Varying mantle sources of supra-subduction zone ophiolites: REE evidence from the Zambales Ophiolite Complex, Luzon, Philippines

TECTONOPHYSICS ELSEVIER

Tectonophysics 262 (1996) 243-262

Varying mantle sources of supra-subduction zone ophiolites: REE evidence from the Zambales Ophiolite Complex, Luzon, Philippines G.P. Yumul National Institute of Geological Sciences, Unit'ersity (~["the Philippines, Dilimcm. Quezon City. Philippim's Received 26 September 1994: accepted 18 December 1995

Abstract

The Zambales Ophiolite Complex, a supra-subduction zone ophiolite, is characterized by three volcanic-hypabyssal rock units: the Coto Block volcanic-hypabyssal rocks, the Coto dikes intruded into the Coto residual peridotites and the Acoje Block volcanic-hypabyssal rocks. The first two groups exhibit transitional mid-ocean ridge-island arc characteristics while the latter reveals island arc affinity. Furthermore, these three volcanic rock suites are characterized by differing bulk REE, major-element, trace-element and mineral chemistries. The Coto Block volcanic-hypabyssal rocks [(Ce/Yb), 0.4 1.0: TiO, 0.50-1.50 wt.%; Zr 31-76 ppm; Y 13-31 ppm] have higher REE, Ti, Zr and Y than the Coto dikes [(Ce/Yb), 0.3-0.8; TiO~ 0.52-0.94 wt.%; Zr 15-55 ppm; Y 10-27 ppm] and the Acoje Block volcanic-hypabyssal rocks [(Ce/Yb),, 0.2-0.3: TiO 2 0.26-0.86 wt.%; Zr I 1-45 ppm; Y 10-23 ppm] arguing for an increasing degree of melting of the sources from the former to the latter. These data suggest the involvement of several mantle sources which have undergone different degrees of partial melting and LREE addition as evidenced by the presence of LREE-enriched basalts [(Ce/Yb) n 3-5] among the Coto Block volcanic-hypabyssal rocks. These results further illustrate the complexities involved in the generation and evolution of supra-subduction zone ophiolites. 1. Introduction

Geochemical studies on several ophiolites have revealed the presence of volcanic and hypabyssal rock suites exhibiting both mid-ocean ridge basalt (MORB) and island arc tholeiite (IAT) affinities (e.g., Alabaster et al., 1982; Coleman, 1984; Shervais and Kimbrough, 1985; Ishiwatari et al., 1990; Pedersen and Fumes, 1991). The varying geochemistry of these rock suites are attributed to evolving mantle sources, different degrees of partial melting, crystallization or other processes operative during magma generation, ascent and extrusion (Arculus, 1987: Fumes et al., 1992; Taylor et al., 1992).

Subduction is an important process during the formation of these ophiolite complexes, that are better known today as supra-subduction zone (SSZ) ophiolites (Pearce et al., 1984; Taylor and Nesbitt, 1988; Shervais, 1990; Jones et al., 1991; Hawkins and Florendo, 1992). Aside from mineral, bulk major- and trace-element chemistry and isotope systematics, rare earth element (REE) geochemistry is a good tool in delineating the evolutionary history of a particular ophiolite suite. In this respect, the different volcanic-hypabyssal rock groups of the Zambales Ophiolite Complex (ZOC), which also exhibit M O R B - I A T signatures, were analysed for their REE contents. It

0040-1951/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S 0 0 4 0 - 195 I ( 9 6 ) 0 0 0 t 3-3

G.P. Yumul / Tectonophysics 262 (1996) 243-262

244

is the purpose of this paper to present REE evidence which will reveal that evolving mantle sources that underwent varying degrees of partial melting were

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responsible lbr the generation of this ophiolite complex. It is hoped that the information presented here will help not only in our understanding of upper

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INDEX

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~T'.~ Cumulates [e.g.Troetolite, - ~ - ~ Gobbro,ete )

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CABANGAN

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UNIT)

15=00 San Antonio ........

actore Zone ~:~3;;~.

-COTO BLOCK SAN ANTONIO

Bay Fault 120"00 Fig. 1. Generalized geologic map of the Zambales Ophiolite Complex. Available geochemical and petrological data show that the Acoje unit and San Antonio Massif are similar; the same can be said for the Coto unit and the Cabangan Massif. See inset for the distribution of the Acoje and Coto Blocks. Map modified from the Philippine Mines and Geosciences Bureau quadrangle geologic maps.

G.P. Yumul / Tectonophysics 262 (1996) 243-262

mantle-crust interactions but will also elucidate and add to our knowledge on the generation and evolution of SSZ ophiolites.

2. Geology 2.1. A geologic outline The ZOC is composed of three massifs, Masinloc, Cabangan and San Antonio, which are distributed from north to south (Fig. !). The Cabangan Massif is the fault-bounded block in the central portion of the ophiolite. Furthermore, the Masinloc Massif is divided into two units, the Acoje and Coto, which on

245

the basis of petrological evidence has been recognized to preserve island arc and transitional mid-oceanic ridge-island arc characteristics, respectively (e.g., Hawkins and Evans, 1983; Abrajano et al., 1989; Geary et al., 1989; Evans et al., 1991; Florendo and Hawkins, 1992). The boundary between these two units is still not clearly defined although a fault is believed to define the contact in some places. Mine geologists of the Benguet Corporation-Masinloc Chromite Operation in the Coto unit of the Masinloc Massif reported what they believe as Coto harzburgites as clasts in Acoje gabbronorite bodies (G. Muerong, pers. commun., 1986). This gives the possibility that some portions of the Acoje "pluton" is intruded into the Coto unit. Geochemical and

BASALT WITHIb MASSIVESULFIDESF

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DIABASE- DIORITE• DIKES r

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:+++++++++4 +++++++++4 +++++++++4

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podiform ~chromites

*++++++++"*++++++++~ ++++++++++ -RED OXENITESITES NSITION ZONE DUNITES iRE[} WEHRLITES

dunite pods~ l

m~crogabbro&l, diabasedikes

pyrO~gabb

HARZBURGITES~'

Coto

;ZBURGITESRZOLtTES

Acole GENERALIZED ROCK COLUMNS Fig. 2. Synthetic stratigraphic column based on observations made on the Acoje and Coto Blocks. Column is not drawn to scale. Hawkins and Evans (1983) estimated the Coto unit of the Masinloc Massif to have 8-10 km thickness of peridotite and approximately 3.5 km thickness of dunite and gabbro section; The Acoje unit was estimated to have 10-12 km thick peridotite and approximately 8 km of ultramafic to mafic cumulate section. Explanations: T.Z.-Transition Zone; pge-Platinum Group Elements. See text for discussion.

