Subduction components and the generation of arc-like melts in the Zambales ophiolite, Philippines: Pb, Sr and Nd isotopic constraints

Chemical Geology 156 Ž1999. 343–357

Subduction components and the generation of arc-like melts in the Zambales ophiolite, Philippines: Pb, Sr and Nd isotopic constraints John Encarnacion ´ a

a,)

, Samuel B. Mukasa

b,1

, Cynthia A. Evans

c

Department of Earth and Atmospheric Sciences, Saint Louis UniÕersity, 3507 Laclede AÕe., St. Louis, MO 63103, USA b Department of Geological Sciences, UniÕersity of Michigan, Ann Arbor MI 48109, USA c Lockheed Engineering and Sciences, Houston, TX 77058, USA Received 23 September 1996; revised 14 September 1998; accepted 3 November 1998

Abstract New Pb, Sr, and Nd isotopic data on mineral separates from the crustal and mantle rocks of the Zambales ophiolite ŽLuzon, Philippines., were obtained to help constrain the origin of its arc-like sections, which include island arc tholeiites as well as minor boninitic rocks. Previous work using major and trace elements showed that the ophiolite consists of similar volumes of laterally contiguous MORB-like crust and arc-like crust, and that the arc-like sections are less than ; 1 Ma younger than the MORB-like sections. It has been suggested that the arc-like magmas were derived from a mantle source that saw previous melt extraction, i.e., they are at least ‘second stage’ melts, whereas the MORB-like magmas are the products of ‘single stage’ melting. The isotopic data presented here exhibit elevated 87Srr86 Sr and 207 Pbr204 Pb, but identical 143 Ndr144 Nd, in the arc-like sections relative to the MORB-like sections, which have Pb, Sr, and Nd isotopic compositions indistinguishable from typical MORB. This suggests that an enriched, aqueous, subduction fluid acted as a flux for the second stage melting that generated the arc-like crust. Possible scenarios for the multiple melting events and addition of the subduction component include: Ža. influx of the subduction fluid directly into the melting zone beneath the MORB-like lithosphere, thus triggering further melting, Žb. hydration of young lithosphere by the subduction fluid followed by reheating Žby a propagating spreading center, for example., and Žc. decompression melting of a passive ‘nugget’ of depleted and subsequently hydrated mantle as it is tapped by a spreading center. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Radiogenic isotope; Island arc; Mantle; Subduction zone; Ophiolite

) 1

Corresponding author. Tel.: q1-314-977-3119; Fax: q1-314-977-3117; E-mail: [email protected] Fax: q1-313-763-4690; E-mail: [email protected].

0009-2541r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 8 . 0 0 1 9 0 - 9

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1. Introduction

Over the past two decades there has been a growing body of evidence to indicate that many ophiolites represent oceanic crust formed above subduction zones Že.g., Miyashiro, 1973; Upadhyay and Neale, 1979; Saunders et al., 1980; Pearce et al., 1984; Stern et al., 1989.. Because ophiolites may represent suprasubduction zone lithosphere, their exposed lithologies provide insight into processes occurring in the mantle wedge and overlying crust. A distinctive geochemical characteristic of many suprasubduction zone magmas, which is most strongly developed in boninitic rocks, is the depletion in high field strength elements ŽHFSE., enrichment in large ion lithophile elements ŽLILE., and common enrichment in light rare earth elements ŽLREE. relative to normal mid-ocean ridge basalt ŽMORB.. Because both HFSE and LILE behave incompatibly during partial melting of mantle that produces basaltic magmas, this feature is difficult to explain by a single stage of melt extraction. Instead, it is widely attributed to at least a two-stage melting history of the mantle source Že.g., Green, 1973; Sun and Nesbitt, 1978; Duncan and Green, 1987; Crawford et al., 1989; McCulloch and Gamble, 1991; Woodhead et al., 1993.. The first melt extraction event depletes the source in both LILE and HFSE leaving a refractory, or depleted residue. Subsequently, a hydrous LILE-enriched but HFSE-poor component, is added to the source before, or during, the second stage of melting. It is realized, though, that the products of either of these two ‘stages’ of melting are unlikely to be simple batch melts, but are probably mixtures of polybaric, near-fractional, andror critical melts Že.g., Johnson et al., 1990; Sobolev and Shimizu, 1993.. Models that invoke magma–mantle interaction to generate the HFSE depletion Že.g., Kelemen et al., 1990. still require that the mantle ‘reactant’ be depleted, i.e., that it saw previous basaltic melt extraction. ŽNote that the term ‘depleted mantle’, commonly used by petrologists, and as used here, should not be confused with the same term used by mantle geochemists, which refers to the MORB source mantle that experienced an ancient depletion of incompatible elements as reflected for example by low 87 Srr86 Sr and high

143

Ndr144 Nd ratios relative to chondritic ratios yet is fertile and capable of yielding MORB.. Here we present new Pb, Sr and Nd isotopic data largely derived from hand-picked mineral separates from the crust and mantle sections of the Zambales ophiolite. The isotopic data, in conjunction with other constraints indicating the ophiolites general tectonic setting, and age relationships between its geochemically distinct blocks, provide insight to the sources and processes occurring in the mantle wedge that may have caused the multiple depletion and enrichment events that are reflected in the ophiolite.

