Geochemical Characteristics And Geological Significance Of Cretaceous Volcanic Rocks In Amdo Area,Tibet

Bangong-Nujiang suture zone is located in the north of Tibetan plateau. It begans from Bangong Lake in west of China, and north west of it runs through the central Tibet. Therefore Qiangtang plate in the north and Lhasa plate in the south was divided by the Suture Zone, which nearly parallels to the line of north latitude 32. The scope of the research area is between 90 ° and 91 ° of east longitude, 32° and 33° of north latitude. We can get the following conclusions by adequate geochemistry, chronology and oxygen fugacity study in the research area:(1) The geochemical study of the volcanic rocks from Gacangjian formation of Amdo area in the middle Bangong-Nujiang suture zone shows that, volcanic rocks are loss of high field strength elements, such as Nb, P and Ti, which suggests that the volcanic rocks characteristics is related to island arc environment. Basic-intermediate volcanic rocks from Gacangjian formation are probably from the partial melting of relative enrichment mantle wedge source area, and the acidic volcanic rock is most likely from the partial melting of the crust.(2)The volcanic rocks of Gacangjian formation in Amdo area has relatively typical characteristics of island arc volcanic rocks. And the research area in the tectonic environment discrimination diagram also falls within the scope of island arc volcanic rocks, fully illustrating that the volcanic rocks was formed in Bangong-Nujiang Tethys subduction stage of the oceanic. From the spatial position, the volcanic rocks of Gacangjian formation are in the north of Bangong-Nujiang suture zone, which supports that Bangong-Nujiang suture zone have subduced to north. Summarizing predecessors’ research achievements we can know that southward subduction of Bangong-Nujiang Suture Zone have a close relationship to the the formation of volcanic rock and granite of northern Lhasa. Therefore, volcanic rocks from Gacangjian formation in Amdo area was formed in the island arc tectonic environment, which was the production of southward and northward subduction of Bangong-Nujiang Tethys.(3)Zircon chronology research shows that, the Mesozoic volcanic rocks of middle Bangong-Nujiang Suture Zone were formed in middle early Cretaceous. Research, for instance, geochemical characteristics of volcanic rock, tectonic environment, source rock characteristics, calculation of volcanic subduction distance and calculation of the earth’s crust thickness, etc., shows that, Bangong-Nujiang suture zone exists bidirectional subduction of south and north in the early cretaceous.(4) The calculation of oxygen fugacity, shows that the volcanic rocks has high oxygen fugacity in the research area, most of which is higher than FMQ+5, reflecting that he magmatic environment belongs to island arc magma environment. Higher oxygen fugacity is an important factor in mineralization of Cu. Because of the high Ce anomaly, there is a possibility of the formation of porphyry copper deposit.