Research On Characteristics Of Phenocrysts In Acidic Volcanics And Its Magmatic Evolution In The Okinawa Trough

Acidic volcanics mainly appear by pumice, including black and white pumice in the Okinawa Trough. The volume of phenocrysts was very low. It was difficult that checking the phenocrysts assemblage and their compositions roundly. Analyzing the phenocrysts assemblage of acidic volcanic and matrix characteristics by utilizing artificial density, backscatter scanning images and electro-probe microanalyzer, mainly concerning the texture, structure and chemical compositions of pyroxene, plagioclase and glass matrix, we discuss the implication of phenocrysts of volcanics to magmatic evolution in back-arc basin.The phenocrysts assemblage and their chemical features show two species of pumice were the result of basaltic magma highly fractional crystallization. The difference in two species of pumice was mainly due to the bubble characteristics of glass matrix and the properties of Fe-Ti oxides.The calculating mineral thermodynamic results show the clinopyroxene crystallized pressures in white pumice vary considerably, and the maximum crystallized depth was in lower crust. Orthopyroxene and plagioclase in two species of pumice crystallized under the similar pressures and the crystallized depth was approximately 10 km, corresponding to the boundary depths of upper and lower crust. The characteristics of the An value in plagioclase in both the black and white pumice imply there was no uniform magmatic evolution process, and meanwhile, indicating there was no uniform magma cooling rate, showing that the magma chamber was recharged in pulses by hotter magma. Core-sieved plagioclase was the product of hotter magma corrosion. The characteristics of glass matrix of two species of pumice show white pumice were the result of magma continuous decompression and black pumice was the result of magma multi- or single-step decompression. From the above, black and white pumice underwent different magmatic evolution.These characteristics of pyroxene, plagioclase and glass matrix conform the dual-layer magma chamber existed in the Okinawa Trough. The shallow reservoir evolved from the basaltic magma developed in the middle Okinawa Trough. The two-layered magma chamber existed in the crust in the northern Okinawa Trough. White pumice crystallized in the deep-seated reservoir and black pumice formed in the shallow reservoir.