY AND ORIGIN OF IGNEOUS ROCKS

Table 1 (continued).

Sample:

Laboratory:Method:

135-835B-7R-2, 75-84 cm

QueenslancXRF, AA

Major elements (wt%):SiO2TiO2A12O3Fe2O3

FeOMnOMgOCaONa2OK2OP2O5SumLOI

50.801.04

15.524.155.750.138.19

11.432.590.120.07

99.79t3.00

Trace elements (ppm):ZnNiCrVCuZrSeYSrBaRbNbCsUThPbCoTaHfWGaTILaCePrNdSmEuGdTbDyHoErTmYbLu

9758

108289

85483321.9

12346

1.90.7

———

0.9836———————————————————

[ WHOIXRF

51.770.85

16.379.12*—

0.138.31

12.862.030.130.08

100.82t—

7679

1212467843

—24

13146

3—————

49———————————————————

BUICP

50.620.86

16.209.26*—0.1217.98

12.712.200.080.10

100.13—

10674

20425275383821

12834—

———————

——1.5——

————

———

DurhamICP-MS

—0.89——

0.125—————

—

9376

193276

7233.839.220.6

13228.3

1.90.450.010.130.120.79

39.80.041.020.06

160.041.574.560.754.671.790.772.70.533.280.762.140.311.910.39

135-836A-3H-3, 33^3 cm

QueenslandXRF, AA

55.511.35

14.952.949.450.203.988.453.390.230.13

100.58f1.68

1131110

431109724030

136652.71.6

31

—

WHOXRF

55.291.34

14.8413.23*

0.214.108.632.890.260.14

99.73t—

972110

42211073

34147586

49

—

1 BUICP

54.341.32

14.6013.14*

0.203.998.423.000.240.16

99.42—

82188

399117683632

15054

2.5

—

DurhamICP-MS

—1.35——

0.20—————

—

122154.9

4121117339.236

156544.10.990.140.090.161.23

34.20.082.020.03

180.102.728.321.368.513.091.11

48.70.855.41.243.770.523.360.73

135-836A-3H-4

QueenslandXRF, AA

57.031.21

14.702.788.930.263.658.123.260.280.13

100.35t2.06

112134

38912076.94032.5

134794.42.1

—

—1.4

34——

————

———

——

WHOIXRF

56.671.19

14.6311.35*

—0.263.828.333.010.290.15

99.70t—

96228

36712477

—36

147857

————

46——

————

—

——

———

, 88-100 cm

BUICP

55.601.16

14.4012.46*

—.249

3.647.963.110.280.17

99.02—

114187

354132723634

14865

—

———————

—3——

————

————

DurhamICP-MS

—1.21

———0.254—————

—

107184.2

37712774.738.936.7

15460.34.11.190.120.130.231.36

34.50.122.190.10

180.372.938.751.479.053.421.184.800.885.671.234.000.603.600.78

no contacts were recovered, and the sample could be a sill or a flow(Shipboard Scientific Party, 1990a). Site 781 is about 100 km east ofthe Mariana active arc.

The recovery of mafic intrusions or flows within forearc basinsediments in three different forearcs indicates that magmatic activitymay be far more common in forearcs than has been recognized. Thereare two possible explanations for the origin of these intrusives. First,they may represent melts generated in the mantle immediately be-neath the forearc. This has been suggested for the Site 781 flow(Marlow et al., 1992). Alternatively, these mafic magmas may beinjected into the forearc from the associated active arc along dikes orsills. Two arguments need to be examined in determining whichexplanation is the more likely. The first concerns the chemical com-position of the intrusives, the second the physical constraints on dikeinjection and melt generation.

The Site 781 intrusives, which occur within late Miocene sedi-ments, are strikingly similar to lavas that were erupting from the activearc in the late Miocene. The intrusives, in all regards, are typicalisland-arc lavas. The same is true of sills and flows from Sites 781and 793 (Fig. 7). Each of the mafic units has element abundances andratios typical of "main" arc eruptives and, in particular, are similar to

lavas erupted from the arc active when the units were likely intruded(Miocene for Site 793; late Pliocene-Pleistocene for Site 781).

If the Lau Ridge is restored against the Tonga platform, where itwould have been in the Miocene, it appears that the crest of the activearc had to be over 100 km from the trench. There are Miocene dikeson 'Eua, but no substantial Miocene volcanic section, which suggeststhat 'Eua was near the eastern side of the active arc province. Giventhat geometry, Site 841 must have occupied a position in the Miocenemuch like that in which it presently occurs. If it was less than 60 kmfrom the trench the surface of the subducting slab was at most 40 kmbelow the site (taking a slab geometry similar to that which nowexists). If the Unit 1 intrusives were generated in the mantle, they hadto be made at very shallow depths, presumably at unusually lowtemperatures, between the top of the slab and the base of the crust.This mantle might be expected to be extremely depleted because ofolder episodes of melt generation. Basaltic melts from that thin cornerwedge should carry some distinctive chemical signatures: a greatersilica saturation because of the low pressure of melting, very highvolatile contents because of the high water concentrations needed tomelt the cold mantle, signs of low degrees of melting because of thecool thermal regime, and very depleted incompatible element com-

631

SH. BLOOMER. A. EWART, J.M. HERGT, W.B. BRYAN

Table 1 (continued).

Sample:

Laboratory:Method:

135-834B-13R-1,

QueenslandXRF, AA

Major elements (wt%):SiO 2

TiO 2

A1 2O 3

F e 2 O 3

FeOMnOMgOCaONa 2 OK 2OP2O5SumLO I

49.971.18

16.843.794.910.158.82

11.552.890.130.10

100.33t4.90

Trace elements (ppm):ZnNiCrVCuZrSeYSrBaRb1MbCsUThPbCoTaHfWGaTILaCePT

\TdSmEuGdTbDyHoErTmYbLu

7105

202210

68S13021.9

16437

1.21.2

———

1.8135

———

———

——————

—

WHOIXRF

50.121.13

16.908.88*

0.148.74

12.002.660.140.12

100.02t—

02106189225

7586

2616146

2

———

48

———

——

—

——

—

130-131 cm

BUICP

48.921.14

16.489.02*

0.1338.44

11.322.800.120.12

98.48,—

7490

243237

847S

3723

17537—

———

—

—

3

—

—

—

—

DurhamICP-MS

—1.15

——

0.136—

———

—

7993

241252

8173.736.823.2

17828.1

1.10.960.030.050.101.31

39.10.091.70.03

160.162.638.321.267.682.671.033.450.613.710.82.30.351.970.42

Notes: Values marked with an asterisk (*) represent total iron asFe2θ3; a dash (—) indicates values that were not determined;a dagger (t) represents major element analyses performed ona volatile-free basis. BU = Boston University, WHOI = WoodsHole Oceanographic Institution, ICP = inductively coupledplasma source spectrometry, ICP-MS = ICP mass spectrome-try, AA = atomic absorption, and XRF = X-ray fluorescence.ND = no data.

positions because of the nature of the mantle. These are, in fact, allcharacteristic of the boninitic volcanic rocks that constitute an impor-tant phase of early arc volcanism in the Izu-Bonin-Marianas (Arculuset al., 1992) and that are generated at low pressures (<6 kb; van derLaan et al., 1989). Similar geometric arguments can be made formelting in the mantle beneath Sites 781 and 793.

None of the forearc basin intrusives in the Tonga, Mariana, orBonin forearcs have such a geochemical signature. They are all, infact, exactly like the lavas erupted from their associated active arcs.An interesting contrast is provided by the sill at Site 793 in theIzu-Bonin forearc and the Oligocene basement there. The sill hasTi/Zr =110, Ba/Ce = 8 and K/Ba = 61, all values typical of the activearc. The Oligocene basement, however, has Ti/Zr = 63, Ba/Ce = 4.6,

and K/Ba = 132 to 600. These values are characteristic of the depletedboninitic and tholeiitic lavas that form the Eocene forearc basement,rocks formed by shallow melting of a depleted, hydrous mantle wedge(Murton et al., 1992; Taylor et al., 1992). The Oligocene basement atSite 793, in fact, formed by rifting and magmatism within the forearc,so it is not surprising that basement has the same geochemical sig-nature as the Eocene volcanics (Taylor, 1992). The sill within the Mio-cene sediments, however, has a chemical signature much more likethe Miocene and recent active arc.

