Geochronology and geochemistry of Penghu basalts, Taiwan Strait and their tectonic significance

Abstract

The Penghu Islands are located in the Taiwan Strait and consist of 64 islets. These islands, except for Huahsu, are composed mainly of basaltic lavas with minor amounts of sedimentary rocks. The basaltic lavas have effused from many vents rather than from fissure eruption. KAr datings of the basalts, and on planktonic foraminifers and calcareous nannofossils in the sedimentary rocks revealed that the volcanic activity was from Middle Miocene to Late Miocene. The basalts in the Penghu Islands are mainly alkalic and tholeiitic. The presence of many half-graben-type basins in the Tertiary sequence in western Taiwan and the paleostress analysis on the Penghu Islands indicate a continental rifting environment in which the intraplate volcanism occurred. REE data and other evidence suggest that the alkali basalt may have derived from relatively deeper mantle which had been metasomatized by LILE enriched fluid through partial melting. But the tholeiite may have originated from unmetasomatized mantle lherzolite at relatively shallow level by 5–10% equilibrium batch partial melting. Contemporaneous volcanism occurred sporadically during the deposition of Miocene sediments in the western foothills of Taiwan. The volcanic rocks of the western foothills of Taiwan and those of the Penghu Islands are similar in geochronology and geochemistry. They are closely related to Cenozoic rift tectonism along the Asiatic continental margin caused by the third heating and rifting episodic evolution of the South China Sea.