2. Tectonic setting and geochemistry of the Zambales ophiolite The Zambales ophiolite is exposed in a north– south trending mountain range and consists of oceanic crust and upper mantle that dips gently to the east along the western side of central Luzon in the northern Philippines ŽFig. 1.. The large northern Masinloc massif, which harbors several economic podiform chromite deposits, has been studied the most. Hawkins and Evans Ž1983. and Evans et al. Ž1991., divided it into two ‘blocks’ partly on the basis of major and trace element geochemistry. The Acoje block in the north is dominated by rocks with an arc-like affinity including island arc tholeiites ŽIAT. and lesser boninitic rocks. These exhibit HFSE depletion and LILE Ž"light REE. enrichment, both relative to MORB. The boninitic lavas and dikes resemble high-Ca boninites, or rocks transitional between high- and low-Ca boninites ŽHawkins and Evans, 1983; Crawford et al., 1989; Florendo and Hawkins, 1991.. The Coto block, which is contiguous with, and just south of the Acoje block, is dominated by MORB-like rocks in terms of major and trace elements. Yumul et al. Ž1990. analyzed chrome spinels in dikes and lavas from all three massifs of the Zambales ophiolite and found that the Cabangan massif had MORB-like Al-rich spinels, similar to the Coto block section of the Masinloc massif, whereas the San Antonio massif has more arc-like, ‘depleted’, Cr-rich spinels similar to the Acoje block section of the Masinloc massif.

J. Encarnacion ´ et al.r Chemical Geology 156 (1999) 343–357

345

Fig. 1. Geology of central Luzon. The northern two-thirds of the Eocene Ž44–45 Ma. Zambales range is the Masinloc massif, which consists of a northern section with rocks having arc-like characteristics Ž‘Acoje block’. and a southern section with MORB-like characteristics Ž‘Coto block’. ŽHawkins and Evans, 1983.. The arrow head southwest of Coto points to a peridotite-gabbro contact running northeast which is the nominal boundary between the two blocks. The central Cabangan massif has characteristics similar to the Coto block and the southernmost San Antonio massif is similar to the Acoje block ŽYumul et al., 1990.. The 48 Ma Eocene Angat ophiolite ŽEncarnacion ´ et al., 1993. exposes mainly gabbro, with sheeted dikes and plagiogranites at its southern end Žnot shown at this scale.. The submarine basaltic lavas and dikes south and east of the Eocene Angat ophiolite are Cretaceous ŽKarig, 1983.. The voluminous volcanics in the southern Sierra Madre represent a late Paleocene to late Eocene Ž; 60 to ; 34 Ma. volcanic arc that covers both the Eocene and Cretaceous ophiolitic basement ŽEncarnacion ´ et al., 1993.. Pliocene and Quaternary volcanics are the result of younger subduction along the Manila trench.

Based on the features outlined above, several specific tectonic settings have been proposed for the Zambales ophiolite. The differences between the Acoje and Coto blocks led Hawkins and Evans Ž1983. to propose that the ophiolite consisted of

tectonically juxtaposed slivers of an island arc ŽAcoje block. and a backarc basin ŽCoto block.. Geary et al. Ž1989. found variable Ta and bimodal Cr and Ni concentrations in some lavas of the MORB-like Coto block, which, along with the discovery of a few