The chemical evidence, then, is most consistent with the derivationof the forearc basin dikes and sills in both the Tonga and Mariana-Boninforearcs from the active arc. Do the physical constraints on melt gener-ation and dike injection give the same answer? Melt generation in adepleted mantle wedge, even under water-saturated conditions, willrequire temperatures in excess of 1000°C (van der Laan et al., 1989).The sites of the Miocene and Pliocene or younger dike injections are45 to 100 km from the trench. There was a well-established island arcto the west of the forearc when each of the dikes or flows was formed;the implication is that there existed a "normal," steady-state subductionzone. In such a case, temperatures at 50 km depth, 40-60 km behindthe trench, 20 Ma after the beginning of subduction, should be on theorder of 500°C (Fryer and Fryer, 1985). This requires a substantialthermal anomaly to generate melts in situ in the corner of the mantlewedge. Ridge subduction could provide a mechanism for such ananomaly, but there is no other evidence suggesting the subduction ofa ridge in any of the three forearcs during the appropriate time period.

If, on the other hand, the dikes and flows are derived from theassociated active arc, they have been injected very large distances intothe forearc. Site 841 was likely at least 70 km, and perhaps over 100km, from the Miocene arc. Site 781 is 100 km from the active arc;Site 793 is 75 km from the active arc and probably occupied aboutthe same position relative to the Miocene arc (Taylor, 1992). Theseare not unreasonable distance for large basaltic dikes and sills. ThePalisades sill is 300 m thick and 80 km long, the Whin Sill in England75 m thick and 125 km long, and Icelandic dikes are not uncommonlytens of meters wide and over 100 km long (Philpotts, 1990). TheHigganum dike in New England runs nearly 250 km through thebasement (Philpotts and Martello, 1986). The emplacement of dikesis likely to be facilitated by fracturing of the forearc. All of theseintraoceanic forearcs are in extension (Mrozowski and Hayes, 1980;HussongandUyeda, 1982; Fryer and Fryer, 1985;Taylor, 1992). Boththe Tonga and the Mariana forearcs are cut by both trench-paralleland trench-perpendicular faults and fractures (Fryer and Fryer, 1985;Herzer and Exon, 1985; Bloomer et al., 1988; MacLeod, this volume).These fractures would facilitate the intrusion of low-viscosity basalticmelts outward from the arc.

It is unlikely that the sills, dikes, and flows sampled have beenemplaced all the way from the arc through the forearc basin sediments.Typical width/length ratios for shallow dikes emplaced by hydraulicfracture in consolidated sediments are I0"2 to I0"3 (Fedotov, 1978).The 18-m-thick dike sequence at Site 841 could only have traveled 18km in such a case. Also, injection through wet sediments would haveproduced very high cooling rates and shorter propagation lengths.However, in the deeper crust and in higher density rocks, basaltic dikestypically have length-to-width ratios of I0"3 to lO~4, or for the same18-m-thick unit distances of 18-180 km (Fedotov, 1978). The preex-isting fractures in the forearc would only increase the distance suchdikes could be injected.

We suggest that the dikes and sills found in Site 841, and in similarpositions in the Mariana-Bonin forearc, are derived from their asso-ciated active arc. They are fed by large dikes propagated through theforearc basement; the sampled intervals are intrusive bodies that havebeen fed from that deeper seated large dike. The intersection ofmultiple sets of faults, like those described on the Tonga platform(Herzer and Exon, 1985), could provide pathways for vertical move-ment of melt from a central feeder dike or sill. A very similar geometryhas been described for en-echelon dike sets in New England, where

632

GEOCHEMISTRY AND ORIGIN OF IGNEOUS ROCKS

i

Korobasagaandθsitβs

Korobasagabasalts

-óO

Tofua

Lau basalts

'Eua

10

3

0.3 0.5Y/Zr

o Averages, arcvolcanic units

Hole 841B,Unit 1

Korobasagabasalts

Korobasagaandesites

0.7 50 100Zr (ppm)

Figure 6. Comparison of Unit 1 basalts and other arc units in the Tonga region. The high-Ba Unit 1 samples are all noted as extensively veined. Thereference fields are shown as averages (dots) and standard deviations (bars) for published analyses of the particular groups. Sources as in Figure 3. 'Euavolcanic rocks are Eocene in age, Lau Group basalts are Miocene, Korobasaga Group rocks are Pliocene, and rocks from 'Ata and the Tofua Arc arePleistocene to Recent.

individual 4 to 10 km near-surface dikes are fed by the deeper andregionally extensive Higganum dike (Philpotts and Martello, 1986).This interpretation is mechanically reasonable and is more consistentwith the chemistry of the intrusives than is an origin by in-situ meltingof the mantle wedge beneath the forearc sites.

PRE-LATE EOCENE SILICIC BASEMENT(UNITS 2-6)

Description

At Site 841,209.55 m of volcanic basement (605.05 to 815.6 mbsf)were drilled; these rocks were divided into five units (Fig. 3). Unit 2,at the top of the rhyolitic basement, is 41.9 m of rhyolites and pumicebreccias, which are divided into three subunits. Subunit 2A is a 7.25-m-thick fault zone, developed within rhyolites, that separates the base-ment from the overlying calcareous sandstones. Subunit 2B (19.51 mthick) is a quartz-plagioclase phyric rhyolite flow or dome; it is theleast altered unit within the basement complex (Figs. 8A-8B). Therhyolite averages 14% modal Plagioclase (An48^0), 7% quartz, 0.6%clinopyroxene (augite, En35Wθ40Fs25 to En32Wθ39Fs29), 0.6% orthopy-roxene (hypersthene-ferrohypersthene En5|Wo3Fs46 to En^WfyFs^),trace amounts of hornblende, and 0.5% opaques (magnetite and il-menite), with 8% vesicles in a hydrated, perlitic groundmass. Repre-sentative analyses of the pyroxene and opaque phases are in Table 5.Locally within the unit, there are transitions to a poorly welded rhy-olitic pumice. A similar poorly welded pumice breccia constitutes the22.4-m-thick Subunit 2C.

Unit 3 (25.6 m) includes rhyolitic lapilli tuffs (Subunit 3A), weldedrhyolitic lapilli tuffs (Subunit 3B), laminated crystal to lapilli tuffs

(Subunit 3C), and rhyolite breccias (Subunit 3D). Subunit 3B isbounded by clearly erosive contacts. The tuffs in Subunit 3C haveconvoluted and steeply dipping laminations that are cut by clasticdikes of sand to silt sized tuffaceous sediment. The tuffs coarsen inthe lower 50 cm of the unit, and there is a concentration of large (2.5cm) lithic clasts at the base. The laminated unit could be a surgedeposit or a reworked ash flow. Subunit 3D includes angular frag-ments of rhyolite, some of which are welded, with pumice andsiltstone clasts. The rhyolites have 3%-5% quartz phenocrysts, 1%-5% Plagioclase, and <l % magnetite in a granular matrix of quartz andfeldspar. Symplectic plagioclase-quartz overgrowths on the pheno-crysts are common in the rhyolite clasts.

Unit 4 is a 10.2-m-thick welded rhyolitic lapilli tuff with clasts ofpumice, rhyolite, and basaltic andesite. The flattened pumice clastsare more concentrated toward the top of the unit, whereas the lithicclasts become more abundant, and larger, toward the base. The baseof the unit is sheared and is gradational into Unit 5, an 18.8-m-thicksequence of sheared pumice breccias. Unit 6, at the base of the hole,is at least 39 m thick. Recovery in the unit was less than 3% and it isdifficult to characterize its lithology. Recovered fragments includewelded tuffs, pumices, and mafic clasts. There are also mafic inclu-sions in some of the tuffs.

Alteration is pervasive throughout the basement cores, with theexception of the rhyolites in Subunit 2B. Illite, kaolinite, chlorite,pyrite, calcite, quartz and albite all are variously developed as replace-ments of the groundmass and as overgrowths and alterations of phe-nocrysts (see Schöps and Herzig, this volume). More detailed petro-graphic descriptions of each igneous unit are in Parson, Hawkins,Allan, et al. (1992).

633

S.H. BLOOMER, A. EWART, J.M. HERGT, W.B. BRYAN

Table 2. Analyses of samples from Hole 841B Unit 1.

