小兴安岭“前寒武纪”风水沟河群的形成时代及其构造意义:来自锆石U-Pb年代学及古生物化石证据

Age of the“Precambrian”Fengshuigouhe Group in Lesser Xing an Range and its tectonic implications:evidence from zircon U-Pb dating and paleontological fossils

为探明中亚造山带东段兴安地块的基底属性及岩石组合,笔者选择以小兴安岭地区前人划定的前寒武纪风水沟河群为研究对象,通过岩相学、锆石LA-ICP-MS U-Pb年代学及古生物化石等分析方法,对风水沟河群的岩石组合、形成时代及成因进行了详细研究。结果表明:风水沟河群主要由变粒岩、片麻岩、大理岩、千枚岩和片岩等一系列变质岩组成,其中黑云斜长片麻岩的原岩年龄为389~387 Ma,片理化二云母红柱石角岩的原岩年龄为(284±5)Ma,黑云斜长片岩的原岩年龄为(261±2)Ma。上述特征结合本次在黑色含矽线红柱堇青二云变粒岩(具角岩化)中发现早二叠世的腕足化石,证明风水沟河群可能形成于晚古生代,并非形成于前寒武纪,可能为古亚洲洋分支俯冲于额尔古纳—兴安地块之下形成的俯冲-增生杂岩。

In order to explore the basement affinity and rock assemblage of the Xing an block in the eastern sector of the Central Asian orogenic belt,the authors have chosen the previously proposed Precambrian Fengshuigouhe Group in the Lesser Xing an Range as the study object,and made a detailed study on the rock assemblage,age and genesis of the Fengshuigouhe Group by means of petrography,zircon LA-ICP-MS U-Pb dating and paleontological fossils.The results show that the Fengshuigouhe Group is mainly composed of metamorphic rocks such as leptynite,gneiss,marble,phyllite and schist,among which the protolith age of biotite plagioclase gneiss is 389-387 Ma,the protolith age of schistose two-mica andalusite hornstone is(284±5)Ma,and that of biotite plagioclase schist is(261±2)Ma.On the basis of the characteristics mentioned above and the Early Permian brachiopod fossils found in the black hornized sillimanite-,cordierite-bearing two-mica leptynite,it is proved that the Fengshuigouhe Group may be formed in the Late Paleozoic rather than Precambrian,and could be a subduction accretionary complex resulted from the subduction of a Paleo-Asian Ocean branch beneath the Erguna-Xing an block.