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samples with light REE enrichment, led them to favor a model wherein the Coto block is forearc lithosphere that was chemically modified by incipient island arc magmatism represented by the Acoje block. Similarly, Evans et al. Ž1991. suggested that the Zambales formed on the backarc, or forearc edge of a young volcanic arc. Recognizing the possible presence of arc-like crust in the San Antonio massif, Florendo and Hawkins Ž1992. suggested that the Acoje and San Antonio areas represent rifted sections of a nascent arc that were separated during intra-arc spreading that generated the intervening Coto block and Cabangan massif. Previous models of the Zambales ophiolite discounted the regional geology and stratigraphy of adjacent areas in Luzon, because earlier work had suggested that the Zambales ophiolite was allochthonous to the rest of Luzon Že.g., Karig, 1983.; Encarnacion ´ et al. Ž1993. dated the different blocks within the Zambales ophiolite using zircon U–Pb geochronology and found ages of 44.2 " 0.9 Ma and 45.1 " 0.6 Ma Žmiddle Eocene. for the arc-like ŽAcoje block and San Antonio massif. and MORBlike ŽCoto block. sections, respectively. They also found that the Angat ophiolite in the Southern Sierra Madre ; 100 km east of the Zambales ophiolite ŽFig. 1., previously thought to be Cretaceous ŽKarig, 1983. and hence unrelated to the Zambales ophiolite, is also Eocene and only slightly older Ž48.1 " 0.5 Ma.. This obviated the need for a major tectonic suture between the Zambales and Angat ophiolites beneath the Central Valley Basin of Luzon ŽKarig, 1983. and suggests that the Zambales and Angat ophiolites form a contiguous Eocene ophiolitic slab that forms the basement to the Central Valley ŽFig. 1.. Additional stratigraphic data suggest that the Eocene Zambales–Angat ophiolite formed adjacent to the older Cretaceous ophiolitic terrane of the Southern Sierra Madre, whose age is constrained by radiolarian biostratigraphy, and hence may not be allochthonous to the rest of Luzon. The key evidence is the presence of the same Eocene volcaniclastic formation overlying both the Cretaceous ophiolitic basement and the Eocene Angat ophiolitic basement ŽEncarnacion ´ et al., 1993.. This autochthonous origin for the Zambales is not inconsistent with the absence of coarse volcaniclastics in the Eocene sedimentary

carapace directly overlying the Zambales ophiolite in the west ŽSchweller et al., 1983.. The associated arc in the Southern Sierra Madre to the east was active from the Paleocene to the Oligocene and produced Eocene–Oligocene calc-alkaline plutons and abundant, thick, and coarse volcaniclastic sediments that thin westward across the Central Valley basin ŽFig. 1.. The volcaniclastics appear to have propagated westward and thus only appear in the Oligocene section overlying the Zambales ophiolite ŽSchweller et al., 1983; Haeck, 1987.. Bathymetric and seismic profiling offshore northeastern Luzon identified an early Tertiary trench, accretionary complex, and forearc basin that were probably associated with the Sierra Madre arc ŽLewis and Hayes, 1983.. The location, and east-facing polarity of the subduction complex led Encarnacion ´ et al. Ž1993. to suggest that the Zambales–Angat ophiolite probably formed in a backarc setting, though this is certainly not required by the data. This in situ origin for the Zambales ophiolite Žrelative to the Southern Sierra Madre. does not violate the available paleomagnetic data which indicate about 908 of rotation of the ophiolite since the Eocene Že.g., Fuller et al., 1991. as there is no paleomagnetic data of the same age from the Southern Sierra Madre to demonstrate relative rotation between the two areas. Thus, although there appears to be a consensus that the Zambales ophiolite formed in suprasubduction zone setting, and there is good evidence that it formed adjacent to the rest of Luzon, there is no agreement as to its specific setting of formation Ži.e., backarc, forearc, etc...

3. Samples and techniques Samples from the upper crust, lower crust and mantle sections of the different blocks of the Zambales ophiolite were analyzed for Pb, Sr and Nd isotopic composition and concentration. The specific rock types that were sampled are indicated in Fig. 2 and Table 1, and the type of mineral analyzed from each rock type is given in Table 1. Samples from the Angat ophiolite have also been analyzed for comparison. We used fresh mineral separates Žpyroxene, hornblende and plagioclase. when possible, but used

J. Encarnacion ´ et al.r Chemical Geology 156 (1999) 343–357

347

Fig. 2. Sample codes, rock-types, and locations of samples in idealized sections through the Zambales ophiolite Žafter Hawkins and Evans, 1983.. The sections are not to scale. Samples 180DWZ, 181B, 191C and 511A are pyroxenite ‘dikes’ a few millimeters to a few centimeters wide generally oriented subparallel to the high temperature foliation in the host harzburgitic mantle. Sample ZA9101-13 Žnot shown. is hbl gabbro from the San Antonio massif Žsee Fig. 1..