Core:Interval (cm):Subunit:Laboratory:

18R-1137-140

1ADurham

18R-228-32

1AShip

23R-417-21

IBBU

23R-48-12

IBShip

25R-1105-109

IDShip

25R-421-22

IDDurham

25R-438^6

IDBU

25R-442-46

IDQueensland

26R-288-91

IEShip

26R-281-86

IEBU

26R-284-86

IFShip

Major elements (wt%):SiO2TiO2A12°3Fe2O3*MnOMgOCaONa2OK2OP 2 O 5

TotalLOI

Trace elements (ppm):ZnNiCrVCuZrSeYSrBaRbNbCsUThPbCoTaHfWTIBeLaCePrNdSmHuGdTbDyHoErTmYbLu

——————————

—

——

—

57

25.6241

76.12.510.630.010.160.250.97

0.041.640.06

—3.139.171.478.692.811.013.970.673.940.872.580.42.40.51

54.131.01

16.0412.530.224.559.522.780.160.15

101.09t1.29

1232014

34512271

23216

9021

—

————

—

50.660.98

15.6811.960.244.549.323.230.150.16

96.93**—

1482026

358114643925

24879

—

3

————

—

52.591.06

16.8313.060.274.76

10.262.840.20.17

102.04t1.01

1732217

37611456

24229256

51

————

—

52.591.05

16.6213.130.244.8

10.182.90.090.15

101.74t0.72

1322112

38712874

24230125

11

—

————

—

—

—

64.3

26.1256123.6

1.40.67

BDL0.180.251.67

0.121.680.07

3.249.081.548.422.840.973.950.643.980.912.690.412.480.48

49.661.04

16.8012.510.234.649.543.600.170.18

98.36**—

1282228

380133664026

250124

——

——

—

3.1

———————————

50.25 52.581.21 1.08

17.36 16.4613.43 13.080.26 0.254.98 4.939.69 9.613.26 3.280.17 0.450.15 0.17

100.76t 101.16t3.39 1.55

186 16618 2026 (24) 17

416 382115 11769 7643 (38) —23.5 25

225 227145 310

2 40.77 10.14 —0.180.15 —4.58 —

37 —0.06 —2.90 —0.09 —1.44 —0.18 —4.17(3.4) —

11.7(9.3) —2.05 —

10.8 (7.8) —3.46 (2.6) —1.16 (.99) —4.08 (3.6) —0.72 (0.6) —4.36 —0.93(0.8) —2.63 —0.38 —2.52 (2.6) —0.41 (0.4) —

51.031.05

16.1512.780.234.809.233.100.920.18

99.47**—

1462227

386140.56740.526.5

241294.5

—————————

——3.4—————————————

51.851.03

15.6412.420.24.369.332.940.260.16

100.98t0.87

901812

35113274

—25

213140

42

————————————————————————

Notes: Single asterisk (*) represents total iron as Fe2O3; double asterisk (**) indicates that the sum does not include volatiles; dagger (t) indicates major elementanalyses perfoimed on a volatile-free basis; dashes (—) indicate values that were not determined. Values in parentheses were determined by instrumentalneutron activation from Australian National University, B.W. Chappell, analyst; other analyses as in text. BDL = below detection level.

Subaerial vs. Subaqueous Origin

The environment of eruption of the Site 841 rhyolites has animportant bearing on the tectonic history of the forearc. Clear evi-dence of welding exists in some of the tuffs (Fig. 8C), indicating asubaerial or very shallow submarine origin. No interbedded sedi-ments are present in any of the units, and all of the units are completelydevoid of microfossils. There is a very large volume of pumice withinthe section, which indicates eruption subaerially or in very shallowwater, or which requires very high volatile contents. However, thephenocryst assemblage in the rhyolites is anhydrous, implying lowconcentrations of water or temperatures too high for the crystal-lization of biotite or amphibole. More important, many of the pumicebreccias are poorly sorted; they include abundant low-density pumiceclasts intimately mixed with higher density rhyolite and andesiticclasts. The pumice clasts would normally be separated from the lithicclasts by flotation in a submarine eruption. Only Unit 4 shows evi-dence of density sorting but is still poorly sorted.

The volcanic facies at Site 841 are similar to facies associated withsubaerial eruptions in places like the Taupo volcanic zone (Ewart,1968). Unit 2 is similar to late-stage ash flow deposits associated withplagioclase-quartz rhyolite domes (Ewart, 1968). Poorly consolidatedash-flow deposits (producing pumice breccias), ignimbrites, and pla-gioclase-quartz rhyolites are a common association throughout theTaupo zone (Ewart, 1968). We consider the textures of the volcanicunits at Site 841 to be indicative of subaerial eruption. This is con-sistent with the interpretation that the environment of deposition forthe calcareous sandstones above the rhyolites was in the photic zone(see Chaproniere, this volume). These sandstones contain fragmentsof rhyolitic debris like those found in the basement rocks.

Geochemistry

Most of the rocks recovered from Site 841 are tuffaceous orpumiceous and are extensively altered. The rhyolites of Subunit 2B,and rhyolite clasts within Subunits 3C-3D and Unit 4, provide the

634

GEOCHEMISTRY AND ORIGIN OF IGNEOUS ROCKS

Table 3. Average compositions of Site 841 Unit 1 lavas compared with compositions of arc lavas of different ages fromthe Tonga region.

Location:

Source:

Major elements (wt%):SiO2TiO2

A12O3

Fe 2 O 3 *MnOMgOCaON a ^K2OP 2 θ 5Sum

Trace elements (ppm):ZnNiCrVCuZrSeYSrBaRbNbPbLaCe

Ratios:Ba/ZrTi/ZrY/ZrBa/La

Hole 84IBUnitl

Table 1Average

51.521.05

16.3312.720.244.679.593.090.280.16

99.65

1442020

376124674025

234157

2.7412.413.499.81

2.394

0.3745

'Eua

Cunninghamand

Anscombe(1985)

53.950.77

16.359.660.184.526.153.330.400.06

95.37

758

31280

22—4021

14072—

1.24—

58

'Eua

Hawkins andFalvey(1985)

54.870.71

18.3010.320.215.368.072.580.200.08

100.70

49

181

42

2611537

1

—

0.9101

0.62—

Lau Group

Cole et al.(1985)

55.161.14

18.089.370.173.628.543.300.840.25

100.47

9014

45973035

331138

11.2226

19

1.470

0.3623

KorobasagaGroup

Cole et al.(1985)

55.760.73

18.587.440.154.418.812.991.240.29

100.40

801627

17947662120

533309

3724.39

20

4.766

0.3034

Tonga Arc

Ewart andHawkesworth

(1987)

53.210.51

17.0910.920.185.37

11.451.570.340.07

100.71

842352

305—18—12

196884.40.561.81.53.7

4.9170

0.6759

'Ata

Valuer,Stevenson,

And Scholl,et al. (1985)

52.280.74

17.1310.980.195.48

11.132.030.520.14

100.62

578651

482—46—19

251159

11—

93.58.9

3.596

0.4145

Notes: Dashes (—) indicate values that were not determined; single asterisk (*) indicates total iron as Fe2O3.

6 0 0 -

Sitθ 793 Zr(ppm) 150

Mariana-Bonin:

I I Eocene

Oligocene

Miocene

Active arc

Forearcintrusivθs

Table 4. Isotopic analyses of volcanic rocks from Hole 841B.

Figure 7. Compositions of forearc basin dikes and sills from Site 781 (inPliocene-Pleistocene sediments; Shipboard Scientific party, 1990a) and Site793 (in Miocene sediments; Shipboard Scientific Party, 1990b) compared witharc volcanic units of different ages in the Mariana system. Eocene analyses forforearc lavas from Bloomer (1987) and Reagan and Meijer (1984); Oligoceneanalyses from Reagan and Meijer (1984) and Wood et al. (1980); Mioceneanalyses from Wood et al. (1980); active arc analyses for selected volcanoesfrom Bloomer et al. (1989).

Core, section:Interval (cm):Subunit:

206pb/204pb

207pb/204pb

208pb/204pb

87 S r / 86 S r

143 N d / 144 N d

18R-1137-140

1A

18.70715.53638.2960.703380

0.513096

25R-3118-119

ID

18.716

15.53738.324

0.513085

25R-421-22

ID

18.709

15.54738.3100.7034660.513085

50R-116-18

2B

18.750

15.53938.341

0.513116

Note: Analytical methods detailed in Hergt and Nilsson (this volume).

only materials suitable for analysis. All of the analyzed samples arehigh-silica, low-K rhyolites (Table 6). The freshest samples, theperlitic glasses in Subunit 2B, average 77.6% SiO2 and 5.6% totalalkalis (Table 6), making them rhyolites sensu strictu, by the criteriaof Le B as et al. (1986). We will refer to these rocks as low-K rhyolites,to emphasize their low K20/Na20 (0.12-0.4; Ewart, 1979). Some ofthe rhyolitic clasts show very high SiO2 (>80%; Table 6); this resultsfrom secondary silica developed in post-crystallization graphic inter-growths with feldspar (Shipboard Scientific Party, 1992). All of therhyolites have low contents of Zr, Y, Ba, and REEs, considering theirhigh SiO2 concentrations. All of the analyzed rocks have similar

635

A cm9 -

11 -

1 3 -

15

17

1 9 -

21 - 1

C cm

24-i

2 5 -

26

2 7 -

28 - 1

Figure 8. A. Plagioclase-quartz phyric rhyolite pitchstone from Subunit 2B (Interval 135-841B-49R-1, 9-21 cm). B. Photomicrograph of Subunit 2B rhyolite; field of view is 6 mm from topto bottom (Sample 135-841B-50R-1, 14-17 cm). C. Welded tuff from Unit 6 (Sample 135-841B-66R-CC, 24-28 cm).

GEOCHEMISTRY AND ORIGIN OF IGNEOUS ROCKS

Table 5. Representative analyses of phenocryst phases in Hole841B Subunit 2B.

