阿拉善地块南缘龙首山岩群及相关岩石的起源和归属——来自LA-ICP-MS锆石U-Pb年龄的制约

The origin of Longshoushan Group and associated rocks in the southern part of the Alxa block: constraint from LA-ICP-MS U-Pb zircon dating

采用LA-ICP-MS方法对采自阿拉善地块南缘龙首山岩群及上覆的墩子沟群底部共6个不同类型的样品进行了锆石U-Pb测年,获得龙首山岩群上部变沉积岩中的碎屑锆石年龄集中在~2.01 Ga和~2.15 Ga,花岗质片麻岩的岩浆锆石年龄主要在2.04~2.17 Ga之间,变质重结晶(增生边)的年龄在1.89~1.93 Ga之间,表明龙首山岩群的形成时代为古元古代。获得龙首山岩群中斜长角闪岩中变质锆石年龄为~1.85 Ga,指示古元古代晚期龙首山岩群经历角闪岩相区域变质作用;上覆的墩子沟群底部变沉积岩的碎屑锆石U-Pb年龄测定集中在2.03~2.05 Ga之间,表明其物源来自古元古代的岩浆岩。这些年龄资料显示龙首山岩群及相关岩石所代表的阿拉善地块南部在早前寒武纪与华北克拉通具有明显亲缘性。

The Longshoushan Group is a part of Precambrian metamorphic basement of the Alxa block.Its origin is particularly important for constraining the western boundary of the North China Craton（NCC）and understanding the evolutionary history of China＇s cratons.In this study,LA-ICP-MS U-Pb dating technique was used to determine the ages of different rocks from Longshoushan Group and Dunzigou Group.The analytical results of detrital zircon grains from the metasedimentary rock in the Longshoushan Group gave two age populations： ~2.01 Ga and^2.15 Ga.The magmatic zircons and metamorphic zircons from the granitic gneiss yield age spans of 2.04~2.17 Ga and 1.89~1.93 Ga,respectively.These ages indicate that the Longshoushan Group was formed in Paleoproterozoic.The metamorphic zircons from amphibolite in the Longshoushan Group yielded a metamorphic age of ca.1.85 Ga,indicating that the Longshoushan Group underwent an amphibolite-facies metamorphism during late Paleoproterozoic.Detrital zircon ages of the metasedimentary rock from the overlying Dunzigou Group vary from 2.03 to 2.05 Ga,suggesting a single Paleoproterozoic provenance.These data imply that the southern Alxa block had an affinity with the NCC during early Precambrian period.