Geochemistry and plate-tectonic significance of the metabasites from the Tananao Schist Complex of Taiwan

Abstract

The pre-Tertiary Tananao Schist Complex of Taiwan forms the basement rocks on which younger rocks are deposited. The Complex is composed of two belts of different lithologic characteristics: the western Tailuko Belt of high T/P metamorphism and the eastern Yuli Belt of high P/T metamorphism (Yen, T.P. 1963. Proc. Geol. Soc. China 6, 72–74). The juxtaposition of these two belts has been thought to have resulted from the Late Cretaceous subduction process. In the Tailuko Belt, abundant metabasites occur as a mappable unit, or as enclaves in Late Mesozoic granitic intrusions. Forty-three metabasite samples were collected from the Tailuko Belt and analysed for the major and trace element compositions. The results show that most metabasites are tholeiitic in nature. These metabasites can be divided into three groups based upon their geochemical characteristics: group 1 samples resemble oceanic island-arc tholeiites; group 2 samples have elemental abundances similar to those of T-type MORBs; and group 3 samples are similar to within-plate oceanic plateau-seamount basalts. Metabasite-marble-metachert association in the belt also suggests environment(s) above/near the carbonate compensation depth (CCD). All of these results fit rather well with a marginal/back-arc-basin tectonic setting. Based on the recent PbPb dating result from the associated marble, it is suggested that the island arc/seamounts were accreted to the Southeast Asian continental margin during Early Jurassic time, and the Tailuko Belt represents a subduction/collision complex which has undergone several periods of metamorphism in association with Late Mesozoic and present tectonic events.