SiO2

TiO2

A12O3

FeO*MnOMgOCaONa2OZnOSum

Fe2O3

FeOSum

T(°C)

!og f02

Magnetite

0.1414.052.11

76.570.650.810.09

BDL0.29

94.71

38.4941.9498.57

Low-Mgilmenite

BDL47.22

0.1449.56

0.801.470.09BDLBDL

99.28

11.8338.92

100.47

906-11.87

High-Mgilmenite

0.0248.02

0.1748.11

0.632.860.03BDLBDL

99.82

11.8237.48

101.01

907-11.85

Average

clinopyroxene(TV = 1 0 )

51.770.210.76

15.380.68

11.4418.970.25BDL

99.46

15.3899.46

Average

orthopyroxene

(TV = 9)

51.210.130.36

30.711.22

15.771.330.05

BDL100.78

0.7130.07

100.85

Note: Data determined on Section 135-841B-50R-1,5-9 cm. Single asterisk(*) indicates total iron as FeO. Fe•203 and FeO partitioning based onStormer (1983). Temperatures and oxygen fugacities from Buddingtonand Lindsley (1964) and Ghiorso and Carmichael (1981). Analyses byelectron microprobe at the University of Queensland. BDL = belowdetection limit.

chemical compositions, indicating that the units at Site 841 are partof a single magma series. However, the rhyolites of Subunit 2B arethe freshest samples and the only ones that are clearly part of a flowor dome. The subsequent discussion of the chemistry of the basementunits will refer to the average composition of the Subunit 2B rhyolites(Table 7).

The two hypotheses advanced for the origin of the basement atSite 841 (Parson, Hawkins, Allan, et al., 1992) were that it was partof an Eocene sequence correlative to rocks on 'Eua, or that it was afragment derived from rifted Cretaceous crust, like that exposed onthe Lord Howe Rise, that was dispersed by the original breakup ofthe Southwest Pacific (Burns and Andrews, 1973). Analyses of LordHowe Rise basement samples from DSDP Leg 21, Site 207 (Table 6)show that they are high-K2O rhyolites similar to other continental riftrhyolitic sequences (Fig. 9). Although they have Nb, Ta, and Tidepletions like the Site 841 rocks, the Lord Howe Rise rhyolites arehighly enriched in incompatible elements (Fig. 10) and have pro-nounced LREE enrichments (Fig. 11). The Site 841 rhyolites clearlyare not materials derived from a crustal fragment similar to the Creta-ceous rhyolites of the Lord Howe Rise.

The other hypothesis is that the Site 841 rhyolites are part of anEocene complex developed during the earliest subduction of thePacific Plate beneath the Indo-Australian Plate. The rhyolites have a

Table 6. Analyses of rhyolitic volcanic rocks from Hole 841B Units 2-6.

Hole:Core, section:Interval (cm):Subunit:Laboratory:

841B49R-110-13

2BShip

841B47R-2

147-1492BBU

841B50R-1

5-92B

Queensland

841B50R-116-18

2BDurham

841B54R-2

142-1463C

WHOI

841B56R-CC

2-33D

Ship

841B57R-110-13

3DBU

841B59R-214-18

3DBU

84 IB62R-CC

11-154

Ship

207A34R-2

110-113Lord Howe Rise

Queensland

207A35R-3

108-111Lord Howe Rise

Queensland

207A40R-198-101

Lord Howe RiseQueensland

207A42R-243-^6

Lord Howe RiseQueensland

Major elements (wt%)SiO2TO;A12O3FeöOj*MnOMgOCaONa2OKiOP2O5SumLOI

Trace elements (ppm):ZnNiCrVCuZrSeYSrBaRbW ,CsUTilPbCoTaHfWTIBeLaCePrNdSmEuGdTbDy

ErTmYbLu

78.460.25

12.352.370.070.251.834.521.460.05

101.61+5.42

35203

10175

—6249

198142———

—

—

—

20

——————————

75.640.24

11.462.150.060.331.714.551.240.04

97.42**—

43<5<5<521

116150

190—

———

—

—

5—

———

———

——

74.520.37

12.873.290.070.82.934.031.230.04

100.20+4.66

571

19(15)34

1134

16(13)4463

19110.4

1.130.25 (0.40)0.280.393.065

4.340.080.79

4.33 (3.5)13.7(11.2)2.57

13.9(11)4.92 (3.6)1.15(0.96)6.42 (5.5)1.24(0.8)7.901.75(1.6)5.200.805.35 (5.4)0.88 (0.79)

78.630.16

12.312.060.090.211.594.310.820

100.00+—

—

—

161—

70179

111.20.370.250.424.94

0.124.760.14

4.2312.952.15

12.614.961.127.51.129.011.985.851.016.231.12

76.310.34

12.433.720.031.811.323.850.680.06

100.19+—

77

3325

148—5.372

7

——

13

—

—

—

———

—

80.50.28

11.562.480.020.951.803.000.380.04

101.01 +2.59

4440

159

184—6476

11682

——

19

—

—

—

—

80.840.24

11.440.920.011.081.433.250.220.05

99.45**—

207

14310534786—

4—

—

77.250.26

11.792.210.010.511.653.450.670.00

97.79**—

3314

22145

94461

104

—

5—

—

—

75.940.23

10.834.350.060.744.973.181.390.06

101.75+5.17

0109

21137—50

1378942

——

20

—

—

—

—

75.970.18

13.522.380.0120.790.302.754.090.02

100.01 +9.08

131

22

3341

5232

19319830.410.73.65

16.125

31.968.31.649.925.1

47.6101

12.344.910.20.48

10.31.579.01.845.020.754.790.77

76.890.20

12.892.010.0140.270.463.234.430.03

100.42+5.24

165

4——

3682

7663

54718837.510.23.96

14.527.9

21.816.631.79

10.75.3

7514219.670.315.41.39

14.82.22

12.52.546.981.056.91.03

77.500.17

1.870.0240.340.55

4.300.02

100.33+9.18

128—53—

25533

2811.33.66

14.520.4

—1.767.451.74

17.85.2

55.3115

14.251.911.60.54

11.61.759.791.97

0.805.170.79

75.770.26

13.152.580 010,161.093.144.140.03

100.36+5.34

174—53—

4926

1451260

17430.210.33.68

13.342.0

—1.63

1.6912.46.5

59.5124

15.255.312.42.23

12.21.81

10.02.045.530.825.250.80

Notes: Single asterisk (*) indicates total iron as Fe2θ3; double asterisk (**) indicates sum that does not include volatiles; dagger (+) indicates that major element analyses were performed on a volatile-free basis;dashes (197 indicate values that were not determined; values in parentheses by instrumental neutron activation from Australian National University, B.W. Chappell, analyst; other analyses as in text. BU = BostonUniversity, WHOI = Woods Hole Oceanographic Institution. BDL = below detection level.

637

S.H. BLOOMER, A. EWART, J.M. HERGT, W.B. BRYAN

Table 7. Average composition of Hole 841B Subunit 2B rhyolites compared with other western Pacific dacites andrhyolites.

Location:

Source:

Major elements (wt%):SiO2

TiO2

A12O3

Fe2O3*MnOMgOCaONa2OK2OP 2 O 5

SumLOI

Trace elements (ppm):ZnNiCrVCuZrSeYSrBaRbNbCsUThPbCoTaHfLaCePrNdStnEuGdTbDyHoErTinYbLu

Site 841Subunit 2B

AverageTable 5

77.580.22

12.042.190.070.261.714.461.170.03

99.685.42

39364

16173126156

19012

1.40.310.270.414.0040.104.554.28

13.322.36

13.264.941.146.961.188.461.875.530.915.791.00

Lord Howe Rise

AverageTable 5

76.530.20

13.042.210.030.390.603.034.240.03

100.207.21

150

43

3733

6168

5381923210.63.74

14.628.82.51.797.81

59.4120

15.355.612.4

1.1612.2

1.8410.32.105.710.865.530.85

SD

0.700.030.350.280.010.240.300.180.130.01

1.92

20

10

7429

46444

11.93.60.40.1318.10.50.120.84

10152.79.31.90.71.60.241.30.270.750.120.810.11

Saipan

BoniniticrhyoliteMeijer(1983)

78.200.17

10.581.470.040.221.283.541.690.12

97.253.00

14

125

9217515

——

0.851.6

5.509.96

7.712.480.543.31

3.42

2.13

2.40.43

Site 786

61R-4,96-102 cm

Pearce etal. (1992)

70.880.32

13.384.630.070.563.524.841.720.08

100.002.93

5748

3857811215

12317528

1.390.360.430.832.35

0.122.94.53

10.91.427.341.880.632.430.372.500.551.420.241.430.23

Site 786

67R-1,3 3 ^ 1 cmPearce etal. (1992)

