MIOCENE BASALTS IN NORTHWESTERN TAIWAN - EVIDENCE FOR EM-TYPE MANTLE SOURCES IN THE CONTINENTAL LITHOSPHERE

Cenozoic extension around the Taiwan Strait resulted in intraplate basalt volcanism in the Fujian-Taiwan region of the southeastern China continental margin. in northwestern Taiwan, the basalt volcanism took place in two distinct periods: the early Miocene (23-20 Ma), with the eruption of alkali basalt only, and the late Miocene (13-9 Ma), marked by the emplacement of various basalt types. The early Miocene basalts have uniform Sr-Nd-Pb isotope compositions comparable with those of the other Fujian-Taiwan basalts, which are believed to have originated from a ''plum-pudding'' type convecting mantle. These basalts display EM2-type lead isotope signature like that observed in seamount basalts from the South China Sea. By contrast, the late Miocene basalts show distinctive isotope characteristics indicating additional involvement of an EM1-type mantle source that has never been identified before for any volcanics in southern China. We propose that the ''enriched mantle'' components (EM1 and EM2) reside in different levels of the continental lithospheric mantle. Reactivation of the unique EM1 source may be ascribed to the are-continent collision in Taiwan during the last 12 million years which terminated the intraplate volcanism around this region.