whole rocks when mineral separation was not practical Že.g., aphyric dikes and lavas.. Prior to crushing, all visibly weathered surfaces of the samples were removed. For most samples, the 150–250 mm fraction yielded monomineralic grains. The hand-picked mineral separates were leached using protocols similar to those described by Reisberg and Zindler Ž1986. prior to sequential dissolution in HF–HNO 3 , 7 N HNO 3 , then 6 N HCl to a residuefree solution. Dissolved samples were 80–300 mg, depending on initial estimates of elemental concentration. For some samples, an aliquot of the solution Ž6 to 20%. was spiked with mixed 208 Pb– 235 U,87 Rbx – 84 Sr and 149 Sm– 150 Nd tracers. Separation of U and Pb followed procedures similar to Parrish et al. Ž1987.. The Rb, Sr, and REE were separated using a procedure similar to that of Hart

and Brooks Ž1977.. The Sm and Nd were separated from other REE using the procedure of Richard et al. Ž1976.. Procedural blanks for Pb are consistently less than 3% of total Pb analyzed in the isotopic composition ŽIC. split for all samples with Pb IC data in Table 1. Following corrections on IC samples for which the blank constituted nearly 3% Ži.e., 180DWZ., 206 Pbr204 Pb,207 Pbr204 Pband 208 Pbr204 Pb are lowered by only 0.24%, 0.04% and 0.09%, respectively, and therefore the blank is insignificant. The blanks for U, Pb and Rb were significant in the isotope dilution ŽID. split constituting from 1–10% of U, 5–70% of Pb and 20–; 100% of Rb. The nominal U, Pb and Rb concentrations reported in Table 1 are blank-corrected with 10, 200 and 50 pg U, Pb, and Rb, respectively. The Nd and Sr procedural blanks

348

Table 1 Isotopic ratio and isotope dilution data from Luzon ophiolites 206

207

208

Rock type

f

Acoje block 1. ZA9007-4

diorite

2. ZA9101-1

gabbro-norite

18.20 18.15 18.16

15.52 15.53 15.51

38.04 37.95 37.93

3. ZA9101-4a,b 4. 176F 5. 176L 6. 184A 7. 276ZB 8. 180DWZ 9. 181B 10. 191C 11. 511A

tonalite cpyx. ol. webst. wehrlite webst. cpyx. cpyx. pyx. webst.

18.29 18.00

15.55 15.51

37.90 37.80

12. 642

troctolite

H P P C P C C C C C C C C O P

S. Antonio Massif 13. ZA9101-13a hbl. gabbro

204

Pbr Pb

18.24

204

Pbr Pb

15.56

204

Pbr Pb

38.09

18.07

15.52

37.90

18.06

15.50

37.83

P

18.28

15.57

38.19

17.89 17.99 17.82 18.09 17.97 18.04

15.46 15.48 15.47 15.49 15.48 15.49

37.71 37.79 37.69 37.98 37.75 37.79

87 86

Srr Sr

143 144

Ndr Nd

0.703685"13

0.513160"17

0.703305"14 0.703329"13 0.704117"14 0.703414"13 0.703410"15 0.703420"14 0.703482"14 0.703736"14 0.703810"18 0.703870"14 0.703442"15 0.703474"24 0.703490"14

