Crustal accretion and metamorphism in Taiwan, a post-Palaeozoic mobile belt

Abstract

Taiwan consists chiefly of Permo-Mesozoic basement unconformably overlain by Cainozoic cover strata; on the east, this complex is juxtaposed along the Longitudinal Valley against the Neogene Coastal Range, the northern extension of the Luzon calc-alkaline arc and intervening fore-arc basin. Accumulation of well-ordered sandstones, shales, limestones and intercalated basaltic units, and outboard argillite melange along the eastern margin of Asia in Permian and early to mid-Mesozoic time was terminated by a major late Cretaceous (85—90 Ma) dynamothermal event that produced the composite basement complex of Taiwan, and also affected much of southeastern China. This basement consists of a westerly Tailuko belt, separated from the easterly Yuli belt by a major fault. The miogeoclinal Tailuko belt is characterized by chloride and biotitic greenschist facies low-pressure assemblages, except in the north where amphibolite facies assemblages are associated with emplacement of remobilized granitic rocks; metamorphic grade increases gradually eastward. The eugeoclinal Yuli belt lacks marble layers and granitic intrusions, and instead contains serpentinite bodies and associated rare highpressure epidote-bearing barroisitic amphibolite tectonic lenses. Tailuko quartzofeldspathic rocks contain mineralogic and geochronologic evidence of Palaeogene and Neogene reheating. Mafic tectonic blocks in the Yuli belt show partial conversion to glaucophanic assemblages; radiometric ages for the blueschist metamorphism are 8-14 Ma. The Cainozoic slate sequence was deposited on the basement complex following renewed Palaeogene rifting. It consists largely of sedimentary strata (and intercalated basalts) laid down on the Asiatic passive margin and seaward in the South China Sea as continental slope deposits. An accretionary wedge was constructed adjacent to the approaching Neogene Luzon arc, marking the non-subducted western margin of the Philippine Sea plate. During Plio-Pleistocene collision of the Luzon arc with the Chinese continental margin, the landward Cainozoic shelf and slope units were imbricated and thrust westward; increased pressure ( P ) and temperature ( T ) during this loading evidently promoted recrystallization of the basement and passive margin cover. Metamorphism ranged from diagenetic and zeolite facies in the Western Foothills to upper greenschist and locally amphibolite facies in the basement complex. No metamorphic hiatus between Mesozoic basement and Cainozoic cover is recognized. The accretionary terrane lying east of the Longitudinal Valley is virtually unmetamorphosed. However, the East Taiwan Ophiolite, occurring as clastic debris and slide blocks in the unmetamorphosed olistostromal, largely Pliocene, Lichi Melange of the Coastal Range fore-arc basin, carries the effects of a mid-Miocene oceanic-ridge recrystallization of actinolite hornfels facies, overprinted by a late Miocene ocean-floor zeolitization. Large portions of the sialic crust making up Taiwan evidently formed approximately in situ , then were deformed and thrust landward during subsequent tectonic events, but far-travelled terranes and allochthonous fragments of oceanic material played an important role in accretion. Recognized and suspected exotics include. (1) Lower or mid-Mesozoic meta-ophiolites, now greenstones, amphibolites and minor serpentinites, previously obducted into the Upper Mesozoic Tailuko belt; amphibolites also occur as enclaves in granitic intrusives 85—90 Ma old; (2) the entire Upper Mesozoic Yuli belt; (3) Mio-Pliocene tectonic blocks of blueschistic meta-ophiolite, emplaced in the east-central part of the Yuli terrane; (4) olistostromal ophiolitic debris (preexisting Miocene oceanic crust of the South China Sea) within the Pliocene Lichi Melange of the Coastal Range; and (5) the Neogene calc-alkaline Luzon arc, which began to collide with Asiatic continental crust in Plio-Pleistocene time. The Cainozoic slate series represents an additional, parautochthonous terrane, which was deposited as the Tertiary miogeoclinal cover along the Asiatic passive margin, and was thrust westward during Plio-Pleistocene arc collision. Among the suspected allochthonous units, Mesozoic amphibolites of both Tailuko and Yuli belts have been identified as of chiefly normal more affinites from their chemical and Nd and Sr isotopic compositions. The clearly oceanic East Taiwan Ophiolite apparently formed along a transform-interrupted spreading centre of the South China Sea marginal basin, based on mineralogic, chemical and isotopic evidence.