246

G.P. Yumul / Tectonophysics 262 (1996) 243-262

petrological data show that the Cabangan Massif and Coto unit (which hereafter will always be referred to as the Coto Block) are geochemically similar. The same can be said for both the San Antonio Massif and the Acoje unit (which hereafter will be referred to as the Acoje Block) (Yumul et al., 1990; Yumul, 1992) (Fig. 1). The ZOC is a north-south trending, east dipping complete ophiolite suite composed of residual refractory and fertile harzburgites and lherzolites, transition zone dunites, layered ultramafic and mafic cumulates, sheeted dike and sill complexes and overlying volcanic rocks. No single coherent stratigraphic column can be made for the ZOC since the different blocks of this complex preserve distinct physical characteristics and geochemical affinities (Fig. 2). The capping pelagic Aksitero Formation and U - P b geochronology of zircons from associated plagiogranites give a minimum Eocene age of formation for this ophiolite complex (e.g., Garrison et al., 1979; Schweller et al., 1983; Encamacion et al., 1993).

2.2. Field exposures of the Zambales Ophioliw Complex t,olcanic and hypabyssal rocks

dikes represent short-lived subduction related magmatisms independent of the magmatism that formed the Coto volcanic-hypabyssal rocks (Hawkins and Evans, 1983: Yumul and Datuin, 1991).

2.3. Petrography 2.3.1. The Coto Block: t'olcanic-hypabyssal rocks The basalts and diabases of the Coto Block exhibit the same mineralogical assemblage of clinopyroxene, plagioclase and olivine. The order of appearance was olivine, plagioclase and then clinopyroxene. Plagioclase and olivine are the most and least abundant minerals, respectively. The textures range from spherulitic to porphyritic to aphyric and from intergranular to intersertal. Porphyritic rocks have clinopyroxene and, to a lesser extent, plagioclase and olivine as dominant phenocrysts. Plagioclase is usually lath-shaped while clinopyroxene is dominantly tabular and euhedral-shaped. Hour-glass extinction

Table I Accuracy and precision of the Rigaku System 3080E3 XRF of ERI, Tokyo University (after Kaneko. 1990) Oxide/element

The Coto and Acoje Block exposures show that the sheeted diabase dikes intruded or cut the overlying pillow basalts. In the east side of the ZOC, specifically in the Tarlac area, these "dikes" are nearly horizontal and conformable with the layering of the pillow basalts. The intrusive relationships among the basalts and diabases within each block suggest that these rocks were nearly contemporaneous. In the Coto Block, specifically in the Subic area of the C a b a n g a n Massif, L R E E - e n r i c h e d diabasic/basaltic dike rocks cut LREE-depleted volcanic and hypabyssal rocks. As observed in numerous outcrops, the Coto Block residual harzburgites are cut by numerous diabasic to gabbroic dikes. These dikes, which are called Coto dikes in this paper, are not related to the overlying Coto Block sheeted diabase dike complex. No continuity between these two rock groups was observed in the field. The Coto dikes, as will be shown later, exhibit transitional M O R B - I A T geochemical signatures while the host Coto Block peridotites are of MORBaffinity (Yumul, 1989). It is believed that the Coto

SiO, TiO, AI~O~ FeO MnO MgO CaO Na~O K ,O P~O: Rb Sr Ba Y Zr V Ni Cu Zn Ga Nb Sc Th

I-o of calibration linc 0.148 0.014 0.178 0.034 0.003 0.046 0.029 0.043 0.012 0.004

c~ eli c~ c~ % % c7, c~ cA :/~

2.64 ppm 10.60 ppm 17.60 ppm 1.47 ppm 6.28 ppm 9.95 ppm 3.25 ppm 2.18 ppm 4.30 ppm 0.86 ppm 1.07 ppm 2.41 ppm 2.20 ppm

I-~r JB-I STD (60 times) 0.083 0.006 0.071 0.020 0.002 0.045 0.014 0.005 0.005 0.003

c/~ c} f/~ c~ c/i c~ :/, ~ (} 9~

0.81 ppm 1.80 ppm 8.51 ppm 0.58 ppm 2.00 ppm 8.18 ppm 0.88 ppm 0.85 ppm 1.34 ppm 0.78 ppm 1.34 ppm 1.56 ppm 1.27 ppm

G.P. Yumul / Tectonophysics 262 (1996) 243-262

and sector zonings are very apparent among the clinopyroxene phenocrysts. Altered olivine is also observed as a phenocryst and groundmass mineral together with clinopyroxene and plagioclase. The extremely high Ni and Cr contents of some porphyritic rocks could be due to olivine and clinopyroxene accumulation. Geary and Kay (1983) have reported that the Coto Block sheeted diabase dikespillow basalts have undergone ridge crest high temperature greenschist-facies metamorphism. Typical secondary minerals noted are quartz, epidote, chlorite and the rare occurrence of metamorphic amphibole. Metamorphic amphiboles, carbonates and chlo-

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rites usually replace clinopyroxene. Quartz fills amygdules. Accessory minerals include magnetite, ilmenite and other opaques. 2.3.2. The Coto dikes

The dominant mineralogy of the dike rocks is plagioclase, clinopyroxene and rare occurrences of orthopyroxene, brown-colored magmatic hornblende and biotite. Altered samples show plagioclase replaced by epidote, clay minerals and clinozoisite, whereas clinopyroxene is altered to bastite, uralite and metamorphic green-colored hornblende. Quartz and epidote veinlets fill cracks. Magnetite is also

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Fig. 3. Plot of selected major and trace element chemistries of the Coto Block volcanic-hypabyssal rocks (symbol-circles), Coto Block LREE-enriched rocks (symbol-filled stars), Coto dikes intruded into the Coto residual peridotites (symbol-open triangles) and Acoje Block volcanic-hypabyssal rocks (dots). Analyses were normalized to 100% when plotted.

G.P. Yumul / Tectonophysics 262 (1996) 243-262

248

present as accessory mineral. Subautomorphic granular and intergranular textures are the dominant rock textures.

hydrothermal alteration on some of the volcanic-hypabyssal rock outcrops is very apparent.

2.3.3. The Acoje Block: volcanic-hypabyssal rocks

3. Analytical techniques

These basalts and diabases exhibit intersertal to intergranular, aphyric to porphyritic, spherulitic and poikilitic textures. The dominant groundmass and phenocryst minerals are clinopyroxene and plagioclase. The order of appearance was clinopyroxene and then plagioclase. Some of the basalts encountered are two-pyroxene basalts. Most of these volc a n i c - h y p a b y s s a l rocks have u n d e r g o n e greenschist-facies metamorphism resulting in the formation of secondary minerals that include chlorite, uralite, epidote, carbonates and clay. Some of the samples utilized in the analyses were collected from an abandoned massive sulfide mine. The intense

Rock chips, with the weathered portions removed, were initially powdered using a tungsten carbide mortar and pestle until the samples were ground to less than 1 mm in diameter. The samples were then transferred to an automated rotating agate mortar and pestle for final powdering. The samples were baked at 1000°C for two hours, cooled and kept overnight in a 110°C furnace with lithium tetraborate flux. For major-element analyses, 400 mg of sample was used. The sample-flux ratio was 1:10. The combined fired samples and fluxes with a drop of lithium bromide (LiBr) for each sample were then made into fused

Table 2 Representative bulk chemical analyses of the Coto volcanic-hypabyssal rocks ZO-2 Bst# SiO 2 TiO 2 A1203 FeO * MnO MgO CaO Na20 K20 P205 Total FeO * / M g O CaO/AI203 Rb Sr Ba Y Zr V Cr Ni Cu Zn Ga Sc