0.5132"8

0.703036"14

0.51309"8

U, ppb

Pb, ppb

16

59

19

92

0.0159

0.0734

14 8 19 14

23 27 17 41

0.169 0.133

0.230 0.184

0.664

0.160 0.495

0.128

0.173

0.51310"8

Sm, ppm

Nd, ppm

Sr, ppm

88.4

5

65

0.513121"22

32

127

0.703025"15 0.703313"15 0.703293"15 0.702684"14

0.513099"12

29

38

0.540

1.59

61.0 5 -1 12 12

9.24 11.0 10.5 12.8

55.4

Coto block 14. ZA9101-10 a 15. ZA9101-12 a

tonalite hbl. diorite

16. ZA9101-11 17. 395C

diabase hbl. gabbro

18. 507C

troctolite

P H P W P H P

Angat ophiolite 19. ANG9101-1 20. ANG9101-3 21. ANG9007-2 22. IPOH

diabase diabase tonalite ol. gabbro

W W W P

17.89 17.73 18.19 18.02

15.55 15.50 15.52 15.49

37.65 37.44 37.97 37.73

0.703385"17 0.703220"14 0.703373"15

0.513108"16 0.513128"12 0.513119"42

68 56 105

242 449 405

1.05 1.00 1.89

3.06 2.75 5.60

W W

18.44 18.20

15.52 15.51

38.03 37.92

0.704468"17 0.703767"17

0.513113"12 0.513104"16

151 149

351 162

0.760

2.12

Cretaceous laÕas 23. ANT90-2 basalt 24. NA9007-1 basalt

Rb, ppb

28.3

63.7

0.513184"24

8.17

J. Encarnacion ´ et al.r Chemical Geology 156 (1999) 343–357

Sample name

J. Encarnacion ´ et al.r Chemical Geology 156 (1999) 343–357

are about 20 pg and 100 pg, respectively, and are both insignificant relative to the amount of sample analyzed. Isotope dilution and composition measurements were performed on a VG Sector multicollector thermal ionization mass spectrometer at the University of Michigan. All Pb isotopic compositions in Table 1 were measured in static mode using Faraday collectors for all masses. Within-run precisions on Pb ICs were better than the uncertainty from fractionation. The Pb isotopic ratios are corrected for mass discrimination at 0.11 " 0.04% amuy1 based on replicate analyses of National Institute of Standards and Technology ŽNIST. standard SRM981. The 87 Srr86 Sr ratios are presented normalized to 86 Srr88 Sr s 0.1194; the 143 Ndr144 Nd data are presented normalized to 146 Ndr144 Nd s 0.7219. The mass spectrometer used gave 143 Ndr144 Nd s 0.511841 " 15 Ž2 s . for the La Jolla Nd standard, and 87 Srr86 Sr s 0.710246 " 15 Ž2 s . for NIST-SRM987.

4. Preservation of primary magmatic isotopic composition Drawing conclusions regarding the isotopic character of the magma sources for the Zambales ophiolite is possible only if the measured isotopic compositions have not been modified since magmatic crystallization. The Pb and Nd isotopic compositions of ocean floor crust are relatively unaffected by sea water interaction due to the extremely low relative concentrations of these elements in sea water Že.g., O’Nions and Pankhurst, 1976; Chen and Pallister, 1981; Calvez and Lescuyer, 1991.. In contrast, the Sr isotopic composition of samples from the upper oceanic crust can be markedly affected Že.g., Spooner et al., 1977; McCulloch et al., 1981.. The degree of

349

Fig. 3. Sr and Pb isotopic diagram for mineral separates from the Zambales ophiolite having both Sr and Pb isotopic composition data. Samples with higher 87 Srr86 Sr tend to have higher 207 Pbr204 Pb, suggesting that the elevated Sr isotopic ratios are not the result of seawater alteration, but are magmatic.

change in 87 Srr86 Sr roughly correlates with the degree of petrographic alteration of the rocks and the effect is generally confined to horizons above the upper level gabbros ŽBickle and Teagle, 1992.. Petrographically fresh gabbros and plagioclase separates from gabbros of the Troodos ophiolite ŽSpooner et al., 1977. yielded 87 Srr86 Sr values equivalent to those derived from fresh glass of the volcanic section ŽRautenschlein et al., 1985.. Hence, acid leached, optically clear, hand-picked, igneous mineral separates from relatively fresh rocks should yield close to primary magmatic values. The Sr isotopic compositions reported here Ž0.70268 to 0.70412. are similar to magmatic values reported for other ophiolites ŽRautenschlein et al., 1985. and active backarc basins Že.g., Volpe et al., 1987, 1990, and references therein.. In addition, the higher 87 Srr86 Sr values of the Acoje block Ž0.70331 to 0.70412. correlate with 207 Pbr204 Pb ŽFig. 3. supporting the contention that the 87 Srr86 Sr values are magmatic. Clinopyroxene

Notes to Table 1: f s type of material analyzed as follows; P—plagioclase, C—clinopyroxene, O—orthopyroxene, H—hornblende, W—whole rock. Abbreviations under ‘Rock type’: pyx—pyroxenite, cpyx.—clinopyroxenite, ol.—olivine, webst.—websterite, hbl.—hornblende. Errors on Sr and Nd isotopic compositions are within run 2 s errors; errors on Pb isotopic compositions are "0.04% amuy1 Ž2 s . based on the reproducibility of NIST-SRM981. a Samples with zircon ages in Fig. 7. b Sr data not plotted in Figs. 3 and 5. The U, Pb and Rb concentrations are corrected for blank contribution. See Section 3 for additional information. Samples 1 and 2 are from the Sual area, sample 3 is from the Barlo area, and samples 4 to 11 are from the Acoje area of the ‘Acoje block’.

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from fresh layered gabbronorites and pyroxenites from the arc-like Acoje area have higher 87 Srr86 Sr than most of the Coto samples, even those from slightly altered shallow level plutons. Some of the Zambales samples were analyzed by isotope dilution for U, Pb, Rb, Sr, Sm, and Nd concentrations ŽTable 1.. The low concentrations of parent isotopes relative to daughter isotopes assures that the measured isotopic compositions have not been modified significantly by radiogenic growth since magmatic crystallization at ; 45 Ma. Note that the Sr isotopic analyses from the Angat ophiolite and Cretaceous lavas in the Southern Sierra Madre are on whole rock samples that have secondary alteration likely associated with sea floor metamorphism. These are probably not representative of magmatic compositions, and therefore are not discussed further.

