Geochemical features and their geological implications of volcanic rocks from the northern and middle Okinawa Trough

In order to remove non-volcanic components from samples and gain the actual geochemical and isotopic compositions for volcanic rocks, authors deal these rocks sampling with strict pretreatment. The isotopic data about volcanic rocks from the northern Okinawa Trough have not been reported in previous works. Based on the data of the U-series isotopic compositions, we distinguished the mass of subduction plate ( Philippine sea plate) from magna as a mixing component. Then, we take some works about the petrology, sources of magmas and magmatic evolution by U-series and Sr-Nd isotopic compositions of these rocks in the Okinawa Trough. The points are follow. Firstly, the Okinawa Trough held widely silicic volcanic activities, and the volcanic rocks belonged to cal-alkali group (sigma < 1. 8). Secondly, the magmas mainly came from mantle source area which has similar isotopic compositions to PREMA. The crystal fractionation was the most important geological process, but the magma also was contaminated with the mass from the upper crust and/or subduction plate. Finally, the volcanic rocks from the Okinawa Trough could be divided into two groups, which were contaminated by the upper crust and upper crust-subduction plate, based on the mixing model. Further, our studies indicated that contaminations occurred at the evening of magmatic evolution, in which SiO2 contents of magma system under the Trough were higher than exotic material.