ZO-6 Bst

ZO-9 Bst

ZO- 13 Bst

* ZO- 176 Bst

ZO- 184 Dia

ZO-8 Dia

ZO-3 Bst

50.85 0.62 11.55 10.44 0.17 8.85 11.98 2.04 2.34 0.36 99.20

57.02 1.35 15.22 10.28 0.19 4.54 4.28 6.16 0.10 0.10 99.24

48.54 0.64 10.52 10.70 0.17 14.33 1 1.60 0.87 1.56 0.45 99.38

51.61 0.52 15.02 8.16 0.18 9.78 12.73 1.36 0.08 0.04 99.48

55.48 1.42 14.58 13.88 0.17 7.91 4.91 0.01 0.05 0.09 98.50

57.45 1.17 15.83 9.18 0.15 8.12 4.02 3.27 0.05 0.07 99.31

53.48 1.50 15.26 I 1.78 0.16 4.47 6.11 6.03 0.1 I 0.09 98.99

58.4 l 1.07 15.45 9.15 0.17 5.18 3.41 6.18 0.12 0.08 99.22

1.18 1.04

2.26 0.28

0.75 I. 10

0.83 0.85

1.75 0.34

1.13 0.25

2.64 0.40

1.77 0.22

27 314 224 15 36 329 226 53 146 92 11 36

1 61 8 29 72 323 23 11 18 85 18 30

15 431 161 14 48 284 622 210 126 78 13 30

2 144 15 31 228 426 95 14 67 12 47

< 1 103 < I 27 66 412 14 < 1 112 86 16 34

< 1 87 24 51 314 14 17 5 31 14 32

2 129 18 31 76 437 21 5 35 67 21 34

< 1 98 5 24 60 297 11 6 20 89 18 33

G.P. Yumul / Tectonophysics 262 (1996) 243-262

249

Table 2 (continued)

SiO~ TiO 2 AI20:~ FeO • MnO MgO CaO Na20 K20 P205 Total FeO ~/Mgo CaO/AI20 ~ Rb Sr Ba Y Zr V Cr Ni Cu Zn Ga Sc

ZO- 180 Dia

ZO- 181 Dia

ZO-402 Dia

ZO-406 Bst

ZO-410 Bst

ZO-411 Bst

ZO-414 Bst

55.26 0.71 16.78 9.13 0.11 6.24 8.25 2.34 0.61 0.05 99.48

53.31 1.06 16.55 11.10 0.11 5.57 5.38 5.69 0.13 0.08 98.98

55.51 0.77 16.80 10.44 0.16 3.43 2.29 7.58 0.11 0.07 97.16

54.63 0.98 15.78 9.49 0.16 5.37 4.75 6.53 0.17 0.08 97.94

58.04 0.96 15.13 7.94 0.16 4.32 4.48 6.79 0.18 0.08 98.08

56.91 1.00 16.52 9.01 0.16 4.77 3.53 7.21 0.15 0.08 99.33

48.84 0.89 15.65 9.51 0.19 7.23 9.54 2.76 1.92 0.37 96.90

~.45 0.49

1.99 0.32

3.04 0.14

1.77 0.30

1.84 0.30

1.89 0.21

1.32 0.61

3 157 6 19 40 308 26 25 7 41 16 27

< 1 15l 2 27 64 361 14 4 5 59 19 39

47 25 49 307 12 7 128 96 18 39

43 25 58 303 33 17 19 87 16 39

59 25 63 254 39 16 44 93 13 28

107 24 64 253 41 17 14 82 14 < 1

20 463 137 22 62 295 177 71 121 88 18 36

FeO = total iron. * * ZO-Z 3, 6, 8, 9, 13, 402, 404, 406, 410, 411 and 414 samples were collected from the Subic-Olongapo Area. ZO-176, 180. 181 and 184 samples were collected from the Tarlac Area. # Bst = basalt; Dia = diabase.

beads. For t r a c e - e l e m e n t analyses, a p p r o x i m a t e l y 5 g o f unfired samples were m a d e into pressed pellets using 3 m m thick plastic P V C rings, 3.2 c m in diameter. The bulk chemistry o f the v o l c a n i c - h y pabyssal rocks were analysed utilizing the Rigaku S y s t e m 3080E3 X R F o f the Earthquake Research Institute (ERI), University o f Tokyo. The standards and p r o g r a m s used in measuring and recalculating results were those o f Professor Shigeo A r a m a k i ' s laboratory at E R I (Table l) (Kaneko, 1990). The bulk-rock R E E chemistry o f the v o l c a n i c - h y pabyssal rocks were analysed by instrumental neutron activation analyses ( I N A A ) . 100 m g of powdered samples were sealed in p o l y e t h y l e n e film and irradiated by neutron activation in the T R I G A - I I nuclear reactor of the Institute for A t o m i c Energy,

R i k k y o University. The thermal neutron flux was 4 - 5 X 1 0 ~ n / c m 2 s and the irradiation time was 24 h. The irradiation flux ratio o f each sample was calibrated using pure iron wires as flux monitors. C o u n t i n g was carried out twice b e t w e e n 5 and 40 days after irradiation by using a 100 cm 3 pure Ge detector for g a m m a rays at the Radioisotope Center o f the University o f Tokyo. Utilizing a B A S I C calculation p r o g r a m " N A C P " which automates the peak and baseline fitting process of each spectrum by the least squares m e t h o d ( W a t a n a b e and Iwamori, 1991), the contents of eight (8) R E E (La, Ce, Nd, Sm, Eu, Tb, Yb and Lu) were determined f r o m the peak area o f each nuclide. JB-1 was used as a working standard. The precision estimated from the statistical counting error was less than 5% for La, Sm and Eu,

G.P. Yumul/ Tectonophysics 262 (1996) 243-262

250

about 10% for Ce, Tb, Yb and Lu and about 15% for Nd. The REE data were then normalized using the Leedey 73 chondrite of Masuda et al. (1973) to eliminate the odd-even effect of the elemental abundances. The normalized REE abundances were then plotted on a semi-logarithmic scale versus atomic number type of plot.

4. Results of analyses 4.1. Bulk major- and trace-element chemistry Utilizing F e O * / M g O as a fractionation index (with FeO* = total iron), SiO 2, TiO 2 and FeO increase while CaO decreases as F e O * / M g O increases. No change is noted in MnO and P205 which plot within narrow ranges while the K~O plot displays no distinct trend. As F e O * / M g O increases,

Na20 and A1203 slightly increase and then decrease. In terms of the trace-element data, as F e O * / M g O increases, there is an increase in Y, Zr, V, Ga and a decrease in Ni and Cr. The Coto Block and Acoje Block volcanic and hypabyssal rocks register a slight decrease in Sc whereas the Coto dikes register limited variation as fractionation proceeded. Data on Rb, Sr, Ba, Cu and Zn are scattered, and define no distinct trend. These minor elements are usually mobile when subjected to greenschist-facies metamorphism and hydrothermal alteration which has been the case for the Acoje Block rock group (Tables 2-4; Fig. 3).

