秦岭岩群早古生代晚期构造热事件及地质意义

A late Early Paleozoic tectonothermal event of the Qinling Group and its geological significance

文章在野外观察基础上,对秦岭岩群黑云斜长片麻岩和侵入其中的长英质脉体进行了锆石U-Pb定年。定年结果表明:黑云斜长片麻岩岩浆锆石年龄变化于476~2181 Ma之间,主要峰值在897 Ma,而3颗变质锆石的206Pb/238U加权平均年龄为(427±14)Ma。长英质脉体锆石普遍具有清晰的振荡环带和高w(Th)/w(U)比值,属岩浆成因,它们的206Pb/238U加权平均年龄为(442±9)Ma,与黑云斜长片麻岩记录的变质时代时间在误差范围内一致,暗示秦岭岩群在早古生代晚期经历了1期构造热事件。结合相关研究成果,可以推断秦岭岩群早古生代晚期变质-岩浆作用是商丹洋向北俯冲的结果。

Based on field observation,a detailed zircon U-Pb dating was performed on the biotite plagioclase gneiss and felsic vein from the Qinling Group.Magmatic zircons of the gneiss showed a wide range of age from 476 Ma to 2181 Ma with a peak at 897 Ma,whereas the remaining three metamorphic zircons yielded a weighted mean 206Pb/238U age of(427±14)Ma.Zircons in felsic vein were generally characterized by clearly oscillatory zoning and high w(Th)/w(U)ratio,typical of igneous zircons.They yielded a weighted mean 206Pb/238U age of(442±9)Ma,which was consistent within errors to the metamorphic age of the gneiss,suggesting that the Qinling Group had experienced a late Early Paleozoic tectonothermal event.Combined with previous researches,it was believed that the late Early Paleozoic metamorphism and magmatism recorded in the Qinling Group resulted from a northward subduction of the Shangdan Ocean.