青海三江北段白马海印支期花岗岩地球化学、锆石微量特征及地质意义

Geochemistry,Zircon Trace Element Characteristics and Geological Significance of the Baimahai Indosinian Granites in the Northern Sanjiang Section,Qinghai

白马海花岗岩位于青海三江北段下拉秀弧后前陆盆地内,侵位于早元古代宁多群(Pt_(1)N),形成于中三叠世早期,岩体锆石U-Pb同位素年龄为244.7~243.4 Ma。为探讨三江北段玉树地区的构造演化,本文对白马海花岗岩岩石地球化学及锆石微量进行测试分析。主量元素特征和岩石类型研究表明,白马海岩体岩石属准铝质-弱过铝质钙碱性系列I型花岗岩,分异指数和固结指数显示岩体具有较高分异程度,同时氧化物-lgSI值的相关关系表明岩体受到了一定程度的围岩混染;岩石地球化学和岩浆锆石微量元素特征及相关判别图解显示岩体形成于俯冲汇聚环境下的大陆地壳重熔结晶。综合研究认为,在岩体形成时的中三叠世早期,金沙江洋盆的俯冲消减即将结束,“硬碰撞”造山随即开启。

The Baimahai granite is located in the Xialaxiu back-arc foreland basin in the northern Sanjiang section,Qinghai Province,and intruded into the Early Proterozoic Ningduo Group(Pt_(1)N).It was formed in the early Middle Triassic period,zircon U-Pb isotopic ages of the rock body were 244.7~243.4 Ma.In order to discuss the tectonic evolution of Yushu area in the north section of Sanjiang,the petrochemistry and zircon trace elements of the Baimahai granite were tested and analyzed in this paper.The characteristics of major elements and rock types shows that the Baimahai rock mass belongs to the the quasi-aluminous to weakly peraluminous calc-alkaline series I-type granite.And the differentiation index and consolidation index show that the rock mass has a high degree of differentiation.In addition,the correlation between the oxide lgSI values indicates that the rock mass was contaminated by the surrounding rock to a certain extent.Petrogeochemistry and magmatic zircon trace element characteristics and related discrimination diagrams show that the rock mass was formed by the remelting crystallization of continental crust under the subduction convergence environment.According to comprehensive research,in the early Middle Triassic when the rock mass was formed,the subduction of the Jinshajiang ocean basin was about to end,and the“hard collision”orogeny started immediately.