4.2. Bulk-rock REE geochemistry 4.2.1. The Coto Block t,olcanic-hypabyssal rocks All of the LREE-enriched basalts and diabases of the sheeted dike complex (ZO-2, ZO-9 and ZO-413)

Table 3 Representative bulk chemical analyses of the Coto dikes ...... intruded into the Coto Block residual peridotites

SiO 2 TiO 2 A1202 FeO * MnO MgO CaO Na20 K 20 P205 Total FeO * / M g O CaO/AI20 Rb Sr Ba Y Zr V Cr Ni Cu Zn Ga Sc

ZO-48 Dia#

ZO-72 Dia

ZO-87 Dia

45.55 0.52 13.44 8.58 0.34 19.76 11.32 0.06 0.04 99.61

52.73 0.79 16.66 10.06 0.15 5.40 8.97 4.33 0.11 0.05 99.25

0.43 0.84

1.86 0.73

1 8 14 35 211 876 469 23 66 10 42

< I 252 11 21 48 351 17 20 75 65 15 42

ZO- 114 Dia

ZO- 115 Dia

ZO- 134 Dia

ZO- 135 Dia

52.19 0.90 17.71 9.75 0.15 4.21 10.55 3.67 0.1 I 0.05 99.33

52.48 0.86 16.61 I 1.06 0.18 5.17 7.92 4.62 0.20 0.05 99.15

57.83 0.92 15.89 9.98 0.16 3.52 8.29 2.88 0.13 (I.07 99.67

53.88 0.68 15.52 9. I 1 I).16 6.54 9.49 3.93 0.14 0.05 99.50

50.11 0.93 15.27 10.36 0.17 6.3 I 13.21 2.68 0.18 0.08 99.30

2.32 0.60

2.14 0.48

2.84 0.52

1.39 61.00

1.64 0.87

2 191 21 25 48 338 16 7 21 52 18 39

2 144 5 19 42 401 20 23 52 84 14 44

I 122 15 27 55 297 19 6 43 67 19 37

3 102 7 18 40 295 23 16 37 61 16 43

3 96 34 24 49 329 12 4 53 89 14 41

G.P. Yumul / Tectonophysics 262 (1996) 243-262

251

Table 3 (continued)

SiO 2 TiO~ A120~ FeO * MnO MgO CaO Na~O K20 P205 Total FeO / M g O CaO/Al20 ~ Rb Sr Ba Y Zr V Cr Ni Cu Zn Ga Sc

ZO-138 Mgb

ZO-141 Dia

51.22 0.73 16.44 8.91 0.15 7.91 1 1.07 2.88 0.15 0.05 99.51 l.l 3 0.67 2 218 8 18 42 292 90 36 48 66 15 40

ZO-145 Mgb

ZO-423 Dia

ZO-425 Dia

ZO-427 Dia

55.23 0.75 14.39 7.94 0.13 7.82 8.81 4.29 0.14 0.06 99.56

55.31 0.77 16.52 10.19 0.15 4.80 5.56 5.70 0.28 0.05 99.33

53.22 0.66 16.58 8.05 0.15 6.79 10.27 2.60 0. I0 0.05 98.47

50.25 0.70 17.16 9.03 0.15 7.18 9.85 2.89 0.20 0.06 97.46

51.49 0.36 12.89 8.37 0.16 12.36 10.91 1.78 0.10 0.03 98.45

1.02 0.61

2.12 0.34

I. 18 0.62

1.26 0.57

0.68 0.85

< 1 109 3 19 49 247 3t3 135 28 60 12 34

2 106 2 18 35 348 28 14 5 24 15 46

112 19 36 264 125 161 I1 57 0 0

0 136 2 20 37 290 116 66 13 65 0 0

74 11 17 223 818 234 22 61 0 0

FeO ~ = total iron. ..... All the Coto Dike samples, except ZO-423, 425 and 427 were collected fi'om open pits and drill holes of the Coto Mine. ZO-423, 425 and 427 were collected from the Bucao River area. # Dia = diabase: Mgb = microgabbro.

came from the Subic-Olongapo area, Cabangan Massif. Geary et al. (1989) also sampled and analysed relatively LREE-enriched basalts from the Tarlac side (eastern-central portion of the ophiolite) of the Coto Block. The LREE-depleted basalts have LREE contents of 2 - 1 5 × chondrite and HREE contents of 6 - 1 8 × chondrite (Fig. 4a). (La/Yb) n ( L a / Y b in chondrite-normalized values) ratios of these LREE-depleted basalts range from 0.3 to 0.7 while their (Sm/Yb)o ratios range from 0.96 to 1.7 (Table 5). The LREE-enriched basalts, on the other hand, have LREE contents of 16-25 × chondrite and HREE contents of 4 - 1 0 × chondrite (Fig. 4b). (La/Ce) n ratios of these LREE-enriched basalts range from 0.99 to 1.1 while their (La/Yb), and (Ce/Yb) n ratios are 3.0 to 5.0 and 3.1 to 4.6 respec-

tively (Table 5). Sheeted diabase dike samples analysed came from both the Cabangan Massic (SubicOlongapo area = ZO-8, 13, 408; Bucao area = ZO435, 439) and the Coto unit of the Masinloc Massif (Tarlac area = ZO-184). The sheeted diabase dike samples have LREE contents of 1-6 × chondrite and HREE contents of 3 - 1 0 × chondrite (Fig. 4c). (La/Yb) n ratios range from 0.15 to 0.61. ( S m / Y b ) n ratios of these rocks range from 0.62 to 1.30 (Table

6). 4.2.2. Coto dikes These diabasic, gabbroic to dioritic dikes intruded into the Coto Block residual peridotites are, as already mentioned, not related to the overlying sheeted dike complex. The Coto dike samples presented here

G.P. Yumul / Tectonophysics 262 (1996) 243-262

252

ophiolite, display LREE contents of 0.9-3 X chondrite and HREE contents of 6 - 9 X chondrite (Fig. 6b). (La/Yb) n ratios range from 0.17 to 0.46 while (Sm/Yb)n ratios range from 0.30 to 0.92 (Table 9). A considerable overlap exists between the Acoje basalts and diabases.

are all diabase. The Coto dikes have a range of LREE content of 1-4.5 X chondrite and HREE content of 3.5-12 X chondrite. ( L a / Y b ) n ratios range from 0.28 to 1.14 while ( S m / Y b ) , ratios range from 0.78 to 1.41 (Fig. 5; Table 7).

4.2.3. The Acoje Block L'olcanic-hypabyssal rocks The LREE contents of the Acoje basalts are 1-2 X 5. Discussion

chondrite while their HREE contents range from 3.5 to 9 X chondrite (Fig. 6a). The ( L a / Y b ) , ratios are considerably low, ranging from 0.14 to 0.35 while their (Sm/Yb)~ ratios range from 0.31 to 0.64 (Table 8). Most of these rocks have Z r / Y < 1.0 similar to boninites (e.g., Brown and Jenner, 1989). The diabases collected from Sual, Barlo and their immediate vicinities, located in the northern portion of the

5.1. --

Zambales Ophiolite Complex REE geochemisto' magmatic or metamorphic signatures?

