内蒙古中部乌拉山-大青山地区富铝片麻岩年代学特征

Geochronologic Characteristics of Al-rich Gneisses in Wulashan-Daqingshan Area,Central Inner Mongolia

内蒙古中部乌拉山-大青山出露的乌拉山岩群是一套变质程度达高角闪岩相-麻粒岩相的早前寒武纪高级变质岩系。为划分乌拉山-大青山地区富铝片麻岩的原岩形成时代与变质时代,通过对侵入乌拉山岩群黑云石榴斜长片麻岩的变质深成体—石英闪长质片麻岩和紫苏花岗质片麻岩变质深成体进行年代学研究。结果表明:紫苏花岗质片麻岩(TW7)的加权平均年龄为(2 462±13)Ma,花岗闪长质片麻岩(D1227)加权平均年龄为(2 512±24)Ma,由于后期构造事件对岩石的改造,岩体的实际年龄应更大一些。由此限定乌拉山富铝片麻岩原岩的形成年代早于2.5 Ga。石英闪长质片麻岩(TWB6)具有两期变质年龄,分别为2.2 Ga和1.9 Ga,指示乌拉山-大青山地区遭受过古元古代早期和晚期构造热事件叠加改造。上述研究结果表明,乌拉山-大青山地区富铝片麻岩最晚形成时代限定在2.5 Ga。

The Wulashan rock group,a set of Early Precambrian high-grade metamorphic rock series with high metamorphic degree of amphibolite facies-granulite facies,has been determined in the Wulashan-Daqingshan Area of central Inner Mongolia.The metamorphic plutons of quartz diorite gneiss and perilla granitic gneiss,intruded into the biotite garnet plagioclase gneiss of the Wulashan rock group,were studied by chronology to divide the protolith formation age and metamorphic age of aluminum-rich gneiss in the Wulashan-Daqingshan Area.The results show as follows.The weighted average ages of perilla granitic gneiss(TW7)and granodiorite gneiss(D1227)are(2462±13)Ma and and(2512±24)Ma,respectively.It could be inferred that,the formation age of the protolith of the aluminum-rich gneiss in Wulashan is earlier than 2.5 Ga,because of the transformation of rock by later tectonic events.Two metamorphic ages of 2.2 Ga and 1.9 Ga,respectively,could be confirmed from the quartz diorite gneiss(TWB6),indicating that the Wulashan-Daqingshan Area has undergone the superposition of early and late Paleoproterozoic tectonic thermal events.Overall it can be concluded that the latest formation age of aluminum-rich gneiss in the Wulashan-Daqingshan area is limited to 2.5 Ga.