柴达木盆地北缘大柴旦地区古生代花岗岩锆石SHRIMP定年

Zircon SHRIMP U-Pb dating of granites from the Da Qaidam area in the north margin of Qaidam basin,NW China.

大柴旦地区是柴北缘古生代超高压带的重要组成部分,与超高压岩石相伴的花岗岩十分发育。这些花岗岩具有两类不同的岩石地球化学特征,Ⅰ类以 Na_2O/K_2O 比值小于1、明显的负 Eu 异常和低 Sr、高 Y 为特征,具有 S-型花岗岩的属性,Ⅱ类以 Na_2O/K_2O 比值大于1、弱负 Eu 异常到正 Eu 异常和高 Sr、低 Y 为特征,具有Ⅰ-型花岗岩的属性,反映了它们的源岩及成因上的差异。锆石 SHRIMP U—Ph 定年结果表明,大柴旦地区花岗岩的年龄可分为三组,第一组年龄为446.3±3.9Ma,第二组年龄分别为408.6±4.4Ma、403.3±3.8Ma、401.8±3Ma,第三组年龄分别为374.5±1.6Ma、372±2.1Ma。结合区域地质特征,我们认为,第一组年龄可能反映了柴达木陆块与中南祁连板块碰撞的时代,第二组年龄可能反映了深俯冲地下的板块由于拆沉而折返的时代,第三组年龄可能反映了碰撞隆起后造山带上不同块体之间的伸展、滑塌的时代。

Abstract The granites associated with ultra-high-pressure rocks are very developed in Da Qaidam area which is an important part in the early Paleozoic UHP belt of the north margin of Qadaim Basin, NW China. Two types of granites in the area can be divided based on their petrology and geochemistry. The first type of the granite is characterized by the ratio of Na2O/K2O to be less than 1, obvious negative Eu abnormal, and very low Sr and high Y content, having an affinity like the S-type granite and the other type is by the ratio of Na2O/K2O to be larger than 1, no negativ.e Eu abnormal to positive Eu abnormal, and very higher Sr and low Y content, like the I-type granite, indicating the differences in their protolith and origin. The zircon SHRIMP U-Pb dating shows that the ages of the granites in Da Qaidam area can be divided into three groups, that is （1） 446.3 ± 3.9Ma, （2） 408.6 ±4.4Ma, 403.3 ± 3.8Ma and 401.8 ± 3Ma, （3） 374.5 ± 1.6Ma and 372± 2.1Ma. Combined with regional geological characteristics, we think that the first group of ages represents the collision time between Qaidam continental plate and middle-southern Qilian plate, the second group the delemination time and the last stretch of different blocks after plate collisional upheaving.