The problem of REE mobility must always be considered first to determine whether the derived REE patterns indicate magmatic or metamorphic sig-

Table 4 Representative bulk chemical analyses of the Acoje volcanic-hypabyssal rocks * * ZO-64 Dia# SiO 2 TiO 2 AI203 FeO * MnO MgO CaO Na20 K 20 P205 Total FeO * / M g O CaO/A1203 Rb Sr Ba Y Zr V Cr Ni Cu Zn Ga Sc

ZO-292 Dia

ZO-293 Bst

ZO-295 Dia

ZO-298 Bst

ZO-302 Dia

ZO-304 Bst

ZO-309 Mdio

ZO-310 Mdio

ZO-315 Dia

58.31 0.86 15.80 9.11 0.11 3.61 11.60 0.00 0.05 0.07 99.52

55.05 0.35 16.77 8.53 0.16 7.00 5.84 4.81 1.03 0.02 99.56

53.78 0.30 14.03 7.96 0.18 11.73 9.49 1.77 0.36 0.02 99.62

56.77 0.43 15.66 9.10 0.16 5.26 7.25 4.29 0.06 0.02 99.00

53.12 0.33 16.22 8.01 0.18 10.66 9.28 0.90 0.70 0.02 99.42

51.38 0.40 16.91 8.84 0.26 10.47 10.18 0.44 0.56 0.02 99.46

57.16 (I.33 15.34 6.99 0.16 7.60 5.77 4.25 0.15 0.03 97.78

54.61 0.50 16.07 7.60 0.18 8.13 10.70 1.35 0.40 0.04 99.58

53.74 0.50 19.26 8.21 0.14 6.07 9.05 2.20 0.53 0.03 99.73

56.18 0.63 16.52 8.48 0.t5 5.37 9.41 2.31 0.31 0.04 99.40

2.52 0.73

1122 0.35

0.68 0.68

1.73 0.46

0.75 0.57

0.84 0.60

0.92 (I.38

0.93 0.67

1.35 0.47

1.58 1.57

<1 212 2 23 45 320 27 11 11 46 15 29

11 94 246 12 17 267 83 49 83 77 15 38

2 59 13 12 13 250 495 127 71 65 12 36

<1 55 12 21 329 23 22 89 54 16 37

4 62 7 12 13 265 416 113 157 134 13 45

2 34 9 15 13 289 354 101 5 132 14 46

68 13 18 224 70 48 4 81 11 40

2 110 14 30 239 228 55 20 104 13 42

4 111 13 26 254 59 27 14 107 17 38

2 102 7 17 31 331 39 21 314 83 16 4l

FeO * = total iron. • " ZO-292, 293, 295, 298, 302 and 304 samples were collected from the Barlo Mine. ZO-64, 309, 310 and 315 samples were collected from Sual, Pangasinan.# Bst = basalt; Dia = diabase; Mdio = microdiorite.

G.P. Yumul/ Tectonophysics 262 (1996) 243-262

natures. Considerable evidence has disclosed that REEs may be remobilized, either due to physical or chemical weathering, involving metamorphic or hyCOTO

10(]

(a)

BASALT

•

431 406 o 410 t, 176

"C. e.,,,.,,.t--~ L

i0

0 r" C)

~

~

x

COTO D I K E S INTRUDED INTO THE COTO U L T R A M A F I C ROCKS

4'CnO

t--

o 87

~

~

LaCe Nd SmEu "Fb COTO

Yb

BASALT

(b)

4-

4# 413 02 m9

"0

cC.) 0 !

i

i

i

La~ Nd ~Eu mb I00

_

~

~ "/2

0

423 134 135 48 114 427

"

Sm Eu

Yb

Fig. 5. Bulk chondrite-normalized REE patterns of the Coto dikes intruded into the Coto Block residual peridotites.

I

100

~

IX:

' '

CtC

_

* • $ • • o

Lo Co

o 0

'~b

COTO D I A B A S E

(C)

¢... 0

100

253

o 184 • 439 • 13 o 408 =8 =, 435

IO

0 o 0 I

i

L

i

i

LaCe hJd Sm Eu

Tb

Y'b

Fig. 4. (a) B u l k c h o n d r i t e - n o r m a l i z e d R E E patterns o f the C o t o

LREE-depleted basalts. (b) Bulk chondrite-normalized REE patterns of the Coto LREE-enriched basahs. (c) Bulk chondrite-norrealized REE patterns of the Coto diabases taken from the sheeted dike-sill complex.

drothermal fluids (e.g., Ludden and Thompson, 1978; Haskin, 1984; Schandl and Gorton, 1991). There is no general agreement to the degree of mobility that REEs undergo although it is deemed wise to always treat the REE patterns of altered rocks with caution (e.g., Humphris et al., 1978; Hajash, 1984; Humphris, 1984). To determine whether the ZOC volcanic-hypabyssal rock REEs have been remobilized or not, the total REE (La + Sm + Yb) contents of the volcanic-hypabyssal rocks of the different blocks of the ZOC are plotted versus two incompatible elements, Ti and Zr (Fig. 7a and b). These elements are generally thought to be immobile during metamorphism or alterations (e.g., Pearce and Cann, 1973; Rubin et al., 1993). Being incompatible elements and assuming that there is no mineral receptor of the REEs, Ti and Zr, these elements should generally increase as fractionation progresses. The plots of the total REEs versus TiO z and Zr show a generally consistent positive correlation. Since the TiO 2 and Zr are believed to be immobile and reflect igneous signatures, the figures suggest that the trends defined by the REEs are magmatic as well (e.g., Coish et al., 1982). Also noticeable in the diagrams are three filled stars (ZO-2, ZO-9 and ZO-413) which do not fall within the dominant ZOC REE trend. These filled stars are LREE-enriched basalts and diabases collected from the Subic area of the Cabangan Massif. These Cabangan Massif LREE-enriched basalts and diabases are products of a melt compositionally different from the melt responsible for the Coto Block

