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CEOSEA '98 Proceed£l2gJ, Ceo L. Soc. Ma LayJ£a BuLL. 45, December 1999; . 85-94 Ninth Regional Congress on Geology, Mineral and GEOSEA '98 Energy Resources of Southeast Asia - GEOSEA '98 17 - 19 August 1998 • Shangri-La Hotel, Kuala Lumpur, Malaysia Subic Bay fault zone: its role in the geologic history of the Zambales Ophiolite Complex, Philippines G.P. YUMUL JR.\ C.B. DIMALANTA1,2, J.v. DE JESUS\ D.V. FAUSTINO\ F.T. JUMAWAN1,3, E.J. MARQUEZ\ J.L. BARRETTO\ R.A. TAMAYO 1,4, K.L. QUEAN01,5 AND F.A. JIMENEZ J R.1 1Rushurgent Working Group, National Institute of Geological Sciences College of Science, University of the Philippines, Diliman Quezon City, Philippines 20 cean Research Institute, University of Tokyo, Nakano-ku Minami-dai, Tokyo, Japan 3Extar Resource Development Corporation, San Juan Metro Manila, Philippines 4UMR Domaines Oceaniques, University of Bretagne Occidentale Brest, France 5Department of Petroleum Geoscience, University of Brunei Darussalam Brunei Abstract: Several models have been forwarded to explain the distribution of the three massifs, Masinloc, Cabangan and San Antonio, of the Zambales Ophiolite Complex. Available information support a model that calls for the separation and southward translation of the San Antonio massif from the Acoje block of the Masinloc massif. Translation occurred along the left-lateral West Luzon Shear/ Subic Bay Fault Zone. This would explain the presence of clinopyroxenite-gabbronorite allochthonous hills scattered along the western edge of the ophiolite complex where the fault zone is thought to have passed. The recognition of the existence of the Subic Bay Fault Zone helps elucidate our understanding on how this crust-mantle sequence had evolved. INTRODUCTION palinspatic reconstruction suggest that the ZOC originated from the south. The source is thought to The Zambales Ophiolite Complex (ZOC) is made be either the Celebes Basin or the Paleocene-Eocene up of three massifs, Masinloc, Cabangan and San southwest sub-basin of the South China Sea (e.g. Antonio, from north to south (Fig. 1). The Masinloc Fuller et al., 1983; Honza, 1991; Yumul, 1994). massif is divided into two blocks, the Acoje and Recent works showed the presence of a left-lateral Coto, which on the basis of geochemical and strike slip fault, the Subic Bay Fault Zone, along petrological evidence are recognized to preserve the western side of the ZOC. The purpose of Jhis island arc and transitional mid-ocean ridge (MOR)­ paper is to present evidence which will argue that island arc (IA)characteristics, respectively (e.g. some present day features ofthe ZOC are related to Hawkins and Evans, 1983; Geary et al., 1989; Yumul movements along this fault zone. and Dimalanta, 1997). This ophiolite complex is an allochthonous terrane bounded on the east and GEOLOGY west by the Central Valley Suture and West Luzon Shear, respectively (Karig, 1983). Previous works The Eocene ZOC is a complete ophiolite suite. showed that the Masinloc, Cabangan and San Geological and geochemical data confirmed the Antonio massifs are cut by very young faults differences between the Acoje and the Coto blocks resulting into the formation of grabens in between of the Masinloc massif. The Acoje block contains a these massifs. Available paleomagnetic data and well-developed layered ultramafic cumulate 86 G.P. YUMUL JR. ET AL. 120 15 Zambales Ophiolite Complex Luzon, Philippines o 10 20 km "COTO BLOCK" {3 Basalt·Dlabase m Cumulates (e.g. Troctolite, Gabbro, etc.) South China Sea !Zl Harzburglte residue "ACOJE BLOCK" ~ Basalt·Diabase El Cumulates (e.g. Pyroxenites, Gabbronorltes, .etc.) [!III Harzburglte residue SUSIC S Y FAULT ZONE SURVEY ..... .. .. ...... AREA ..' ·' .. , .. ' .. .... .13 ". :.: ..... .. -_... '--_"';;;''''--l. ' ' Figure 1. Geologic map ofthe Zambales Ophiolite Complex showing the different massifs and the location ofthe Subic Bay Fault Zone. CEO SEA '98 Procee(JingJ (CSM BuLL. 4,)) SUBIC BAY FAULT ZONE: ITS ROLE IN THE GEOLOGIC HISTORY OF THE ZAMBALES OPHIOLITE COMPLEX, PHILIPPINES 87 sequence and a transition zone dunite which cannot block and Cabangan massif mafic cumulate be said for the Coto block. The Acoje block is also sequence. The hills were left behind during the characterized by metallurgical chromitites (spinel southward translation of the San Antonio massifto Cr# [Crt(Cr+Al)] > 0.60), the presence of platinum its present position. For that matter, the location group minerals and pyroxene crystallizing ahead of of the clinopyroxenite-gabbronorite hills mark the plagioclase in the crystallization order. Mineral translation path of the San Antonio massif and chemistry analyses of chromites and clinopyroxenes gives a good idea where the Subic Bay Fault Zone show the distinct geochemical difference between passes through. the Acoje and Coto blocks (e.g. Yumul, 1993) (Figs. 2a-2d). Whole rock together with mineral chemistry SUBIC BAY FAULT ZONE