柴北缘构造带深部岩石的LA-ICP-MS锆石U-Pb年龄及微量元素研究:对全吉地块基底演化的启示

Zircon LA-ICP-MS geochronology and trace element features of rocks deep in northern margin of Qaidam tectonic belt:implications to evolution of Quanji massif basement

通过LA-ICP-MS方法对柴北缘全吉地块基底的斜长角闪岩和花岗闪长岩进行了锆石U-Pb年代学及锆石微区微量元素的研究。斜长角闪岩中的岩浆锆石上交点年龄为2 396±26 Ma,代表了锆石结晶的年龄,下交点为905±140 Ma,代表了锆石发生铅丢失事件的年龄。其岩浆锆石具有U/Yb较高(0.1~1),Hf值较低(<10 000×10^(-6)),U值较低(<150×10^(-6))的特点,显示其岩浆源区与富集地幔密切相关。在锆石微量元素判别图解中数据主要落在陆弧区和洋岛区,指示其形成的构造环境为弧后盆地。花岗闪长岩中岩浆锆石的加权平均年龄为484±21 Ma,岩浆锆石具有U/Yb较高(>1)和U值较高(平均值为640×10^(-6))的特点,显示岩浆源区为典型陆壳。锆石微量元素判别图解显示其形成于陆弧。研究结果表明,全吉地块基底除前寒武纪岩石外,还包含早古生代的岩石组合。全吉地块基底内部岩石组合与形成时代的复杂性,与全吉地块经历的多次洋壳俯冲、陆陆碰撞密切相关。

LA-ICP-MS zircon U-Pb geochronology and trace elements of amphibolite and granodiorite from the Quanji massif basement in the northern margin of Qaidam tectonic belt were studied.The upper and lower intersection point ages of magmatic zircons from amphibolite are 2 396±26 Ma and 905±140 Ma respectively,of which the former represents the crystalline age of zircon and the latter represents the age of the partial Pb-loss in the zircons.Magmatic zircons from amphibolite are characterized by high U/Yb（0.1～1）,low Hf（〈1 000×10^-6）and U（〈150×10^-6）,indicating the magma is related to the enrichment of mantle.On discrimination plots of zircon trace element,all samples fall into continental arc and ocean-island,showing that they are formed in back-arc basin.Magmatic zircons from granodiorite yield weighted average age of 484±21 Ma.Magmatic zircons from granodiorite are characterized by high U/Yb（〉1）and high U（average value=640×10^-6）,indicating that the magma is from typical crust source.Discrimination plots of zircon trace elements indicate that these zircons are formed in continental arc.The results of the study show that the basement of the Quanji massif consist of Precambrian and Early Paleozoic rocks.The complexity of the basement of the Quanji massif is related to the subductions of ocean crust and continental collisions in Quanji massif.