The Tongbai granulites are present mainly as xenoliths in granodioritic gneisses. The xenoliths with a zircon age of 470Ma are older than the host rocks of granodioritic gneisses which yield a zircon age of 435Ma. It ...The Tongbai granulites are present mainly as xenoliths in granodioritic gneisses. The xenoliths with a zircon age of 470Ma are older than the host rocks of granodioritic gneisses which yield a zircon age of 435Ma. It is suggested that the granulites were transported from the lower crust to the upper level along with granodioritic magma. Geothermometrical and geobarometrical studies based on the coexisting minerals (Opx-Cpx and Opx-Gar) show that the granulites were crystallized at 818-840℃ and 9.5-9.8 ×108 Pa corresponding to the lower crust. Tectonically, the Shangdan suture zone conshtutes the boundary between the North China and Yangtze plates. The zone is characterized by the occurrence of ophiolites in the western part and by that of granulites in the eastern part. So the western part marks the upper crustal level of the Qinling belt, while the eastern part represents the exposure of a deeper level. The results of isotopic dating and the geochemical characteristics of the xenoliths are consistent with those of meta-tholeiltes of the ophiolites in the western part.Therefore, it is assumed that both ophiolites found in the west and granulites found in the east all represent the renmants of the ancient Qinling ocean plate. The difference is that the ophiolites are pieces of obductal fragments from the ocean floor during the subduction in the Early Palaeozoic.However, in the Tongbai area, when the ocean floor was subducting towards the lower crust, it underwent a granulite facies metamorphism. Subsequently, granodioritic magma formed by partial melting trapped some fragments of granulite upwards.展开更多
文摘The Tongbai granulites are present mainly as xenoliths in granodioritic gneisses. The xenoliths with a zircon age of 470Ma are older than the host rocks of granodioritic gneisses which yield a zircon age of 435Ma. It is suggested that the granulites were transported from the lower crust to the upper level along with granodioritic magma. Geothermometrical and geobarometrical studies based on the coexisting minerals (Opx-Cpx and Opx-Gar) show that the granulites were crystallized at 818-840℃ and 9.5-9.8 ×108 Pa corresponding to the lower crust. Tectonically, the Shangdan suture zone conshtutes the boundary between the North China and Yangtze plates. The zone is characterized by the occurrence of ophiolites in the western part and by that of granulites in the eastern part. So the western part marks the upper crustal level of the Qinling belt, while the eastern part represents the exposure of a deeper level. The results of isotopic dating and the geochemical characteristics of the xenoliths are consistent with those of meta-tholeiltes of the ophiolites in the western part.Therefore, it is assumed that both ophiolites found in the west and granulites found in the east all represent the renmants of the ancient Qinling ocean plate. The difference is that the ophiolites are pieces of obductal fragments from the ocean floor during the subduction in the Early Palaeozoic.However, in the Tongbai area, when the ocean floor was subducting towards the lower crust, it underwent a granulite facies metamorphism. Subsequently, granodioritic magma formed by partial melting trapped some fragments of granulite upwards.
