Os同位素在花岗岩物质来源示踪中的初步研究:以湖南骑田岭岩体为例

Re-Os同位素在基性-超基性岩物质来源示踪方面已得到较广泛应用,但在酸性岩方面尚属少见。与其它同位素体系不同,Re、Os均为高亲铁元素,在岩浆分异演化过程中,Re属于中等不相容元素,Os属于强相容元素,地壳和地幔两端员Os同位素组成差别较大,因此,Re-Os同位素是研究花岗岩中是否存在幔源物质贡献的灵敏示踪剂。通过采自湖南骑田岭复式岩体三个阶段15件花岗岩全岩样品Re-Os同位素的分析,这些样品中Re、Os含量极低(Re含量0.0053～0.4539ng/g,Os含量0.0011～0.0328ng/g),Os同位素初始比值为0.3543～1.728,波动较大,显示骑田岭岩体成岩物质具有壳幔混合来源的特征。其中,早阶段与新田岭钨成矿关系密切的中粗粒似斑状角闪石黑云母花岗岩中壳源物质贡献更多一些,而晚阶段与芙蓉锡矿关系密切的细粒黑云母花岗岩幔源物质相对更多一些。Re-Os同位素的研究,不仅能够为花岗岩物质来源与成因机制的研究提供新的手段,而且,对于获取南岭重点矿集区深部成矿的地球化学信息、探索南岭矿集区壳幔相互作用过程以及幔源岩浆在不同类型矿化花岗岩形成过程中的作用也具有重要意义。

A Premilinary Study of Os Isotope in Tracing of Sources for Granitoids: A Case Study of the Qitianling Pluton

Incompatible lithophile elements and their isotopic systems,such as Rb-Sr,Sm-Nd,and U-Pb,are widely used in the genesis study of granite,but they are less useful as indicators of mantle sources because they are vulnerable to incorporation of continental materials.In contrast,Re and Os provide unique tools for tracing mantle sources because of their chalcophile and siderophile nature,and because of the compatible behavior of Os during mantle melting.The Re-Os isotopic system is thus a highly sensitive monitor of the extent of mantle involvement during granite genesis.In this study,Re and Os concentration and Os isotopic composition have been obtained for 15 granite samples from three major stages in order to assess the role of mantle involvement in the genesis of each stage granite from the Qitianling granite batholith(QGB).These samples have low Re concentrations(0.0053 to 0.4539 ngg,whole rock),and low common Os concentrations(0.0011 to 0.0328 ngg,whole rock).The initial Os isotope ratios of individual stage granite vary considerably in range of 0.3543 to 1.728,showing crust and mantle contributions to the granite genesis.The initial Os isotope ratios of the latest stage fine-grained biotite granite,which closely related to the Furong Sn deposit both spatially and genetically,are generally lower than those of the other two stage granites,indicating more mantle contributions.The initial Os isotope ratios of the earliest stage hornblende-biotite monzonitic granites,which closely related to the Xintianling W deposit,are generally higher,indicating more crustal contributions.Os isotope study is important for researches on crust-mantle interaction process and the mantle-derived magma in the formation of different types of mineralized granite in the Nanling range.