G.P. Yumul / Tectonophysics 262 (19961 243-262

254 Table 5

Bulk REE analyses of the Coto basalt (n values = normalized values) ZO-2

La Ce Nd Sm Eu Tb Yb

ZO-9

Z0-176

ppm

n

6.58 15.51 11.36 2.61 0.84 . . 1.02

17.41 8.65 15.89 20.35 15.86 13.51 11.35 2.90 9.70 0.89 . . 4.10 1.14

22.88 20.85 18.87 12.61 10.28

1.10 4.20 3.90 1.50 1.40 2.80 10.21

1.10 5.00 4.60 1.80 1.60 2.80 12.69

(La/Ce)n (La/Yb)n (Ce/Yb) n (La/Sm)n (Ce/Sm)n (Sm/Yb)n L a + S m + Yb

ppm

n

ppm 1.62 13.26 2.44 0.82 0.40 1.59

4.58

ZO-406

ZO-410

ZO-413

JB-I

JB-1

i+

ppm

n

ppm

n

ppm

n

ppm

n

ppm

ppm

4.28 18.52 10.61 9.47 6.79 6.38

1.07 3.54

2.83 3.63 9.22 9.93 6.79 9.56

1.79 8.91 2.32 0.91 0.50 2.27

4.74 9,13 10.09 10.51 8.49 9.12

7.98 21.25 9.43 3.24 1,09

21.11 21.77 13.17 14.09 12.59

4.29 14.88

I 1,35 15.24

7.07

22,43 17.09 15.11 16.55

38.00 67.00 27.00 1.52 5.00

I,76

5.16 1.48 0.89 4.12

39.99 63.03 27.02 1.49 5.55 2.27

2.10

2.12 0.86 0.40 2.38

0.70 0.40

(I.78 0.30 0.40 0.30 0.40 0.96 5.57

1.70 5.65

(I.52 0.5(/ 1.00 0.50 0.9(I 1.10 6.38

Z0-43 I

(I.99 3.00 3.10 1.50 1.5(I 2.00 12.98

(I.74 0.70 0.90 0.50 (/.70 1.40 13.57

Note: ( - ) = b e l o w detection limit. + JB-1 as unknown. * * GSJ R e c o m m e n d e d value (Geological Survey o f Japan: A n d o et al., 1987).

LREE-depleted volcanic-hypabyssal rocks. Field evidence shows that the LREE-enriched basalt and diabase dikes cut across the LREE-depleted volcanic and hypabyssal rocks in the Subic area of the Cabangan Massif. The addition of LREE-enriched fluids from the subducting slab to the mantle source can

account for the presence of these LREE-enriced basalts and diabases. Another possibility is the tapping of LREE-enriched basanitic veins or ocean island basalt sources in the upper mantle wedge above the subducting slab (e.g., Morris and Hart, 1983; Stern et al., 1991: Arculus, 1994).

Table 6 Bulk REE analyses of the Coto diabase (Sheeted D i k e - S i l l C o m p l e x ) (n values = normalized values) ZO-8

Z O - 13

ppm La Ce Nd Sm Eu Tb Yb (La/Yb)n (Ce/Yb) n (La/Sm)n (Ce/Sm)n (Sm/Yb)n La + Sm + Yb

0.84 2.85 . 2,44 1.10 . 2.03

n

ppm 2.22 2.92

1.13 .

.

. 10.61 12.70

.

. 8,15 0.27 0.36 0.21 0.28 1.30 5.31

. .

1.31 0.46 . 1.38

Z O - 184 n 2.99 . . 5.70 5.31 5.54 0.54 0.52 1.03 3.82

ZO-408

ZO-435

ppm

n

ppm

2.23

5.90

1.00 3.93

2.64 4.03

0.41 1.08

. 2.16 0.68

. 9.39 7.85

2.39

9,60

1.96 0.60 0.60 2.36

8.52 6.93 10.19 9.48

.

0.61 0.63 0.98 6.78

17

0.28 0.42 0.31 0.47 0.90 5.32

ppm

ZO-439 n

ppm

n

1 .(18 1.11

1.54

4.07

1.03 0.38

4.48 4.39

1.81

7.27

2.15 0.86 0.40 (/.83

9.35 9.93 6.79 3.33

0.15 0.15 0.24 0.25 0.62 3.25

1.22 0.44 2.81 4.52

G.P. Yumul / Tectonophysics 262 (1996) 243-262 ACOJE BASALT

255

(a)

lO0

(D 4-

"V_

15

c

o

10

to

~----~

y.

-

~

~

~

:, 30D o 306

t (a) 10~

• 298

....

,I,

*

,It

a 293

O

o (3c 1 O

i F

A-

tf

La' ' Ce

Nd

%

Sm ' ' Eu Tb

0.0 ACOJE DIABASE

100

(b)

15 7 (D

¢..

0~0 -

~

-

-

O

/

f~_

_

/~

~

~

|

o

1.2 (wt. )

(b)

,I,

• 64 ~ 315 • 297 • 292

/ /'°

e--7° ~ .

0.8 TiO

04

16

20

S

°

302

(..)

o

Oc

[-.

0

''

''

La Ce

Sm Eu

L 0

qb

Fig. 6. (a) Bulk chondrite-normalized REE patterns of the Acoje basalts. (b) Bulk chondrite-normalized R E E patterns o f the Acoje diabases taken from the sheeted d i k e - s i l l complex.

F 20

i

I t I 40 60 Zr (ppm)

t

I 80

E 100

Fig. 7. (a) Total bulk REEs vs T i O . . See text for discussion. Symbols as in Fig. 3. (b) Total bulk REEs vs. Zr. See text for discussion. Symbols as in Fig. 3.

Table 7 Bulk R E E analyses of the Coto dikes intruded into the Coto residual peridotites (n values = normalized values) ZO-40

La Ce Nd Sm Eu Tb Yb

ZO-48

ppm

n

. . 3.64 1.51 2.01

. . 2.98 5.08 . 6.57 1.09 0.34 0.17 8.07 0.98

(La/Yb)n (Ce/Yb)n (La/Sm)n (Ce/Sm)n (Sm/Yb)n 0.81 La + S m + Yb

ppm

Z O - 114

Z O - 134

Z O - 135

ZO-423

ZO-427

n

Z0-72 ppm

n

ppm

n

ppm

n

ppm

n

ppm

n

ppm

n

ppm

3.05

0.99 2.62

2.62 2.68

1.48 2.61

3.92 2.67

1.50 2.56

3.97 2.62

1.97 0.80

8.56 9.24 9.48

1.28 0.64 0.34 0.98

5.56 7.39 5.77 3.94

1.08 3.65 5.75 1.28 0.60 0.24 1.78

2.86 3.74 8.03 5.56 6.93 4.07 7.15

2.46 2.84 8.54 1.61 0.63 0.40 1.42

6.5l 2.91 11.93 7.00 7.27 6.79 5.70

1.57 4.33 . . 1.57 (I.69 . . 1.89

4.15 0.40 4.44 1.1 I . . 6.83 0.69 7.97 0.34 . . 7.59 0.75

3.01

0.55 0.58 0.61 0.65 0.90 5.03

0.35 0.38 0.35 0.38 1.00 1.84

. 4.74 3.93 2.89 3.94 0.77 0.64 1.20 -

Note: ( - ) = below detection limit.

.

