Mid-ocean ridge or marginal basin origin of the East Taiwan Ophiolite: chemical and isotopic evidence

Abstract

The East Taiwan Ophiolite (ETO) occurs as allochthonous fragments embedded in the Plio-Pleistocene Lichi Melange in the Coastal Range of eastern Taiwan. It is probably the youngest among the known ophiolitic complexes of the world. The overall geochemical and isotopic characteristics of the representative rock types are very “oceanic” and strongly argue for a mid-ocean or marginal basin origin of the ETO. The best criteria include: (1) major element compositions; (2) REE distribution patterns; (3) Nd isotopic ratios (some have ɛNd≧+12); (4) Pb isotopic ratios; and (5) the presence and nature of the red shale. This conclusion is supported by previous studies of metamorphic evolution of the ETO. The occurrence of both N- and P-type basalts in the ETO is firmly recognized for the first time by their REE distribution patterns. Neither type of basalt could be derived by fractional crystallisation from the other, nor by various degrees of partial melting from a common mantle source. Consequently, the genesis of basaltic magmas of the ETO has probably involved melting and mixing of a highly depleted asthenosphere and an enriched plume-type or hot-spot source. Because of the young age of formation (about 15 Ma) and the very recent tectonosedimentary emplacement (olistostromal origin) through arc-continent collision (about 4–5 Ma), the ETO was most likely generated in the spreading center of a small ocean or marginal basin (the South China Sea).