5. Zambales mantle source characteristics Zambales Nd isotopic data ŽFig. 4. from the crustal samples and pyroxenite and gabbroic dikes from the mantle section fall within the range of values reported by Evans et al. Ž1991. on volcanic

rocks and mafic crustal dikes of the Masinloc massif Ž143 Ndr144 Nd s; 0.5131.. We also report Nd data from the Eocene Angat ophiolite and Cretaceous ophiolitic lavas east of the Angat ophiolite ŽFig. 1.. These are nearly identical to the Nd isotopic composition of the Zambales samples. The Luzon ophiolite samples are very uniform in Nd isotopic composition Ž ´ Nd s q9 to q10. in contrast to many ocean island and arc basalts from single islands and orogenic peridotite samples from a single massif Že.g., Reisberg and Zindler, 1986; Zindler and Hart, 1986; Mukasa et al., 1991.. Unlike the 143 Ndr144 Nd ratios which reveal an ancient, relatively uniform depletion in more incompatible elements ŽNd relative to Sm. throughout the ophiolite, Pb data exhibit significant differences in 207 Pbr204 Pb between the areas underlain by arc-like crust and MORB-like crust ŽFigs. 5 and 6.. Samples from the MORB-like areas ŽCoto block. overlap with the MORB field in 207 Pbr204 Pb– 206 Pbr204 Pb space, whereas samples from the arc-like areas form an array rising above the MORB field. Two whole rock Pb isotope analyses reported by Hamelin et al. Ž1984. from a harzburgite and dolerite sample from the Zambales ophiolite Žthe locations of samples from

Fig. 4. Nd isotopic data on mineral separates and whole rocks from the Zambales ŽAcoje and Coto blocks., Angat, and Cretaceous ophiolites plotted in their relative positions, west to east, across Luzon Žcompare Fig. 1.. Note the relative uniformity across Luzon and the lack of any major difference between the arc-like Acoje and MORB-like Coto block of the Zambales ophiolite. The uncertainty bars represent "2 s within run precisions.

J. Encarnacion ´ et al.r Chemical Geology 156 (1999) 343–357

351

Fig. 5. Ža. 207 Pbr204 Pb and Žb. 87 Srr86 Sr data from various horizons in the Zambales ophiolite. Pb and Sr from the arc-like Acoje block and San Antonio massif Žfilled and patterned symbols. are more radiogenic than the MORB-like Coto block Žopen symbols.. Errors on the Pb data are "2 s and are based on the reproducibility of a standard. The "2 s errors for the Sr data are smaller than the data symbols.

within the ophiolite were not provided. are within uncertainty of the fields defined by the data reported here. The Sr isotopic ratios on acid-leached mineral separates in the arc-like sections of Zambales are also enriched, exhibiting higher 87 Srr86 Sr Ž0.70331 to 0.70412. than the MORB-like areas Ž0.70268 to 0.70331. ŽFigs. 3 and 5.. Previous work has shown that minor and trace element systematics of the arc-like lavas and dikes of the Zambales ophiolite, especially the boninitic suite,

are consistent with their derivation from a relatively depleted source that experienced previous melt extraction. The source was then later enriched in LILE before, or during the succeeding melting event Že.g., Hawkins and Evans, 1983; Evans et al., 1991; Yumul, 1996.. Such ‘second stage’ melts have long been recognized from major and trace element systematics in ophiolitic lavas Že.g., Sun and Nesbitt, 1978.. The array generated by the arc-like samples in 207 Pbr204 Pb y206 Pbr204 Pb space ŽFig. 6. rising

352

J. Encarnacion ´ et al.r Chemical Geology 156 (1999) 343–357

sediment- " slab-derived component was probably involved in the generation of arc-like magma. The occurrence of the enriched Pb and Sr isotopic signature in the same areas that also experienced LILE enrichment suggests that the LILE-enriched component that affected the arc-like areas was at least partly derived from the subduction component rather than from small degree partial melts from the mantle wedge. The samples with enriched Pb and Sr isotopic compositions do not show an inverse correlation with Nd isotopic composition. That is, high 87 Srr86 Sr is not coupled with low 143 Ndr144 Nd, which might be expected if subducted ancient sediment underwent melting and was mixed with the source region of the arc-like magmas. The Nd isotopic compositions indicate derivation from a relatively uniform MORB-like mantle wedge with no indication of lower 143 Ndr 144 Nd in the arc-like areas. This suggests that at some point during its transfer from the slab to the melting zone in the mantle wedge, the subduction component was probably an aqueous fluid. This would account for the transport of Pb and Sr and the apparently limited mobility of Nd Že.g., Tatsumi et al., 1986; Arculus, 1987..