76.030.23

12.512.600.030.401.194.252.710.05

100.000.69

1053

112238771412

10243024

0.85——0.522.46

0.062.223.066.160.814.721.350.511.420.191.530.290.920.181.160.13

Marianaforearc

Boniniticdacite

Bloomer(1983)

67.450.19

13.205.570.093.553.973.781.370.06

98.851.4

—249885—

76—7

427—————————7.02—3.961.000.300.98—0.86—0.48———

Marianaforearc

Tholeiiticdacite

Bloomer(1983)

67.540.56

13.487.140.130.814.003.341.200.16

98.362.24

—8

2021

—99

—34

1183110—————————

11.76—8.152.620.813.98—4.52—3.18—3.62—

Notes: Single asterisk (*) indicates total iron as Fe2O3; dashes (—) indicate values that were not determined. SD = standard deviation.

clear subduction signature, with enrichments in LILEs, low compat-ible-element concentrations, depletions in Nb and Ta, and pronouncedPb enrichments (Fig. 10). They have low abundances of REEs as wellas LREE depletions (Fig. 11). Temperatures calculated from coexist-ing magnetite and ilmenite in the rhyolite give temperatures of 906°Cand oxygen fugacitiesof 1 0 " 82 (using the curves of Buddington andLindsley [1964] with the recalculation procedure of Stormer [1983]and the fitting procedure of Ghiorso and Carmichael [ 1981 ]; Table 5).These data, as argued for similar Eocene rhyolites from Saipan, aremost consistent with derivation of the rhyolites by fractional crystal-lization of a more mafic parent (Meijer, 1983). Rhyolites are not theproduct of hydrous melting of the mantle (Green, 1973) and thecompatible element concentrations in the rhyolites are far too low forthem to be mantle-derived melts. Melts of the downgoing slab shouldproduce dacites (Ringwood, 1974) with pronounced heavy-rare-earth(HREE) depletions, because of equilibration with garnet eclogite, andLILE enrichments.

Melting of the altered lower oceanic crust beneath the Eoceneforearc under hydrous conditions could produce rhyolites at 680°-720°C (Helz, 1976). These melts, if the crust were in amphibolite facies

conditions, should have higher A12O3 (Helz, 1976), more pronouncedEu depletions, greater LREE enrichments, and possibly higher HREEelement "tails" than the Site 841 rhyolites. The crystallization tempera-tures for the rhyolites are also much higher than those for anatectitesfrom a basaltic amphibolite (680°-720°C; Helz, 1976; Meijer, 1983).

However, Beard and Lofgren (1991) have recently shown thatdehydration melting, rather than water-saturated melting, of green-stones at low pressures does produce melts very similar to low-K2Oarc rhyolites at 850°-900°C. The restite from this melting lacks am-phibole and includes Plagioclase (>50 modal%), orthopyroxene andclinopyroxene. The experimental rhyolites with 78% SiO2 have 12%A12O3, 2% CaO, 0.5% TiO2, and 0.5% MgO, values very similar tothose in the Site 841 Subunit 2B rhyolites. The Eu anomalies in theSubunit 2B rhyolites are unusually small if they are indeed derivedfrom partial melts that were in equilibrium with a plagioclase-richresidue. Also, production of the rhyolites by partial melting of thecrust might be expected to produce a bimodal rock suite. However,the few analyses of arc clasts contained within the Site 841 rhyolites(see below) show a diverse range of SiO2 from 50% to 70%, suggest-ing that the basement is not bimodal. However, the evidence for the

638

GEOCHEMISTRY AND ORIGIN OF IGNEOUS ROCKS

Site 841 RhyoliteLord Howe Rise Rhyolites

CDSDP Site 207)O Mt Somers Rhyolites

WhitsundayRhyolite - dacite field

Ab OrFigure 9. Q-Or-Ab normative projection for rhyolite compositions from the Southwest Pacific. LordHowe Rise rhyolites (this paper), Whitsunday rhyolites in Queensland Province, Australia (Ewart etal., in press) and Mt. Somers rhyolites are all examples of Cretaceous rhyolitic fields associated withthe fragmentation of the Southwest Pacific continental margins. Note the distinct compositions of theSite 841 rhyolite. Curve I is the water saturated curve at 100 MPa for compositions of An3, after Jamesand Hamilton (1969); curves 2 and 3 are the calculated 4-phase surfaces in the system Ab-An-Or-Q-H2O for aw = 1.0 and 0.2, respectively, at 200 MPa (after Nekvasil, 1988).

104

103

GOccO 1Q2

Q. 10

(O

1

0.1

a Lord Howe Rise rhyolite

O Hole 786B, boninitic rhyolitβ

• Hole 841B Unit 2

I I I I I I I I I I I I I I I I I I T I I I I I I I I ICs Ba Th Ta La Pb Sr Sm Hf Ti Tb Y Er Yb

Rb U Nb K CΘ Pr Nd Zr Eu Gd Dy Ho Tm Lu

Figure 10. N-MORB-normalized compositions of Site 841 Subuit2B analyses,Lord Howe Rise rhyolites, and Site 786 boninitic dacite (Murton et al., 1992).Normalizing values from Sun and McDonough (1989).

basement composition is fragmentary and the possibility that the Site841 rhyolites represent dehydration melting of lower arc crust cannotbe ruled out.

The Site 841 rhyolites are, therefore, most reasonably interpretedas derivatives of a more mafic magma, requiring a substantial volumeof mafic cumulates at depth, or as dehydration melts of lower arc crust,leaving a substantial granulite facies mafic residue. In either case, the

parent material must have been very depleted in incompatible ele-ments, given the low incompatible element abundances in the rhy-olites. There are two mafic-silicic associations described from Eocenesequences in the Izu-Bonin-Mariana and Tonga forearcs. Boniniticseries lavas have been described from a number of places in the IBMforearc (e.g., Hussong and Uyeda, 1982; Bloomer and Hawkins, 1987;Meijer, 1980; Reagan and Meijer, 1984; Ishii, 1985; Arculus et al.,1992; Pearce et al., 1992). These boninitic associations include daciticand rhyolitic lavas (as on Saipan, first described by Schmidt [1957])that are interpreted to have developed by fractional crystallization of aboninitic magma (Table 7; Meijer, 1983; Bloomer, 1987; Murton et al.,1992). Associated with these boninitic series, in time and space, aredepleted island-arc-tholeiitic series lavas. These also include dacitic torhyolitic fractionates (e.g., Bloomer, 1987; Murton et al, 1992) thatare not as depleted in incompatible elements as their boninite seriescounterparts. Similar tholeiitic series basalts, andesites, dacites, andrhyolites comprise the Eocene basement on 'Eua (Ewart and Bryan,1972; Hawkins and Falvey, 1985; Cunningham and Anscombe, 1985).

The rhyolites from Site 841 Subunit 2B show a closer similarityto tholeiite series lavas than to the boninite series. They are not asdepleted in HREEs and HFSEs as are high-silica rocks derived fromboninitic parents, such as those on Saipan and in Site 786 (Figs. 10and 12). They are, however, very similar in element abundances andratios to tholeiite series dacites from 'Eua (Fig. 12). In fact, they havetrace element abundances nearly identical to the 73% SiO2 volcanicson 'Eua (Table 7 and Fig. 12).

Stern et al. (1991) noticed that boninitic-series lavas in the Mari-ana forearc were characterized by high K/Ba and low Ba/La, in

SH. BLOOMER, A. EWART, J.M. HERGT, W.B. BRYAN

100

T3co

-CO

Q.ECO

CO 10

Site 841 Rhyolite

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Figure 11. Chondrite-normalized, rare-earth-element diagrams for Site 841 rhyolite, Lord Howe Rise rhyolites, andrhyolites from the East Queensland Cretaceous Province (striped field, from Ewart et al., in press).

100

CD

DC 10 +

D Saipan boninitic rhyolitβ

Q Mariana forearctholθiitic dacitβ

• Hole 841B, Unit 2

0-1 <—\—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—h-Rb U Nb K2O Ce Nd Zr TiO2 Dy Er Lu

Ba Th Ta La Sr Sm Eu Gd Yb

CO

uu

10

1

0 1

-π\ /r\\ fi

3- H 1 1 1

g

\ ÅD

O

•

I 1 1 1 1 1 1 1 H

'Eua, 62% SiO2

'Eua, 73% SiO2

Hole 841B, Unit 2,78% SiO2

—I 1 1 1 1 H-Ba Ta La Pb Nd Hf TiO2 Dy Yb

Th K2O Ce Sr Sm Eu Gd Y Lu

Figure 12. Hole 84IB rhyolite compared to other siliceous Eocene forearclavas. Normalizing values from Sun and McDonough (1989). Saipan analysisfrom Meijer (1983), Mariana forearc dacite from Bloomer (1987), and 'Euaanalyses from Cunningham and Anscombe (1985).

