ACCRETIONARY TECTONICS OF PROTO-TAIWAN: CLUES FROM HEAVY MINERALS ON EARLY DEVELOPMENT OF AN INTRAOCEANIC FOREARC

Heavy-mineral analyses of Late Miocene to Pleistocene sandstones from southern Taiwan document changes in sediment sources during opening of the South China Sea, subduction, and ensuing collision of rifted continental blocks with the Luzon arc. Four sandstone members in the middle to upper Miocene Mutan Formation have generally distinct heavy-mineral compositions. Of these units (Shemen, Loshei, Shezetou, and Lilongshan, from oldest to youngest), only the Shezetou is similar in terms of heavies to the relatively mature "generic" Mutan. Shemen samples contain appreciable mafic minerals, especially clinopyroxene and serpentine. Loshei sandstones uniquely contain abundant amphibole. Lilongshan sandstones contain sparse stable grains and very abundant epidote, and mafic pebbles in the Lilongshan contain abundant orthopyroxene and clinopyroxene. Overlying the Mutan Formation, distinctive green sandstones in the Kenting Formation (latest Miocene to Pleistocene) contain abundant mafic minerals.

Regionally, heavy minerals in Mutan sandstones are dramatically more mature than those of age-equivalent sandstones in rifted-margin units exposed by the modern collision in the western foothills of Taiwan. The bulk of the Mutan and its Shezetou member appear to have high continental affinities, despite the inferred distance from Asia at the time. Mafic heavy minerals indicate that sources of the Shemen, Lilongshan, and parts of the Kenting probably included mafic and/or ultramafic rocks. Clearly, mafic and continental crustal components were juxtaposed in a complex accretionary prism before the modern Taiwan collision began at about 5Ma. None of the units analyzed show evidence of high-grade metamorphic sources, despite exposure of such rocks at present to the north on Taiwan. Specific source identities remain enigmatic, but sediment sources have clearly varied dramatically through the history of this key terrane.