Mineral Chemistry Indicates the Petrogenesis of Rhyolite From The Southwestern Okinawa Trough  

作　　者：CHEN Zuxing ZENG Zhigang WANG Xiaoyuan ZHANG Yuxiang YIN Xuebo CHEN Shuai MA Yao LI Xiaohui QI Hanyan 

机构地区：[1]Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences,Qingdao 266071, P. R. China [2]Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology,Qingdao 266071, P. R. China [3]University of Chinese Academy of Sciences, Beijing 100049, P. R. China

出　　处：《Journal of Ocean University of China》2017年第6期1097-1108,共12页中国海洋大学学报（英文版）

基　　金：supported by the National Key Basic Research Program of China(No.2013CB429700);the National Natural Science Foundation of China(Nos.41325021 and 41306053);the Strategic Priority Research Program of the Chinese Academy of Sciences(No.XDA11030302);the Special Fund for the Taishan Scholar Program of Shandong Province(No.ts201511061);the AoShan Talents Program Supported by Qingdao National Laboratory for Marine Science and Technology(No.2015ASTP-0S17);the Innovative Talent Promotion Program(No.2012RA2191);the Science and Technology Development Program of Shandong Province(No.2013GRC31502);the Scientific and Technological Innovation Project Financially Supported by Qingdao National Laboratory for Marine Science and Technology(No.2015ASKJ03);the National High Level Talent Special Support Program;the CAS/SAFEA International Partnership Program for Creative Research Teams;the Qingdao Collaborative Innovation Center of Marine Science and Technology

摘　　要：To reveal the petrogenesis of rhyolite from the southwestern Okinawa Trough, the mineral chemistry of plagioclase, orthopyroxene, amphibole, quartz and Fe-Ti oxide phenocrysts were analyzed using an electron microprobe, and in suit Sr and Ba contents of plagioclase analysed by LA-ICPMS were chosen for fingerprinting plagioclases of different provenances. Results indicate an overall homogeneous composition for each of the mineral phases except for plagioclase phenocrysts which have a wide range of composition(An=39~88). Plagioclase crystals characterized by An contents of >70 are not in equilibrium with their whole-rock compositions, and coarse-sieved plagioclase phenocryst interiors record high An contents(>70) and Sr/Ba ratios(>7), which are similar to the those of plagioclase crystals in basalt. Therefore, these crystals must have been introduced to the rhyolitic magma from a more mafic source. Equilibrium temperatures estimated using orthopyroxene-liquid, iron–titanium oxide, titanium-in-quartz and amphibole geothermometers show consistent values ranging from 792 to 869℃. The equilibrium pressure calculated using amphibole compositions is close to 121 MPa which corresponds to an approximate depth of 4 km. The fO_2 conditions estimated from Fe-Ti oxides and amphiboles plot slightly above the NNO buffer, which indicates that the rock formed under more oxidized conditions. Our results suggests that petrogenesis of the rhyolite due to basaltic magma ascend with the high An and Sr/Ba plaigoclases from deep magma chamber into the shallow chamber where the fractional crystallization and crustal assimilation happened. It also indicates that a two-layer magma chamber structure may occur under the southwestern Okinawa Trough.To reveal the petrogenesis of rhyolite from the southwestern Okinawa Trough, the mineral chemistry of plagioclase, orthopyroxene, amphibole, quartz and Fe-Ti oxide phenocrysts were analyzed using an electron microprobe, and in suit Sr and Ba contents of plagioclase analysed by LA-ICPMS were chosen for fingerprinting plagioclases of different provenances. Results indicate an overall homogeneous composition for each of the mineral phases except for plagioclase phenocrysts which have a wide range of composition (An=39~88). Plagioclase crystals characterized by An contents of >70 are not in equilibrium with their whole-rock compositions, and coarse-sieved plagioclase phenocryst interiors record high An contents (>70) and Sr/Ba ratios (>7), which are similar to the those of plagioclase crystals in basalt. Therefore, these crystals must have been introduced to the rhyolitic magma from a more mafic source. Equilibrium temperatures estimated using orthopyroxene-liquid, iron-titanium oxide, titanium-in-quartz and amphibole geothermometers show consistent values ranging from 792 to 869°C. The equilibrium pressure calculated using amphibole compositions is close to 121 MPa which corresponds to an approximate depth of 4 km. The fO₂ conditions estimated from Fe-Ti oxides and amphiboles plot slightly above the NNO buffer, which indicates that the rock formed under more oxidized conditions. Our results suggests that petrogenesis of the rhyolite due to basaltic magma ascend with the high An and Sr/Ba plaigoclases from deep magma chamber into the shallow chamber where the fractional crystallization and crustal assimilation happened. It also indicates that a two-layer magma chamber structure may occur under the southwestern Okinawa Trough.

关 键 词：GEOTHERMOBAROMETRY oxygen FUGACITY XENOCRYSTS MAGMA evolution RHYOLITE OKINAWA Trough 

分 类 号：P588.141[天文地球—岩石学] P736.3[天文地球—地质学]