400-

* _

100"

mm

H

N-MORB•

Mariana forearc/boninites

HSite 786 HCB

"@Sitθ786lCB

TEua

Site 786 L C B W

Site 841

o• Ata

rhyolitθ11

•Tofua

10 Ba/La 60

Figure 13. Trace element ratios for Site 841 lavas compared with boninite andlava on an island-arc-tholeiitic series. Average compositions for Ata and TofuaArc from Ewart and Hawkesworth (1987); average and standard deviation forMariana boninites from Stern et al. (1991), averages and standard deviationsfor 'Eua from Cunningham and Anscombe (1985), averages for Site 786boninites from Murton et al. (1992; LCB = low-Ca boninite, ICB = intermedi-ate-Ca boninites, and HCB = high-Ca boninite); N-MORB average from Sunand McDonough (1989).

contrast to the normal arc signature of high Ba/La and low K/Ba (Fig.13). They suggested that this signature was unique to the boniniticeruptives that characterized the early phases of subduction. Indeed,the boninitic series at Site 786 show a low Ba/La signature, althoughK/Ba varies between high-Ca and low-Ca boninites (Fig. 13). TheSite 841 rhyolites have high Ba/La and low K/Ba, suggesting anaffinity to an arc-tholeiitic, not boninitic, magma stem.

Volcanic Clast Compositions

The extreme degree of fractionation of rhyolitic rocks makes itdifficult to be specific about the nature of the parental lavas in theEocene forearc. Furthermore, no information is available about thebasement on which the silicic edifice was built. However, a varietyof mafic clasts are present within the section at Site 841 that mayprovide a clue about the nature of both the parental lava for therhyolites and about the basement beneath them.

We have examined three populations of clasts to search for infor-mation about the Eocene history of the forearc (Table 8). The first

GEOCHEMISTRY AND ORIGIN OF IGNEOUS ROCKS

Table 8. Analyses of clasts from Hole 841B basement and sediments.

Core, section:Interval (cm):Sample wt (g):Type:

48R-129-310.0059

Arc

Major elements (wt%):SiO2TiO2A12O3Fe2O3*MnOMgOCaONa2OK2OP2O5Sum

Normalizedfrom:

50.810.62

15.7413.130.32

10.282.565.401.13—

lOO.OOt

90.10

Trace elements (ppm):ZnNiCrVCuZrSeYSrBa

489—————

—27—

48R-129-310.0059

Arc

51.441.13

13.4117.500.437.743.673.371.31—

lOO.OOt

94.90

433——

296——52—

152—

51R-324-280.0008

Arc

54.320.81

20.507.410.082.788.644.860.611.43

lOO.OOt

117.19

——————————

51R-324-280.0012Arc?

53.681.11

18.988.830.102.319.903.851.23—

lOO.OOt

104.93

———

————

52R-166-690.2427

Arc

75.680.38

11.253.900.0270.902.184.990.330.09

99.64

109

102142

147—

24210372

Inclusions from rhyolitic basement complex

52R-1126-1290.2514ORB

49.141.51

10.8611.590.267

11.9711.32

1.910.100.20

98.67

95137218251

1603743

12223

52R-1126-1290.0553ORB

46.461.48

16.5015.660.224

12.025.921.460.150.11

99.87

29292

233273

764429

16458

62R-153-570.1051

Arc

82.610.548.514.320.0331.002.842.081.540.046

103.47

77

1587

4845

119

63R-191-940.0439ORB

48.872.67

17.4916.820.0985.743.723.252.180.28

100.84

542

132467197

155

63R-191-940.2204ORB

46.912.21

17.6114.630.0975.762.573.391.990.42

95.167

890

—295

1274797

10696

63R-1108-1100.2663ORB?c

54.211.55

19.059.790.0813.394.395.091.070.35

98.62

394<IO<IO177<151043170

16794

65R-CC25-270.0009

Ar

70.961.08

10.838.940.0493.151.582.780.64

lOO.OOt

109.70

—

————

65R-CC25-270.0023

Arc

62.10.77

17.678.750.0575.290.883.690.78

lOO.OOt

105.83

———

—————

65R-CC25-270.2019

Arc

84.770.339.192.820.0210.570.633.060.430.028

101.82

471712<l17

1258

438451

67R-CC0-5

0.24477

61.941.45

15.589.390.052.140.527.100.120.16

98.44

7441273215389411513642

68R-CC12-150.2529

Arc

53.401.05

15.4810.940.117.144.473.040.160.09

95.88

7815263863052422210450

Table 8 (continued).

Core, section:Interval (cm):Sample wt (g)Type:

Inclusions fromEocene sediments

45R-138^0

: 0.0045Arc?

Major elements (wt%):SiO2TiO2

A12O3Fe2O3*MnOMgOCaONa2OK2OP 2 O 5

Sum

Normalizedfrom:

51.501.07

14.3113.060.1364.885.823.321.53—

95.63

Trace elements (ppm):ZnNiCrVCuZrSeYSrBa

252——

322——

53—

208278

46R-CC

0.0012Arc

48.280.87

13.7115.420.1227.375.09———

90.86

— —— —— —— —— —— —— —— —— —— —

Inclusions from Oli;

41R-CC15-200.0047

Arc

50.460.88

16.1311.450.2747.286.463.201.11—

97.24

263———272———207318

sediments

42R-235^00.0025

Arc

65.600.99

16.996.750.530.714.272.461.830.66

lOO.OOt

109.69

———————

——

jocene

43R-1116-1190.0057Arc?

54.311.14

14.3911.640.1775.329.182.821.020.46

lOO.OOt

105.00

232

355695

65

189269

22R-281-850.0056

7

49.231.04

18.3910.910.2236.499.764.030.19

100.26

180229180

148258

Inclusions from Miocene volcaniclastics with reworked Eocene fauna

22R-281-850.0022

9

51.241.10

15.6214.870.2416.774.175.000.990.67

lOO.OOt

107.50

—

22R-281-850.0115ORB?

49.221.81

16.5214.220.1633.83

10.423.130.770.09

lOO.OOt

105.12

8218339

392

40

264261

22R-281-850.1257

Arc

64.000.48

16.484.590.0521.292.677.370.420.07

97.35

3177

8419381415

93

22R-281-85

0.0022Arc

57.250.75

14.4211.640.134.094.804.182.738.01

lOO.OOt

108.70

—

———

—

——

22R-281-850.0829

Arc

64.660.58

12.0810.660.133.383.182.223.120.19

lOO.OOt

94.64

901835

1872735223879

697

22R-2125-1290.2482

Arc

50.400.74

15.2012.000.2529.311.715.820.220.25

95.65

190115413181304532349841

22R-2125-1290.2451ORB?

45.601.29

18.6414.370.22

15.452.102.090.230.15

lOO.OOt

91.95

99153470323

59045216342

22R-2125-1290.1033

Arc

54.320.71

18.7111.370.163.192.608.700.250.06

lOO.OOt

93.15

772013

206434230115663

group comprises clasts contained within the rhyolites. These includeaphyric greenstones, mafic grains and pebbles, and silicic pebbles.Similar clasts occur within the Oligocene and Eocene sediments imme-diately overlying the rhyolites. A third group of clasts comes from aMiocene breccia/conglomerate sequence in which there is a reworkedEocene fauna, indicating that Eocene basement was locally exposed

during sedimentation. Many of these clasts are extensively altered,and their mobile element concentrations are suspect; also many weretoo small to allow analysis of trace elements.

The compositions of the clasts are shown in Figure 14, comparedto fields for their most likely sources. The field for 'Eua representsbasaltic to rhyolitic Eocene arc compositions, whereas fields for E-

641

S.H. BLOOMER, A. EWART, J.M. HERGT, W.B. BRYAN

Table 8 (continued)

Core, section:Interval (cm):Sample wt (g):Type:

22R-2125-1290.0234ORB?

Major elements (wt%):SiO2

TiO2

A12O3

Fe2O3*MnOMgOCaONa2OK2OP 2 O 5Sum

Normalizedfrom:

48.351.03

14.6115.670.318.255.904.110.700.25

98.93

Trace elements (ppm):ZnNiCrVCuZrSeYSrBa

150110360165

99404781

187

Inclusions from Miocene Volcaniclastics with reworked Eocene fauna

23R-112-140.2149

Arc

51.380.62

17.0216.150.296.471.925.410.740.03

lOO.OOt

92.06

153347

251

49241552

133

23R-112-140.0058

Arc

53.160.56

20.789.550.153.194.157.421.04

—lOO.OOt

94.56

152

—

74400

23R-2113-1150.0056

7

53.781.03

14.9012.320.193.716.364.580.840.21

97.71

171

331

49179380

24R-CC13-150.2604ORB?

50.131.13

16.5112.930.295.16

10.273.480.110.26

lOO.OOt

90.75

1222931

436109804330

25787

24R-CC13-150.1141Arc?

