南祁连哈拉湖地区中奥陶世花岗岩地球化学特征及年代学意义

Geochemical characteristics and chronological significance of Middle Ordovician granite in Halahu area of southern Qilian Mountain

哈拉湖地区奥陶纪花岗岩相对富钠(Na2O>K2O),具有中钾-高钾钙碱性系列特征,岩石类型以辉长闪长岩和花岗闪长岩为主。岩石稀土及微量元素特征均显示物质来源有地幔物质参与,构造环境判别图显示形成于大洋俯冲的环境。利用LAICP-MS进行锆石U-Pb同位素年龄测定,样品锆石206Pb/238U年龄加权平均值为466±4 Ma,限定该花岗岩的形成时代为中奥陶世,该时期北祁连洋发生双向扩张俯冲,南部中祁连陆块遭受俯冲挤压,导致地幔物质上涌,同时陆壳部分熔融,岩体为两者混合形成的产物,该岩体年龄的确定为祁连洋的多阶段俯冲提供了新的证据。

The Ordovician granite in the Hala Lake area is characterized by relatively rich sodium(Na2O>K2O)and medium potassiumhigh potassium calc-alkaline series.Rock types include gabbroic diorite and granodiorite.The whole-rock rare earth and trace element characteristics indicate that the material source was derived from mantle material,and the tectonic environment was most likely the island arc granite formed during the ocean subduction.The zircon U-Pb isotope ages were determined by LA-ICP-MS.These zircons yielded a 206Pb/238U weighted mean age of 466±4 Ma,suggesting that the granite was formed in Middle Ordovician.In the two-way expansion and subduction stage of the North Qilian Ocean,the southern Zhongqilian block was subjected to subduction and extrusion,which caused the mantle material to migrate upward.At the same time,the continental crust was partially melted and the intrusions were formed by the mixture of the two melts.The age of the intrusion provides new evidence for multi-stage subduction of the Qilian Ocean.