青藏高原冈底斯北缘卡穷岩群形成时代的确定

冈底斯北缘的卡穷岩群为一套中高级变质岩系,其形成时代至今未得到有效限定。通过高精度的LA-ICP-MS(激光剥蚀等离子体质谱)锆石微区原位U-Pb同位素测年,获得西藏八宿县同卡乡北卡穷岩群条带状斜长角闪岩(正变质岩)上交点年龄为(1082±18)Ma,将该岩群原岩形成时代厘定为中元古代末,即晚蓟县世。同时,获得卡穷岩群中片麻状二长花岗岩的上交点年龄为(549±18)Ma,属于泛非期构造-热事件的岩浆作用记录;结合野外地质和岩相学特征,认为该岩体为卡穷岩群变质火山-沉积岩系部分熔融的产物,岩浆未发生远距离运移侵位。泛非运动之后,卡穷岩群还经历一次(416±23)Ma强烈的构造作用,这次构造作用的地质意义值得进一步研究。

The formation age of Kaqiong Rock Group in the northern margin of Gangdese,the Qinghai-Tibet plateau

The Kaqiong Rock Group is consisted of intermediate to high grade metamorphic rocks in the northern margin of Gangdese, the formation age of which was not well constrained. Recently, high resolution LA-ICP-MS in situ dating yielded an upper intercept age of （1082±18） Ma from banded plagioclase amphibolite （orthometamorhpites） of Kaqiong Rock Group in northern Tongka area, Baxoi County, Tibet, and this age limits the formation age of the protolith to late Mesoproterozoic （that is, Late Jixian）. The gneissic monzogranite of Kaqiong Rock Group yields an upper intercept age of （549±18） Ma, which belongs to tectonothermal event caused by magma events in Pan-African. Combined with the field geology and petrographics, we believe that these rocks were the materials formed from the meta-volcanic and sedimentary rocks of Kaqiong Rock Group during partial melting, and the magma emplaced nearly autochthonous. After Pan-African Event, Kaqiong Rock Group underwent a strong tectonic deformation at （416±23） Ma, the significance of which is worth to further discussion.