54.271.01

14.8512.760.264.128.383.870.200.07

99.72

972428

229

633724

213112

29R-152-540.0366ORB?

52.541.40

15.9915.140.223.773.645.911.380.09

100.001

94.23

168132

51713623530

11798

29R-152-540.0147

9

50.881.17

14.6815.550.245.844.626.000.650.42

99.63

15612021

238——72—5388

29R-171-730.1367

Arc

53.240.74

13.7715.880.236.953.924.091.160.16

lOO.OOt

91.47

1955271

30315252439

2232930

29R-171-730.0079ORB

48.812.19

15.3115.630.209.394.553.820.041.00

lOO.OOt

108.00

161147181228——30—21—

Notes: Single asterisk (*) represents total iron as Fe2O3; dashes (M) indicate values that were not determined. Some smallsamples yielded sums between 90% and 120% because of weighing errors or because of very high volatile contentscaused by alteration. These analyses were normalized to 100% (noted by t); the original sum is noted below eachanalysis. Volumes of small samples were insufficient to allow complete trace element analysis. Type designations arebased on empirical comparison to the fields in Figure 13 and discriminant analysis using Ti, V, CaO, Fe2O3, MgO,SiO2, and Sr. ORB = ocean ridge basalts, Arc = compositions similar to Eocene volcanics on 'Eua, and ? = clasts thatcould not be assigned a definitive origin.

MORB and N-MORB from the subducting Pacific Plate east of theTonga Trench (Bloomer and Fisher, 1987) represent reasonable com-positions for the basement on which the Eocene arc might have beenbuilt. The origins assigned to the clasts in Table 8 are based oncomparison of the clast compositions to these fields (empirically andby discriminant analysis).

Two groups of clast types can be identified (Table 8 and Fig. 14);some of the clasts cannot be confidently assigned to either group. Theprincipal population includes basalts, andesites, and dacites with lowTiO2 and low Ti/V that are clearly of arc origin (Table 8). None of thispopulation has the low Ti, Ca, and Al signatures typical of boniniticseries lavas, supporting the conclusion that the Site 84 ̂ volcaniccomplex is part of an arc-tholeiitic magma series.

The second population is small, but appears to comprise clasts ofocean ridge basalt composition. These clasts have TiO2 greater than1.4% (some greater than 2%), high Ni and Cr concentrations, and highTi/V. Some of the highest Ti samples may represent enriched MORBor intraplate compositions, which do exist in the offshore plate (Bloomerand Fisher, 1987). These ocean-ridge-like clasts occur in both thereworked Miocene sediments and within the rhyolitic basement. Thelatter group is very significant, as it means that mid-ocean-ridge crustexisted beneath the rhyolitic arc edifice that built the Site 841 basement.

Discussion

The rhyolitic complex found at Site 841 was one of the mostsurprising discoveries of Leg 135. There are a number of similaritiesbetween this arc sequence and Eocene arc lavas that have been drilledin the forearcs of the IBM systems. The Site 841 rhyolites appear tobe mid-Eocene in age, on the basis of the age of the overlying

sediments and preliminary K-Ar ages. The IBM basement is all partof a middle to late Eocene construct (Taylor, 1992). The rhyoliticmagma at Site 841 was derived by fractional crystallization of a moremafic, arc-tholeiitic lava, very similar to volcanic rocks in the Eocenebasement on 'Eua or by dehydration melting of similar Eocene vol-canic rocks in the arc basement. The IBM forearc is a composite oflow-Ca to high-Ca boninites and depleted arc-tholeiite-series lavas(NatlandandTarney, 1981;Bloomer, 1983,1987; Arculusetal., 1992;Murton et al., 1992; Taylor, 1992). Boninitic lavas have not beenfound with any of the igneous rocks on 'Eua or at Site 841; they havebeen dredged from the northern termination of the Tonga arc, butthe measured age of those boninites is much younger than Eocene.Boninitic rocks have not been found in most of the Tonga forearc,though the volume of dredge and drill sampling in most of the forearcis still so sparse that their existence cannot be ruled out.

The basement at Site 841 has subsided over 5000 m since the lateEocene. This remarkably large subsidence is a consequence both ofthermal subsidence and of subduction erosion. Substantial subsidencehas been documented for forearc sites in the IBM system (Lagabrielleet al., 1992; Taylor, 1992). The boninitic volcano drilled at Site 786in the Bonin forearc is also inferred to have been above sea level inthe Eocene (Lagabrielle et al., 1992).

The results from Site 841 also support hypotheses for forearcevolution proposed in the IBM system. There the initiation of sub-duction was accompanied by the development of a very wide (up to300 km) and extensive (up to 3000 km long) zone of arc volcanismin the middle to late Eocene (Natland and Tarney, 1981; Taylor, 1992;Stern and Bloomer, 1992). This style of volcanism is required by thedistribution of middle to late Eocene arc volcanics in the forearcs andfrontal arcs. This Eocene volcanism developed in an extensional en-

GEOCHEMISTRY AND ORIGIN OF IGNEOUS ROCKS

0.8 -

CO

OCM

1ü

0.4 -

LÜ

i

1

1TTTrmEED

•

IE

]

2 4Fθ2θ3* / MgO

400

Q.

a.

200

E-MORB offshore

Eua

N-MORB offshore

S1

1.0TiO2

T2.0

100 -

Figure 14. Compositions of clasts in Site 841 sections. Solid circles = clasts within the rhyolitic basement (Units 2-6); open circles = clastsfrom Eocene-Oligocene calcareous sandstones; and dotted squares = clasts within Miocene sediments that contain reworked Eocene fauna.Fields for Eocene arc lavas on 'Eua from Cunningham and Anscombe (1985); fields for N-MORB and E-MORB, which form the Pacific Platein the Tonga Trench, from Bloomer and Fisher (1987).

vironment, and in places may have occurred as true seafloor spreading(Stern and Bloomer, 1992).

A similar model can be applied to the Tonga forearc. Site 841 onthe trench-slope break, and the basement on 'Eua at the edge of theTonga platform, bracket a zone at least 65 km wide, on both sides ofwhich identical lavas were erupted at 46-40 Ma. Arc lavas and plu-tonics are the principal materials dredged from the landward trenchslope from the Louisville Seamount Chain intersection all the way tothe north end of the trench (Fig. 1), a distance of over 1000 km. Theoccurrence of ocean-ridge basalt clasts within the rhyolites at Site 841is significant, because it requires that the older oceanic crust on whichthe Eocene arc was building had been rifted, but that crust was notcompletely replaced by juvenile arc volcanism. The age of the base-ment in the forearc is only well constrained at Site 841 and on 'Eua.Additional sampling is essential if we are to confirm that the Tongaforearc is indeed constructed in the same manner as the IBM forearcsystems. However, the available evidence is completely consistentwith the model for forearc evolution that has been proposed by Sternand Bloomer (1992) (Fig. 15).

This model supposes that subduction zones begin by sinking ofold, unstable lithosphere, perhaps initiated at a fracture zone. Theinitial "subduction" is entirely a vertical movement (Fig. 15). Aeon-sequence of that sinking is a strong trench "suction" that extends theupper plate and begins to rift apart the preexisting oceanic crust.

Dehydration of the sinking slab initiates melting in the extendinglithosphere, augmented by an influx of asthenosphere driven by thesinking lithosphere. This melting generates boninitic and arc-tholei-itic sequences. The dominant magma product depends upon the initialthermal structure of the mantle and the degree of depletion of thepreexisting lithosphere. Highly depleted lithosphere produces bonin-itic-series melts; less depleted, perhaps deeper lithosphere and theinflowing asthenosphere produce arc-tholeiitic melts. These arc erup-tives build atop the rifted oceanic crust. Zones of particularly highmagma supply may yield subaerial volcanic edifices like those foundat Site 841 and on Saipan (Meijer, 1980). If the degree of extensionis sufficiently high, the arc volcanism may proceed in a seafloorspreading mode (Stern and Bloomer, 1992). That stage was appar-ently not reached beneath Site 841.

The results from Site 841 provide strong support for the modelsof forearc evolution that have been proposed for the IBM system. Theearliest phases of subduction in the Tonga region appear to have beenaccompanied by broadly distributed island arc-tholeiitic volcanism.

SUMMARY

The basement at Site 841 comprises over 200 m of rhyolites,pumice breccias, and tuffs that are part of a middle to late Eocenesubaerial island-arc volcano. The rhyolites were derived by fractional

643

S.H. BLOOMER, A. EWART. J.M. HERGT. W.B. BRYAN

kmVerticalexaggeration = 1 o 50 100

see detail below.

