Ar-Ar dating and Geochemistry of Volcanic Rocks Dredged from the Middle Okinawa Trough

• Main Authors: Wang, Shing-Lin, 王興麟
• Other Authors: Sun-Lin Chung
• Format: Others
• Language: zh-TW
• Published: 1998
• Online Access: http://ndltd.ncl.edu.tw/handle/05556277078274640025

碩士 === 國立臺灣大學 === 地質學系 === 86 === Okinawa Trough is the only one backarc basin, which is still in the rifting stage of western Pacific. From results of complete petrographic, 39Ar-40Ar dating, and geochemical analysis of the volcanic rocks dredged from the Middle Okinawa Trough, we could recognize the age of the magmatism and the geochemical characteristics of the melts. Besides, we could further discuss the petrogenesis of magmas and its relationship with extensional tectonic setting in the central part of Okinawa Trough. The volcanic rocks dredged from the Middle Okinawa Trough show a bimodal volcanism with basalt and rhyolite in dominance. 39Ar-40Ar dating results suggest that there are two episodes of magmatism. The early episode commenced from the early Pleistocene, about 1.5 Ma, and extensive magmasim occurred in the recent episode, younger than 1 Ma. Including the analytical result of major and trace element and the Sr-Nd isotopic composition, we could delineate two types of basalt. Type (a) basalt has higher MgO (MgO>7 wt.%) and lower REE concentration (LaN = 10.6-12.2) than type (b) basalt. Rhyolite also can be recognized to two types, type 1 and type 2. The REE pattern of the type 1 rhyolite is higher than the type 2 rhyolite, although they both have Eu anomaly. From the Rayleigh''s fractionation modeling, type 1 rhyolite could be formed by the process of small amount of crustal contamination and large degree of fractional crystallization of type (a) basalt. On the other hand, type 2 rhyolite may be the remelting product of basic lower crust constructed by the basaltic magma underplating. Besides the basalt dredged from Aguni graben which has more enriched Sr-Nd isotopic composition (87Sr/86Sr = 0.704311-0.704540; 143Nd/144Nd = 0.512709- 0.512760), the basalt from Iheya graben (87Sr/86Sr = 0.703892-0.704044; 143Nd/144Nd = 0.512926-0.512827) is similar to those from the Western Pacific Marginal Basin OIB-like basalts, and doesn''t change a lot in the 1.5 my. The depletion of HFSE and enrichment of LILE and Pb of basalts are the same with Ryukyu volcanic rocks. This suggest that mantle source region of Okinawa Trough is metasomatised by the fluxing which is produced by the subduction of Philippine Sea Plate. The two component , E- MORB type upwelling asthenospheric mantle and subduction component form the mantle region in Middle Okinawa Trough and partial melt to produce the magma in the extensional setting of central part. The tectonics and magmatism of southwestern margin of Okinawa Trough is relating to the "postcollision collapse "stage of Northern Taiwan. According to the geochemical study of Kuishantao, northeastern offshore of Taiwan, andesitic volcanic rocks, I suggest that it is one of the Ryukyu volcanic front, and consistent with the geophysical study, about 110 km depth above the Wadati-Beneoff Zone.