6. Generation of arc-like crust in the Zambales ophiolite Fig. 6. Lead isotope correlation diagrams for the Zambales, Angat, and Cretaceous ophiolites. Fields for MORB and late Tertiary– Quaternary volcanoes and plugs in central Luzon are shown for comparison ŽMukasa et al., 1994.. Shown in panel ‘b’ are the shifts in Pb isotopic compositions for present day U–Pb reservoirs with various 238 Ur204 Pb Žm . ratios when age corrected to 45 Ma. The Zambales samples have 238 Ur204 Pb ratios of about 4 to 40. The elevated 207 Pbr204 Pb of arc-like samples Žsolid symbols. over MORB-like samples Žopen symbols. of Zambales is a robust feature.

above the MORB field suggests the involvement of subducted ancient sediment in the source of the arc-like magmas Že.g., Hawkesworth et al., 1993, and references therein.. The radiogenic Sr signal in the same samples provides additional evidence that a

A model for the origin of second stage melts and formation of arc-like crust in the Zambales ophiolite must be consistent with the following observations. Ža. Geochemical data from the arc-like magmas, indicating HFSE and REE depletion, require generation from a relatively refractory source, most likely residual from previous melt extraction, that was subsequently enriched in LILE " LREE ŽHawkins and Evans, 1983; Evans et al., 1991; Yumul, 1996.. Žb. Zircon U–Pb data from the various blocks of the ophiolite indicate that the arc-like sections are nearly the same age, or are slightly younger—but not more than ; 1 Ma younger—than the MORB-like sections as shown in Fig. 7 ŽEncarnacion ´ et al., 1993.. Žc. The isotopic data presented here suggest that the LILE enrichment in the arc-like areas is at least

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Fig. 7. Zircon 206 Pbr238 U ages of silicic plutons Žplagiogranites. of the Zambales ophiolite Ždata from Encarnacion ´ et al., 1993.. Ages from the Acoje and San Antonio areas are from single samples; the ages from the Coto area are from two different samples. The sample from the Acoje block ŽBarlo area. is a tonalite that may be an extreme differentiate of a boninitic melt ŽFlorendo and Hawkins, 1993..

partly derived from a subduction component and is not just a product of small degree melts. Žd. The presence of an enriched Pb and Sr isotopic signature throughout the crust and mantle of the Acoje block, and the presence of a dike complex suggest formation of the arc-like sections in an extensional setting. There is no evidence that the arc-like magmas were built onto an older MORB-like basement. The arclike magmas were possibly generated at the same oceanic spreading center from which the adjacent MORB-like crust formed. The lack of a mature arc edifice directly overlying the ophiolite is consistent with its formation in an oceanic extensional environment separate from the main arc. Že. In spite of the broad geochemical differences between the Acoje and Coto blocks of the Zambales ophiolite, there is no evidence for a major tectonic contact between them. Indeed, geochemical and field evidence support a transitional contact ŽAbrajano et al., 1989; Rossman et al., 1989; Evans et al., 1991.. This observation is consistent with the previous point, and with the nearly indistinguishable ages of the arc-like and MORB-like sections. Three basic scenarios for the origin of arc-like crust in the Zambales ophiolite are illustrated in Fig. 8. In the first scenario, an influx of hydrous fluids carrying the enriched Pb and Sr fingerprint as well as

