论文部分内容阅读
报道了藏北羌塘中部驼背岭蛇绿岩中斜长花岗岩的锆石U-Pb定年结果和岩石地球化学资料以及锆石Hf同位素成分.斜长花岗岩的SIMS和LA-ICP-MS锆石U-Pb定年结果分别为504.8±4.2和491.6±1.5 Ma,相当于中-晚寒武世.斜长花岗岩中锆石具有正的?Hf(t)值(11.46~15.16),反映其源区为亏损型地幔.地球化学分析结果表明,斜长花岗岩具有高SiO2、富Na2O、贫K2O的特点,同时具有较低的稀土元素含量和平坦的稀土元素球粒陨石标准化曲线.综合地球化学资料以及野外观察结果可知,斜长花岗岩为洋壳运移过程中含水条件下辉长质岩石发生角闪岩相变质作用后部分熔融形成的,其形成年龄略晚于洋壳的形成时代.这一成果表明,龙木错-双湖-澜沧江洋的开启时限为中寒武世以前.
The zircon U-Pb dating and geochemical data of zircons and the Hf isotopic composition of zircon from the Mesozoic long-plagioclase granites in the hunchieling ophiolite, central Qiangtang, northern Tibet, are reported. The SIMS and LA-ICP-MS zircon U -Pb dating is 504.8 ± 4.2 and 491.6 ± 1.5 Ma respectively, which is equivalent to the middle-late Cambrian.The zircons in the plagiogranite have positive values of Hf (t) (11.46 ~ 15.16) Depleted mantle.The results of geochemical analysis show that the plagiogranite has the characteristics of high SiO2, rich Na2O and poor K2O, meanwhile has lower REE content and flattened normalized curve of REE chondrite.Comprehensive geochemical data as well as field The observation shows that the plagiogranite was partially melted by amphibolite facies metamorphism under the conditions of aquifuge during the migration of oceanic crust, and was formed slightly later than the formation of oceanic crust. This result shows that, Long Mu wrong - Shuanghu - Lancang ocean open time before the Middle Cambrian.