Carboniferous-Permian island arc orogenesis in the Gangdise belt, Tibet, China: evidence from volcanic rocks and geochemistry
WANG Li-quan, PAN Gui-tang, ZHU Di-cheng, ZHOU Chang-yong, YUAN Si-hua, ZHNAG Wan-ping Chengdu Institute of Geology and Mineral Resources, China Geological Survey, Chengdu 610082, Sichuan, China
The authors have not only systemically studied the spatio-temporal distribution, petrography, element and Sr-Nd-Pb isotope geochemistry, tectonic environments and characteristics of source regions of Carboniferous-Permian volcanic rocks in the Gangdise belt, Tibet, but also made a comparative study of them and the Permian volcanic rocks in the Himalaya belt. The Carboniferous-Permian volcanic rocks are mainly distributed in a nearly east-west direction in the north-central segment of the Gangdise belt. In terms of space, the intensity and magnitude of volcanism decreased gradually from east to west, and in terms of time they increased from the Early Carboniferous to Late Permian. The Carboniferous-Permian volcanic rocks of the Gangdise belt formed in an active continental-margin island-arc tectonic environment and the island arc orogenesis went through the evolution process of the primitive island arc, early-stage island arc and mature island arc from the Early Carboniferous to Late Permian. The original magma of the Carboniferous-Permian arc volcanic rocks in the Gangdise belt was derived from partial melting of the enriched mantle (EM) and was strongly contaminated by subducing oceanic crust, attendant deep-sea sediments and crustal substance into the mantle due to recycling during its formation and evolution, which is markedly distinct from the Permian continental-margin rift-type volcanic rocks in the Himalaya belt contaminated strongly by crustal substance. By integrating the latest information and research results of the Gangdise belt and adjacent areas, the authors have constructed an evolutionary model of the Carboniferous-Permian arc-basin system, consisting from north to south of the Gangdise island arc, Yarlung Zangbo back-arc rift basin and Himalayas continental-margin rift basin, and discuss the coupling relation of the Carboniferous-Permian active continental-margin island arc orogenesis along the northern margin of Gondwanaland with the evolution of the Paleo-Tethys of the Qinghai-Tibet Plateau and its dynamic mechanism. They also discuss the formation process of the Sumdo eclogite in the Gangdise belt.