北山地区古元古界北山岩群中长英质脉内锆石U-Pb年龄及其地质意义

LA-ICP-MS U-Pb ages of zircon from the felsic veins within the Paleoproterozoic Beishan Group in the Beishan Orogen,and their geological significance

深熔作用是大陆地壳分异、元素迁移富集的重要地质过程。位于中亚造山带南缘的北山地区,笔者等对前寒武系北山岩群进行了研究,结果表明该岩群普遍经历了角闪岩相变质及深熔作用,长英质浅色脉体分布广泛,却鲜有文献报道。绵山地区出露的古元古代北山岩群斜长角闪岩及相关的长英质浅色脉体的野外地质特征、相互关系共同指示了北山岩群变质地层经历了部分熔融,熔体在原地分凝聚积。锆石LA-ICP-MS U-Pb测定结果表明,顺北山岩群变质地层片理产出的长英质浅色脉体锆石n(^(206)Pb)/n(^(238)U)加权平均年龄约为370 Ma,表明深熔作用发生时间为晚泥盆世;一测点的n(^(207)Pb)/n(^(206)Pb)年龄为1623±12 Ma,与前人报道的北山岩群测年结果高度相近,虽此年龄值不足为证,但结合产出特征,可能指示长英质浅色脉体的源岩为北山岩群。此外,北山岩群中深熔作用普遍发育,露头可见大量的石榴子石、电气石、云母类矿物,指示区内部分稀有元素成矿潜力巨大。

Crustal anatexis is a significant geological process that plays a crucial role in the differentiation of continental crust and the migration of elements.The Beishan Group,located in the southern edge of the Central Asian Orogenic Belt(CAOB),underwent amphibolite facies metamorphism and anatexis.The relationship between the Paleoproterozoic amphibolite of the Beishan Group and the related felsic veins suggests that the metamorphic strata of the Beishan Group experienced partial melting,and the melt was segregated and accumulated in situ.Zircon U-Pb chronology indicates that anatexis occurred in the Late Devonian,with the n(^(206)Pb)/n(^(238)U)weighted average ages of zircons from felsic veins being approximately 370 Ma.The age of n(^(207)Pb)/n(^(206)Pb)at one spot is 1623±12 Ma,which is highly consistent with the previous dating results of the Beishan Group reported by others.Although this age is not sufficient to prove it,combined with the other evidence,it may indicate that the source of the felsic veins is the Beishan Group.Furthermore,anatexis is widely developed in the Beishan Rock Group,with a large number of garnet,tourmaline,and mica minerals visible in the outcrop,indicating a high potential for rare-metal mineralization.