AND THE analyses for the Acoje block show island arc affinity. On the other hand, the Coto block is known for ZAMBALES OPHIOLITE COMPLEX its refractory chromitites (spinel Cr# < 0.60), The Subic Bay Fault Zone is a strike-slip fault plagioclase crystallizing ahead of pyroxene in the believed to be responsible for the transport and crystallization order and the presence of troctolite accretion of the San Antonio Massif southward. in its mafic cumulate sequence. The Coto From the Acoje block ofthe Masinloc massif all the chromitites have lower spinel Cr20 3 and FeO but way to the Cabangan Massif, the fault zone trends higher NiO and Al20 3 with respect to the Acoje N-S which then swings to a NNW-SSE direction chromitites (Figs. 2a-2b). At the same XMg (Mgt towards the Subic Bay area (Fig. 1) (Yumul and Mg+Fetotal) number, the Coto volcanic rock Dimalanta, 1997). Two possibilities can be thought clinopyroxenes contain higher Ti02 and Na20 of with regards to the origin and configuration of compared to their Acoje counterparts (Figs. 2c-2d). the fault zone. The Subic Bay Fault Zone can be a Although there is an overlap in the rare earth splay of the West Luzon Shear Zone of Karig and element contents of the Acoje and Coto mafic others (1986) or it may be related to the San Antonio cumulate rocks, the Coto block gabbros are Fracture Zone (Yumul et al., 1990). Recent characterized by higher olivine forsterite and lower magnetics and gravity data do not support the anorthite contents as compared to the Acoje block presence of the San Antonio Fracture Zone mafic cumulate rocks (Yumul et al., 1998) (Figs. (Dimalanta, 1996). It is more viable to model the 3a-3c). Whole rock and mineral chemistry analyses fault zone as the southeastern extension of the exhibit a transitional MOR-IA characteristics for West Luzon Shear as supported by available the Coto block. geophysical evidence (Dimalanta et al., this issue). Interestingly, the geology ofthe ZOC show that Preliminary structural measurements showed there are similarities between the Acoje block and different paleostress fields that include both left the San Antonio massif; the Coto block and the lateral and right lateral sense of motions. No cross­ Cabangan massif display the same lithological cutting relationship was observed in the field, thus, assemblage (e.g. Yumul et al., 1990). It is believed making it difficult to establish the chronology of that the Coto block of the Masinloc massif and the faulting. Whether the measured stresses were Cabangan massif are just one contiguous body. This generated before, during or after the movement has also led to the model that the San Antonio along the SBFZ must be looked into in future works. massif originated from the Acoje block. It then The possibility also exists that the measured separated and was translated southward to its stresses may not even be caused by movements present position through the Subic Bay Fault Zone. along the fault zone. Furthermore, future works This fault zone passes through the western edge of must also attempt to determine the continuation the Zambales Range and swings southeastward (onshore and offshore) and subsurface configuration towards the Subic Bay area (Fig. 1). of the fault if we are to fully grasp its role in the The presence of the tectonized, layered evolution of the Zambales Ophiolite Complex. clinopyroxenite and gabbronorite hills along the Whether it is active or not will also be important to western boundary of the Cabangan massif lends know. credence to this model (Figs. 4a-4d). These hills, which expose rocks geochemically and lithologically CONCLUSIONS similar to the Acoje block and San Antonio massif, are allochthonous units with respect to the The present day configuration of the Zambales Cabangan massif(Yumul et al., 1996)(Fig. 1). This Ophiolite Complex can be explained with the is for the simple reason that the in situ cumulate recognition of the role played by the Subic Bay sequence ofthe Cabangan massif does not include Fault Zone in the evolution of this crust-mantle a well-developed layered ultramafic cumulate rock sequence. The Coto block and the Cabangan massif suite. Gabbronorites are also not part of the Coto are believed to be a contiguous body. On the other December 1999 88 G.P. YUMUL JR. ET AL. 4~1----------------~----------------~----------------' CHROMITES o .31- 00 0 @1O 8 dO 00 @ ! 0 NiO @ ., (wt %) .2 C:OS~ 0 0 Q) 0 0 0 0 • 00 f60o& 0 • •• 000 00 • o 00 8 0 ..... o 0 .!.. •• • 0 •• .1 ocP ...·••• \ i.;. .• . ·.i. co • o30 o .:.,.::1":~ .. •• . ... , 40 ,"'." 50 60 Cr203 (wt%) Figure 2a. Chromite Crpa (wt %) versus NiO (wt %) shows that the Acoje block chromitites are higher in chromium and lower in nickel compared to the Coto block chromitites. Aside from being a function of melt composition, the difference is a reflection of the degree of partial melting the source region/s of each block had undergone.

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