The Volcanic Activities And Their Implications In The Okinawa Trough

By different analyzing methods, author holds kinds of petrochemical data, for example, major and trace elements, Sr-Nd and U -Th isotopic composition and isochronal dating, for volcanic rocks to detail and systemic study the origin and evolution of magmas and the relationship between magmatism and tectonism in the Okinawa Trough. (1) The magmas came from PREMA source area. The fractional crystallization is the main geological process in magmatic evolution, and contamination is weak. Two exotic components come from upper crust and/or subduction plate. (2) Volcanic rocks have similar petrochemical composition from the northern and middle Okinawa Trough, and hold same or similar magmatic source area. In addition, the volcanic rocks in the Okinawa Trough and Ryukyu Arc hold similar geochemical characteristics. The most important difference i s that rocks from the Okinawa Trough have higher degree of fraction than them in the Ryukyu Arc. (3) Compared to the middle Okinawa Trough, magmatic features hold lower temperature and density and higher pressure and viscosity in the northern Okinawa Trough. Because of smaller density and viscosity in Trough than in other areas, the magma in Trough has stronger activity. (4)In late Quaternary (<0.1Ma), silicic volcanic activities exist widely in the whole of Trough. The volcanic period divided into four subgroups, 10-20ka, about 30ka, about 50ka and 70-90ka. The black pumice formed at the latest sub-period (10-20ka). (5) The subduction of Philippine Sea Plate and collision between Luson Arc and Taiwan made deep mark on volcanism in the Okinawa Trough. The collision changed geodynamical conditions from extension to contraction in this region during 5 -10 Ma, and stopped temporarily the opening of southern Okinawa Trough. During the last Pliocene- early Pleistocene (2.5-2.8Ma) the whole Okinawa Trough started extension and opening as the northern Taiwan mountain belt collapsed.