Magmatic Evolution of Volcanic Rocks in Kueishantao and Its Tectonic Implication

Kueishantao (KST) is a young volcanic island located at the southernmost part of the Okinawa Trough. It is mainly composed of andesitic lava flows and pyroclastic flows. Based on major-element geochemistry, KST andesites can be divided into two subgroups, in which Type I Andesites are defined as possessing lower MgO and SiO2, and Type II Andesites possessing higher MgO and SiO2. In general, both of them reveal similar trace-element compositions except that Type II Andesites exhibit high Ni and Cr contents. In primitive-mantle-normalized incompatible element diagram and chondrite-normalized REE pattern, Type I and II Andesites have highly similar patterns. They are characterized by enrichment of large ion lithophile elements (LILEs) and Th, U and Pb, and depletion of high field strength elements (HFSEs). Type I Andesites are in the lower part of core, and Type II Andesites are distributed in the topper part, indicating that KST magmatic evolution from low MgO magmas transferring into high MgO magmas.
The amount of xenoliths in the drill cores increases from the bottom to the top, inferring that the degree of crustal contamination increases progressively. Base on the combination of results of chemical analyses, isotopic compositions and petrographic observation, we propose that magmatic evolution of KST is as follows. Firstly, Type I Andesites were produced by about 45% fractional crystallization of basaltic magmas, which were derived from melting of the mantle wedge induced by hydrous fluids released during dehydration reactions in the subducting Philippine oceanic lithosphere, prior to the opening of the southern Okinawa Trough. Secondly, as a result of the opening of the Okinawa Trough, quick ascending of magma may shorten its storage time in magma chamber and experienced less fractional crystallization. Finally, magma assimilated the continental crust materials (about 15-25%) during it ascended through the upper continental crust and then produced Type II Andesites.