中祁连苏里地区奥陶纪石英闪长岩地球化学特征及年代学意义

Geochemical Characteristics and Chronology Significance of Ordovician Quartz Diorite from Suli Area in Middle Qilian Mountains

苏里地区奥陶纪石英闪长岩具贫铝、高钾(K2O>NaO)特征,属于钙碱性系列,具偏铝质钾玄岩系列岩石的特点,明显具I型花岗岩的特征。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,构造环境判别显示为形成于板块碰撞前的岛弧花岗岩。利用LA-ICP-MS法进行锆石U-Pb同位素年龄测定,样品的锆石206Pb/238 U加权平均年龄为(474±2.9)Ma(MSWD=0.12),限定该石英闪长岩的形成时代为早奥陶世,表明在北祁连大洋发生双向扩张俯冲阶段,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,岩体为两者混合而形成的产物,该岩体年龄的确定为北祁连洋的俯冲时代提供了新的证据。

In Suli area, the Ordovician quartz diorite has poor aluminum and high potassium （K2O〉NaO）, which belongs to calc alkaline series, with the characteristics of partial aluminium shoshonite series and obvious Ⅰtype granite.Rare earth and trace elements characteristics show that a mantle material was involved in the material source of quartz diorite. The tectonic environment discrimination suggests that the quartz diorite belongs to the island arc granite formed before the plate collision.The zircon UPb isotopic age has been tested by LAICPMS method, the obtained zircon 206Pb/238U weighted average age is （474±2 9）Ma （MSWD=0 12）, which limit the formation age of the quartz diorite era to early Ordovician, suggesting that the north Qilian Ocean experienced two way expansion and subduction stage, while the southern Qilian continental subduction extrusion cause the mantle material upwelling andthe partial melting of continental crust. And then, the rock mass of quartz diorite was formed. The age determination of this rock mass provides a new evidence for the subduction era of the north Qilian Ocean.