天水地区陇山群斜长角闪岩的年代学及其地质意义

Geochronology of amphibolite from Longshan Group in Tianshui area and its geological significance

为限定北祁连构造带古老基底组成的形成时代和构造属性,笔者对天水地区陇山群中的斜长角闪岩进行了LA-ICP-MS锆石U-Pb年代学研究。斜长角闪岩中的锆石均呈自形-半自形,内部结构清晰,发育有典型岩浆成因的振荡生长环带,结合较高的Th/U比值,表明锆石的岩浆成因特征。测年结果显示:斜长角闪岩的加权平均年龄为(444±2)Ma,是陇山群中首次获得的早志留世火山岩的年龄。结合区域地质资料,认为北祁连构造带陇山群应解体为古元古代-新元古代古老结晶基底岩系和早古生代变质火山-沉积岩系两套地质体。

In order to define the formation age and structural properties of the ancient basement in the North Qilian orogen,LA-ICP-MS zircon U-Pb dating of amphibolite in Longshan Group in Tianshui area has been studied.Zircons in amphibolite are euhedral-subhedral with clear internal structure,and typical magmatic oscillatory growth zones are developed.Combined with the high Th/U ratios,zircons are of magmatic origin.The results show that the weighted average age of amphibolite is(444±2)Ma,which is the first age of Early Silurian volcanic rocks in Longshan Group.Based on the regional geological data,the authors suggest that the Longshan Group in the North Qilian orogen should be disintegrated into two sets of geological bodies:Paleoproterozoic-Neoproterozoic ancient crystalline basement rocks and Early Paleozoic metamorphic volcanic-sedimentary rocks.