A NEW GENETIC MODEL FOR THE EAST TAIWAN OPHIOLITE AND ITS IMPLICATIONS FOR DUPAL DOMAINS IN THE NORTHERN-HEMISPHERE

The Miocene East Taiwan Ophiolite (ETO) has been suggested to have been generated at an "abnormal" mid-ocean ridge along a transform fault in the South China Sea [1]. As a result of arc-continent collision, the ETO was fragmented and incorporated into the Pliocene Lichi Melange of the Coastal Range, eastern Taiwan. Based on the structure of the slow-spreading Mid-Atlantic Ridge and the Tethyan Ophiolites, a new genetic model is proposed for ETO: it was formed in a normal slow-spreading axis environment where serpentinized peridotites and gabbros had been exposed onto the ocean floor. Further, we propose that ETO basalts which overlie pelagic red shales represent products of near-ridge seamounts erupted by the end of South China Sea spreading (approximately 15 Ma ago). Basalt samples collected from small areas near Kuanshan in the Coastal Range, eastern Taiwan are characterized by simple mineralogy, general lack of hydrothermal alteration, abundance of commonly fresh glass (up to 95% by volume), and high MgO content averaging 8.8 wt.%. In addition, they have considerably heterogeneous incompatible trace element (e.g. La/Sm ratios) and Nd and Pb isotope compositions. These characteristics are consistent with a magma origin from near-ridge seamounts. The Pb isotope compositions of ETO basalts and post-spreading seamount basalts from the South China Sea have Dupal anomaly characters, containing higher Pb-208/Pb-204 ratios (with DELTA-Pb-208/Pb-204 = 31-74) than MORB of the Northern Hemisphere. The South China Sea, from which the ETO originated, is the only domain where EM2 Dupal-type Pb composition has been found in well-developed spreading centres of the Northern Hemisphere. Since volcanism in this region was not associated with mantle plume activity, the observed Dupal Pb property is probably a result of melting of convecting mantle which has been contaminated by thermal erosion and delamination of continental lithosphere with EM2 character prior to or during the formation of the South China Sea.