PHILIPPINE OPHIOLITES AND THEIR TECTONIC SIGNIFICANCE

Ophiolites are the fragments of subducted oceanic plates.There are more than 20 SSZ-types of ophiolites or ophiolitic complexes in the Philippine archipelago.Most of them are late Mesozoic in age,excluding some Eocene parts,such as the Coto Ophiolite in the Zambales,the Angat Ophiolite in the Southern Sierra Madre,the Central Cordillera basement,and Palawan ophiolites,and also except the Oligocene Amnay Ophiolite in the Mindoro Island.Philippine ophiolites show typical ophiolitic sequence,starting from serpentinized mantle rocks in the lower part,through layered to massive ultramafic and mafic cumulated gabbros and dike-sill complexes to volcanic rocks on the top.The serpentinized mantle rocks include residual lherzolites,harzburgites,websterites,lenses of dunite,whiles the volcanic succession consists of pillow basalts and massive lavas covered by deep-sea cherts.The volcanic rocks are characterized by a relative flat REE pattern of MORB-like,enriched in LILE and middle to weak depleted HFSE(such as Nb and Ta depleted relative to La and Th).
Based on their formation ages,emplacement mechanisms,geochemical characteristics and tectonic settings, the Philippine ophiolites could be subdivided into two distinct belts:the Philippine Mobile Belt Ophiolites and the Philippine micro-Continent Ophiolites.
We suppose that there should be a huge incipient arc and backarc basin,named the Philippine Incipient Arc and the Proto-Philippine Sea respectively,for the Philippine Mobile Belt Ophiolites.The former is comprised of late Mesozoic IA type ophiolites of the Philippine Mobile Belt and the Amami Plateau in the West Philippine Sea Basin.But the later represent by late Mesozoic MORB-like ophiolites of the Philippine Mobile Belt.And the both are formed by the subduction of the Meso-Tethys to the Proto-Pacific in Mesozoic. However,the Philippine Incipient Arc sequently split into 3 slices by opening of the West Philippine Sea Basin and Central Valley Basin in Eocene due to subduction of the Pacific and the Neo-Tethys.
The Philippine micro-Continent Ophiolites consist of the ophiolites of the Palawan micro-Continent and the Zamboanga Peninsula.The late Mesozoic ophiolites of the Palawan micro-Continent and the Darvel Bay Ophiolite of the NE Borneo share a same origin,all source from the Proto-South China Sea.The Eocene-Oligocene ophiolites of the Palawan micro-Continent,however,should be sourced from the Proto-South China Sea subduction-related Cenozoic marginal sea basins such as the Sulu Sea and the South China Sea.Since lacking valuable data to make more restrictions,the late Mesozoic Zamboanga Peninsula ophioliteis are probably fragments of the Proto-South China Sea,or as like as the Meratus Ophiolite in SW Borneo and the Balantak Ophiolite in east Sulawesi island,which are formed by subduction of the Neo-Tethys Ocean beneath the Borneo.