ZO-87

. 1.80 0.65 2.30

. 7.83 7.50 9.24 0.28 0.29 0.33 0.34 0.85 5.09

2.36

0.41 0.28 0.46 0.31 0.90 5.81

1.01 0.66 0.71 0.47 1.41 3.76

0.40 0.52 0.51 0.67 0.78 4.14

1.14 0.51 0.93 0.42 1.23 5.49

n 2.86 3.74 3.00 3.93

G.P. Yumul / Tectonophysics 262 (1996) 243-262

256

Table 8 Bulk REE analyses o f the Acoje basalt (n values = normalized values) ZO-293

La Ce Nd Sm Eu Tb Yb

ZO-298

ZO-300

ppm

n

ppm

n

ppm

-

-

0.51

1.35 -

0.44 1.61

0.54 0.23

2.35 2.66

0.56 0.26

2.43 3.00

0.83 0.42

1.88

7.55

0.95

3.82

2.07

(La/Ce)n (La/Yb)n (Ce/Yb)n (La/Sm)n (Ce/Sm)n (Sm/Yb)n La + Sm + Yb

0.56 0.64 2.02

ZO-306

n

ppm

n

ppm

n

1.16

0.32

1.19

1.38

0.85 1.41

0.45

1.65

1.88

1.93

3.61 4.85 8.31

0.60 0.33 0.27 1.20

2.61 3.81 4.58 4.82

0.70 0.24 0.14 1.60

3.04 2.77 2.38 6.42

0.70 0.14 0.20 0.32 0.46 0.43 3.34

0.35

0.31

ZO-304

0.60 0.18 0.29 0.32 0.54 0.54 2.12

0.62 0.19 0.30 0.39 0.63 0.47 2.75

Note: ( - ) = below detection limit.

5.2. Zambales Ophiolite Complex volcanic-hypabyssal rocks." crystallization products from the same melt or extracts from different partial melting events ?

(Ce/Yb), ratio from the Coto volcanic-hypabyssal rocks [(Ce/Yb) n = 0.4-1.00] to the Coto dikes intruded into the Coto peridotites [(Ce/Yb) n = 0.280.77] up to the Acoje volcanic-hypabyssal rocks

[(Ce/Yb)n0.20-0.30]. Although there are considerable overlaps among the REE patterns of the different ZOC volcanic-hypabyssal rocks, there is a consistent decrease of

Choosing the least fractionated basalt samples among the sample sets, which correspond to a M g # of 50, and comparing them show that the chondrite-

Table 9 Bulk REE analyses o f the Acoje diabase (Sheeted Dike-Sill Complex) (n values = normalized values) ZO-64

La Ce Nd Sm Eu Tb Yb

ZO-292

ZO-297

ZO-302

ppm

n

ppm

n

ppm

n

2.10 6.11 6.29 3.18 1.22 . 4.09

5.56 6.26 8.78 13.83 14.09

0.66 0.57 0.26 . 1.32

1.75 2.48 3.00

0.32 1.69 0.80 0.36

0.85 1.73 3.48 4.16

0.58 0.24

1.27

5.10

2.11

.

. 16.42

(La/Yb)n (Ce/Yb) n (La/Sm)n (Ce/Sm)n (Sm/Yb)n La + Sm + Yb Note: ( - ) = below detection limit.

0.34 0.38 0.40 0.45 0.84 9.37

.

ppm

ZO-315 n

ppm

n

1.11

2.94

2.52 2.77

2.28 1.35 0.55

3.18 5.87 6.35

8.47

1.58

6.34

. 5.30 0.33 0.70 0.47 2.55

0.17 0.34 0.24 0.50 0.68 2.39

0.46 0.50 0.30

0.92 4.04

G.P. Yumul / Tectonophysics 262 (1996) 243-262 ,= 0 zx •

100

.

ZO-9 ,AM9# 51 Z0-13 , M9# 48 Z0-427, Mg# 53 ZO-'298, Mgff 51

...,.

s'm

r'b

v'b

Fig. 8. Comparison of the bulk chondrite-normalized REE patterns of the Coto and Acoje basalts and a Coto dike sample with approximately Mg# 50.

normalized values decrease from the LREE-depleted Coto basalt (ZO-13; Mg#48) to the Acoje basalt (ZO-298; Mg#51) (Fig. 8). The Coto dike sample (ZO-427; Mg#53) cuts across the Acoje basalt REE pattern (Fig. 8). This, at a first order of approximation, can be taken as an indication of varying mantle sources, from a relatively fertile mantle source responsible for the Coto volcanic-hypabyssal rocks and more refractory mantle sources for the Coto dikes and Acoje volcanic-hypabyssal rocks. The variation in the geochemistry of these three rock groups cannot be due to crystallization of a single parental magma (Yumul, 1992). 5.3. Coto Block LREE-enriched basalts and diabases - - subduction signature?

The filled stars in the different binary diagrams (Fig. 7a and b) correspond to the Cabangan Massif LREE-enriched basalts and diabases of the Coto Block. These rocks are believed to be derived from a melt or melts extracted from a source region that had possibly undergone addition of LREE-enriched fluids. These LREE-enriched fluids are assumed to be subduction-related since the LREE-enriched basalts and diabases manifest island-arc characteristics. The introduction of these fluids from the subducting slab to the overlying upper mantle wedge is governed by several parameters as modelled and shown by other workers (e.g., Hawkins et al., 1984; Tatsumi, 1989; Davies, 1994; Nichols et al., 1994). Petrographic examination revealed that most of these rocks are

257

generally fresh. Although there exists the possibility that the LREE-enriched patterns of these rocks are metamorphic signatures, the pattern shapes and (Ce/Sm) n ratios suggest otherwise. Enriched typeMORB with primary LREE enrichment have (Ce/Sm) n > 1.5 and the LREE display a smooth increase from Eu to La (Geary et al., 1989). The Coto LREE-enriched basalts-diabases manifest the same characteristics (Table 5). As pointed out by Geary et al. (1989), samples affected by seawater alteration usually have (Ce/Sm) n < 1.0 and an irregular increase in the LREE from Eu to La. Furthermore, hydrothermal or alteration processes cannot convert a LREE-depleted rock to a LREE-enriched rock (e.g., Humphris, 1984). The enrichment of LREEs in basalts can be attributed to several factors that include: (1) the existence of an enriched source (enriched MORB or ocean island basalt source); (2) metasomatism; (3) disequilibrium melting of an LREE-rich accessory phase in the mantle; (4) contamination or assimilation of country rocks; and (5) introduction of LREE-enriched fluids during subduction (e.g., Hickey-Vargas and Reagan, 1987; Volpe et al., 1987; Muenow et al., 1991; McDermott et al., 1993; Eissen et al., 1994). Isotopic evidence on the mantle (e.g., Campbell and Gorton, 1980), in general, does not favor the long-term existence of enriched sources. Petrographic examination of the ZOC rocks does not reveal the presence of any accessory phase that may be an LREE receptor. The bulk and mineral chemistry of the different rocks do not, in addition, manifest evidence that indicates contamination by the country rocks. Considering the available data, it appears that the most plausible mechanism for the presence of the LREE-enriched Coto basalts and diabases is the addition of a LREE-enriched fluid into the mantle source region during the onset or incipient stage of subduction. The least fractionated LREE-enriched basalt (ZO-9, Mg# 51) sampled from the Coto Block could have not been generated through simple fractional crystallization of the Coto Block LREEdepleted volcanic hypabyssal rocks (Fig. 8). The Coto Block exhibits transitional MORB-IAT characteristics consistent with the premise that the addition of the LREE-enriched fluids occurred during a sub-

258

G.P. Yumul / Tectonophysics 262 (1996) 243 262

duction event (e.g., Geary et al., 1989; Evans et al., 1991 ; Tatsumi et al., 1992).