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LILE " LREE trigger a second melting event in the residual peridotite that has just yielded MORB-like melts at the spreading center ŽFig. 8a.. The resulting melts have a depleted arc-like signature but are variably enriched in LILE and inherit the high 87 Srr86 Sr and 207 Pbr204 Pb from the fluid. The second stage and any subsequent melting events is facilitated by the near-solidus temperature of the residual depleted peridotite. This model obviously requires that the spreading center of the ophiolite was above an area of subducting slab where hydrous fluids were being released Žcf. Meijer, 1980; Stern and Bloomer, 1992.. The slab may have been the one associated with the Sierra Madre arc in the east, or an as yet unidentified subduction system. This scenario raises the following questions: What caused the influx of fluids at one time, and not earlier when the MORB-like Coto section was formed? Why did the generation of arc-like crust cease, and not continue to develop into a more mature arc edifice? A second model, shown in Fig. 8b, involves heating and remelting of hydrated, depleted lithospheric mantle. This may happen during propagation of a spreading ridge within backarc lithosphere as is happening for example in the Lau backarc basin Že.g., Falloon et al., 1992.. In the case of the Zambales ophiolite, this scenario requires that the ridge propagate into lithosphere that is not more than ; 1 Ma old. In addition, the depleted lithosphere needs to be impregnated by the subduction fluid after generation of the MORB-like Coto section, and before arrival of the propagating ridge that forms the arc-like crust less than ; 1 Ma later. One also might expect to see some structural manifestation of the disruption caused by the ridge propagation in the area between the Acoje and Coto blocks. However, no major continuous fault zone, or intrusive boundary, has been identified between the two ‘blocks’; the contact is transitional in well-mapped areas ŽAbrajano et al., 1989; Rossman et al., 1989.. A third possible scenario, which does not require direct fluxing by the subduction fluid, nor a special tectonic event in the suprasubduction zone lithosphere is shown in Fig. 8c. In this model the suprasubduction zone spreading center taps a depleted Ži.e., residual. mantle domain that has been hydrated by a subduction fluid. This depleted and subsequently enriched mantle domain undergoes simple

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decompression melting at the spreading center thus generating the arc-like crust. The first melting event that depletes the mantle source, and the re-enrich-

ment by the subduction zone fluid occurs much earlier and elsewhere in the mantle wedge, most likely near the source of the active arc. Depleted, harzburgitic, mantle xenoliths impregnated with phlogopite veins that carry an enriched subduction signature have been recovered from arc lavas ŽVidal et al., 1989.. The survival of such potential arc mantle source domains and their availability for tapping by a backarc spreading center would depend on the stability of phlogopite. If phlogopite breaks down and triggers partial melting in the host mantle, then arc-like melts would invade the backarc lithosphere, rather than erupt at the spreading ridge. Experimental work suggests that phlogopite could exist in the mantle wedge at depths of 50 to 150 km if T does not exceed ; 11508C to ; 13508C, respectively ŽSato et al., 1997.. If the Zambales ophiolite formed in a backarc area then such an arc-like mantle source could end up beneath the backarc spreading center if backarc spreading is occurring as a response to slab roll back. In such a setting, as the arc moves trenchward, the backarc spreading center will also migrate trench-ward at half the velocity of the arc. The backarc spreading center will eventually migrate over

Fig. 8. Three different scenarios for the formation of arc-like and MORB-like crust in the Zambales ophiolite. The arc-like sections are very close in age, or are only slightly younger than the MORB-like sections as shown in Fig. 7, and both appear to have formed in a suprasubduction zone, oceanic, extensional environment. The subducting slab is not shown, but it may have been associated with the Sierra Madre arc Žsee Fig. 1.. Ža. Hydrous subduction fluids carrying the radiogenic Sr and Pb, as well as other LILE"LREE, are added to residual mantle that has just yielded MORB-like melts. Addition of the hydrous fluid to the near-solidus mantle triggers additional melting that generates the arc-like crust Žcf. Meijer, 1980.. Žb. Young MORB-like lithosphere that had been created at a nearby spreading center is subsequently impregnated by the subduction fluid. The hydrated, depleted mantle is then heated by rising asthenospheric mantle during thinning associated with a propagating ridge Žcf. Falloon et al., 1992.. Žc. A passive domain of hydrous depleted mantle undergoes decompression melting as it is tapped by the spreading center. Previous extraction of basaltic magma from this mantle domain and subsequent impregnation by enriched subduction fluids occurs elsewhere in the mantle wedge—perhaps beneath an associated volcanic arc. Shaded regions represent the sources of arc-like magmas.

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an area formerly beneath the arc. The arc source in the mantle wedge is, of course, unlikely to remain stationary because of induced convection and inflow of fresh mantle into the wedge. It is not inconceivable, though, that such an arc-like source can survive in the wedge until tapped by a suprasubduction zone spreading center. Variable degrees of depletion and re-enrichment of the mantle source domains as well as mixing of primary melts may explain the variety of crustal rocks in the Zambales ophiolite, from MORB-like to boninitic.

Acknowledgements Various versions of this work benefited from reviews by A.J. Crawford, E.J. Essene, A.N. Halliday, J.W. Hawkins, P.J. Patchett, and R.J. Stern. Encarnacion ´ thanks the National Institute of Geological Sciences of the Philippines for its hospitality. This study was supported by U.S. National Science Foundation grant EAR9018967 to Mukasa, and the Scott Turner Fund of the University of Michigan. Some of the Zambales samples were collected by Evans under NSF grants OCE7519148, OCE7920483, and OCE8024894. [PD]

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