•*tt> ExtensionT TrenchM Mantle flow

I U Old ocean crust

E23 Old lithosphere

| Infant arc crust

[ 3 Zone of intensemelting

: : • • : : •

Figure 15. Model for the initial phases of subduction (after Stern and Bloomer,1992). Subduction is initiated by sinking of old lithosphere, producing exten-sion in the overlying oceanic plate and melting in the mantle wedge (lithosphereand inflowing asthenosphere) above the sinking plate. Arc lavas erupt on, andcover, extended ocean crust. Sites of large magma supply may develop silicic,subaerial edifices.

crystallization of a more mafic arc-tholeiitic parent (implying a sig-nificant volume of mafic cumulates at depth) or by dehydrationmelting of lower arc crust (implying a substantial granulite faciesmafic residue at depth). The Site 841 rhyolites are nearly identical tohigh-SiO2 lavas in the Eocene basement on 'Eua and are not the sameas the Cretaceous rhyolites on the Lord Howe Rise, and are not partof a pre-Eocene crustal fragment.

The age and composition of the Site 841 rhyolites, and their posi-tion 65 km outboard of same-aged rocks on 'Eua supports models forforearc development that require an early episode of broadly distrib-uted arc volcanism. Clasts of ocean-ridge basalt within the rhyolitessuggests that the Eocene volcano was built on rifted oceanic crust.

Unlike the Mariana forearc, no evidence was found at Site 841 forextensive exposures of serpentine in the forearc. Such diapiric bodiesmay not exist here, or they may be too far away to have shed debrisinto the sedimentary section at Site 841.

Basaltic dikes and sills intrude upper lower Miocene and mid-Miocene sediments at Site 841. These basalts appear to be part of oneintrusive episode; they are identical to upper Miocene lavas eruptedon the Lau Ridge and show no geochemical signature to suggest thatthey were generated by anomalous melting in the sub-forearc mantle.We consider it most likely that these intrusions were fed from base-ment dikes propagating out from the Miocene active arc. A similarorigin is suggested for forearc basin intrusions in the Mariana andBonin forearcs.

ACKNOWLEDGMENTS

We thank the technicians, crew, and drillers aboard Leg 135 fortheir enthusiasm and skill during the drilling at Site 841. Our ideasabout forearc evolution have benefited from discussions with BobStern, Julian Pearce, Brian Taylor, Kristen Nilsson, Jim Hawkins,Patty Fryer, and Tony Crawford. Shawn Dolan and Jennifer Chaseprovided invaluable assistance with the analyses of the Site 841samples. We would also like to thank members of the Western PacificPanel; David Scholl, Tracy Vallier, and Andy Stevenson of the U.S.Geological Survey; and Jim Hawkins for their efforts in including Site841 in the Leg 135 prospectus. Careful reviews by Robert Stern,Richard Arculus, and James Hawkins improved the final manuscript.This work was supported by a grant from JOI-USSAC.

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Ewart, A., and Bryan, W., 1972. Petrography and geochemistry of the igneousrocks from 'Eua, Tonga Islands. Geol. Soc. Am. Bull., 83:3281-3298.

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Fisher, R.L., and Engel, CG., 1969. Ultramafic and basaltic rocks dredgedfrom the nearshore flank of the Tonga Trench. Geol. Soc. Am. Bull.,80:1373-1378.

Fryer, P., Ambos, E.L., and Hussong, D.M., 1985. Origin and emplacement ofMariana forearc seamounts. Geology, 13:774-777.

Fryer, P., and Fryer, G.J., 1987. Origins of nonvolcanic seamounts in a forearcenvironment. In Keating, B.H., Fryer, P., Batiza, R., and Boehlert, G.W.(Eds.), Seamounts, Islands, and Atolls. Am. Geophys. Union, Monogr.Ser., 43:61-72.

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Haggerty, J.A., 1987. Petrology and geochemistry of Neogene sedimentaryrocks from Mariana forearc seamounts: implications for emplacement ofthe seamounts. In Keating, B., Fryer, P., Batiza, R., and Boehlert, G.W.(Eds.), Seamounts, Islands, and Atolls. Am. Geophys. Union., Geophys.Monogr. Ser., 43:175-185.

Hawkins, J.W., and Falvey, D.A., 1985. Petrology of andesitic dikes and flowsfrom 'Eua, Tonga. In Scholl, D.W., and Valuer, T.L. (Eds.), Geology andOffshore Resources of Pacific Island Arcs—Tonga Region. Circum-Pac.Counc. Energy Miner. Resour., Earth Sci. Ser., 2:269-280.

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Hussong, D.M., and Uyeda, S., 1982. Tectonic processes and the history of theMariana Arc: a synthesis of the results of Deep Sea Drilling Project Leg60. In Hussong, D.M., Uyeda, S., et al., Init. Repts. DSDP, 60: Washington(U.S. Govt. Printing Office), 909-929.

Ishii, T, 1985. Dredged samples from the Ogasawara fore-arc seamount or"Ogasawara paleoland"-"fore-arc ophiolite." In Nasu, N., Kobayashi, K.,Kushiro, I., Kagami, H. (Eds.), Formation of Active Ocean Margins:Tokyo (Terra Sci. Publ.), 307-342.

James, R.S., and Hamilton, D.L., 1969. Phase relations in the system NaAlSi3O8-CaAl->Og-SiO^ at 1 kilobar water vapour pressure. Contrib. Mineral. Petrol.,21:111-141.

Johnson, L.E., Fryer, P., Taylor, B., Silk, M., Jones, D.L., Sliter, W.V., Itaya,T., and Ishii, T, 1991. New evidence for crustal accretion in the outerMariana forearc: Cretaceous radiolarian cherts and MORB-like lavas.Geology, 19:811-814.

Karig, D.E., and Ranken, D., 1983. Marine geology of the forearc region,southern Mariana island arc. In Hayes, D.E. (Ed.), The Tectonic andGeologic Evolution of Southeast Asian Seas and Islands (Pt. 2). Am.Geophys. Union, Monogr. Ser., 27:266-280.

Lagabrielle, Y., Sizun, J.-P., and Arculus, R.J., 1992. The constructional anddeformational history of the igneous basement penetrated at Site 786. InFryer, P., Pearce, J.A., Stokking, L.B., et al, Proc. ODP, Sci. Results, 125:College Station, TX (Ocean Drilling Program), 263-276.

Le Bas, MJ., Le Maitre, R.W., Streckeisen, A., and Zanettin, B., 1986. Achemical classification of volcanic rocks based on the total alkali-silicadiagram. J. Petrol., 27:745-750.

Lonsdale, P., 1986. Amultibeam reconnaissance of the Tonga Trench axis and itsintersection with Louisville Guyot Chain. Mar. Geophys. Res., 8:295-327.

Malahoff, A., Feden, R., II, and Fleming, H.S., 1982. Magnetic anomalies andtectonic fabric of marginal basins north of New Zealand. J. Geophys. Res.,87:4109-4125.

Marlow, M.S., Johnson, L.E., Pearce, J.A., Fryer, PB., Pickthorn, L.G., andMurton, B.J., 1992. Pleistocene volcanic rocks in the Mariana forearcrevealed by drilling at Site 781. In Fryer, P., Pearce, J.A., Stokking, L.B.,et al, Proc. ODP, Sci. Results, 125: College Station, TX (Ocean DrillingProgram), 293-310.

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Murton, B.J., Peate, D.W., Arculus, R.J., Pearce, J.A., and van der Laan, S., 1992.Trace-element geochemistry of volcanic rocks from Site 786: the Izu-Boninforearc. In Fryer, P., Pearce, J.A., Stokking, L.B., et al., Proc. ODP, Sci.Results, 125: College Station, TX (Ocean Drilling Program), 211-235.

Natland, J.H., and Tarney, J., 1982. Petrologic evolution of the Mariana arcand back-arc basin system—a synthesis of drilling results in the SouthPhilippine Sea. In Hussong, D.M., Uyeda, S., et al., Init. Repts. DSDP, 60:Washington (U.S. Govt. Printing Office), 877-908.

Nekvasil, H., 1988. Calculated effect of anorthite component on the crystal-lization paths of H7O-undersaturated haplogranitic melts. Am. Mineral.,73:966-982.

Packham, G.H., 1985. Vertical tectonics on the Tonga Ridge from the Ton-gatapu oil exploration wells. In Scholl, D.W., and Vallier, T.L. (Eds.),Geology and Offshore Resources of Pacific Island Arcs—Tonga Region.Circum-Pac. Counc. Energy Miner. Resour., Earth Sci. Ser., 2:291-300.

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Pelletier, B., and Louat, R., 1989. Seismotectonics and present day relativeplate motions in the Tonga-Lau and Kermadec-Havre region. Tectono-physics, 165:237-250.

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Philpotts, A.R., and Martello, A., 1986. Diabase feeder dikes for the Mesozoicbasalts in southern New England. Am. J. Sci., 286:105-126.

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Date of initial receipt: 22 June 1992Date of acceptance: 7 June 1993Ms 135SR-129

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