that the Coto Block basalts-diabases (TiO 2 0.521.50 wt.%; Zr 31-76 ppm: Y 13-31 ppm) have higher Ti, Zr and Y than the Coto dikes (TiO 2 0.52-0.94 wt.%: Zr 15-55 ppm; Y 10-27 ppm) and the Acoje Block basalts-diabases (TiO+ 0.26-0.86 wt.%; Zr 11-45 ppm; Y 10-23 ppm). These may signify either origin from different mantle sources or if these three volcanic-hypabyssal rock suites came from the same source, the Acoje Block volcanic-hypabyssal rocks and Coto dikes are products of higher

5.4. Zambales Ophiolite Complex rao, ing mantle sources and differing degrees of partial melting - trace-element and REE constraints Comparisons of the major- and trace-element data among the Coto volcanic-hypabyssal rocks, the Coto dikes and the Acoje volcanic-hypabyssal rocks show

lo

(a)

(b)

w

~ ~

TiO2 wt %

WPB

Zr/Y

-

---/ //

I

.~---

~/. "~ A

VAB

1

i

i

i

n

i

i

i

10

/I

~

/

i

J

i

i

i

100

500

Zr

#

\\ \

,

L , l l,lLll

i

t I IA_

Zr 30

ppm

600

,

,

~ ,

~

0

,

25

500 2O

0 • 0

A

v

300-

o

/ %

~

. . /

o o~ ~ o

.

~

~

"

,~ ," ,A ,,,

°

..-.

. t / ~

0

0 0

. /?

400

ppm

,,

,4 t ~,.::. •

o

10

200

/'

/ // /"~,KOMATIITE$ /(~----~I~NINITE$

1O0 ~

0 0

~

1

i

I

I

I

I

I

I

I

2

3

4

5

6

7

8

9

Ti ppm

10

i0

5

I

I

100

150

I

200

I

250

TilSc

x 1000

Fig. 9. The TiO 2 vs. Zr (9a), Z r / Y vs. Zr (b) and Ti vs. V (9c) tectonic discrimination diagrams show the transitional MORB-IAT nature of the different ZOC rocks. The T i / V vs. Ti/Sc diagram (9d) shows the affinity of the Acoje Block volcanic-hypabyssal rocks to boninites while the Coto Block volcanic-hypabyssal rocks manifest MORB characteristics. The Coto dikes consistently plot between the Acoje and Coto Block samples. The diagram is modified after Hickey and Frey (1982). Symbols as in Fig. 3.

G.P. Yumul/ Tectonophysics 262 (1996) 243-262

degrees of partial melting than the Coto Block volcanic-hypabyssal rocks (e.g., Hawkins and Evans, 1983; Yumul, 1989, 1993). It was also pointed out that the decreasing Ti content from the LREE-enriched and LREE-depleted Coto Block volcanic-hypabyssal rocks, then to the Coto dikes and to the Acoje Block volcanic-hypabyssal rocks is a mantle source region signature (Yumul and Datuin, 1991). The increasing degree of refractoriness of the mantle source(s) of the Coto Block rocks to the Coto dikes and then to the Acoje Block rocks can be explained by the increasing degrees of partial melting from the former to the latter source(s). The Coto Block volcanic-hypabyssal rocks [(Ce/Yb), 0.4-1.0] have higher REE contents than the Coto dikes [(Ce/Yb), 0.3-0.8] and the Acoje Block volcanic-hypabyssal rocks [(Ce/Yb) n 0.2-0.3] which, like the trace element data, also argue for an increasing degree of melting of the sources from the former to the latter. It is not possible to derive the three groups of rocks from a mantle source that underwent a single melting event. The available geochemical evidence also do not warrant the derivation of the three groups of rocks through fractionation of a single parental magma. All of the above mentioned information is consistent with the available mineral chemistry data of the chromitites and residual peridotites - - cumulate rocks which showed that the Coto Block cumulate and volcanic-hypabyssal rocks are products of lesser degrees of partial melting as compared with the Acoje Block cumulate and volcanic-hypabyssal rocks (e.g., Hawkins and Evans, 1983; Yumul, 1992, 1993). 5.5. Tectonic setting o f the Zambales Ophiolite Complex - - bulk chemistry

Plotting the sample sets in several tectonic discrimination diagrams (TiO 2 vs. Zr, Z r / Y vs. Zr and Ti vs. V) demonstrate that the Acoje Block volcanic-hypabyssal rocks mostly plot in the IAT field while the Coto Block volcanic-hypabyssal rocks and Coto dikes consistently cluster in the overlapping field of MORB and IAT (Fig. 9a-c) (e.g., Pearce and Norry, 1979; Shervais, 1982). The T i / V vs. T i / S c diagram also shows that the Acoje Block volcanic-hypabyssal rocks have more affinity to boninites and komatiites while most of the Coto

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Block volcanic-hypabyssal rocks manifest MORB characteristic (Hickey and Frey, 1982). The Coto dikes consistently plot in between the Acoje and Coto Block samples (Fig. 9d). Considering the spatial relationships of these different volcanic groups and the probable close temporal relationships among them, these kinds of transitional MORB-IAT geochemical characteristics could have been generated in a marginal basin that had undergone a subduction-related rifting ( = suprasubduction zone ophiolite) (e.g., Hawkins, 1977; Pearce et al., 1984; Saunders and Tarney, 1984; Shervais, 1990). The ZOC formed sufficiently far from any land source to account for the deposition of its clean pelagic sediment capping (e.g., Schweller et al.. 1983).

6. Conclusions The consistent decrease in REE patterns, which have magmatic signatures, and the trace-element (e.g., Ti, Zr and Y) contents of the Coto Block basalts-diabases to the Coto dikes and then to the Acoje Block basalts-diabases is in accord with the increasing degree of refractoriness of the mantle source/s from the former to the latter. The intragroup REE variations of these samples can be attributed to fractional crystallization. The geochemistry of these different groups of rock makes it impossible for them to have been derived either from a mantle source region through a single partial melting event or a single parental magma through fractional crystallization. The bulk major-element. trace-element and REE chemistry suggest the involvement of mantle sources that underwent multistage and differing degrees of partial melting. This is consistent with the generation of the ZOC in a subduction-related marginal basin. The available geochemical information offers insight on the complexities involved in the generation of SSZ ophiolites and the evolution of mantle source regions from a MORB-like to an IAT-like source.

Acknowledgements Most of the analytical work presented here was done at the Geological Institute, Tokyo University.

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Discussions with I. Kushiro, M. Toriumi, T. Fujii, K. Ozawa, N. Shikazono and H. Iwamori, are very much appreciated. R. Matsumoto, K. Fujimoto, Y. Watanabe and Y. Katoh all helped in the analyses of the REE samples. Comments on an earlier version of this paper by A. Ishiwatari is very much appreciated. Constructive comments and helpful suggestions by M.P. Ryan and J.W. Hawkins greatly improved the paper. Field, laboratory and desk works were facilitated by C. Dimalanta, R. Manuel and the personnel of the different mines visited. Financial support for this research work came from a Japanese Ministry of Education and Culture (Monbusho) scholarship grant, University of the Philippines-National Institute of Geological Sciences Research Grants and U.P. Diamond Jubilee Junior Faculty Grants.

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