川西石棉花岗岩的锆石U-Pb年龄和岩石地球化学特征:岩石成因与构造意义

Zircon U-Pb Age and Petrochemical Characteristics of Shimian Granite in Western Sichuan:Petrogenesis and Tectonic Significance

扬子西缘新元古代岩浆岩分布广泛,目前对其成因和构造背景还存在很大争议.对扬子西缘康滇裂谷北段石棉花岗岩体进行了系统的SHRIMP锆石U-Pb年龄、岩石学和元素-Nd同位素地球化学研究,结果表明该岩体是弱铝质的高钾钙碱性Ⅰ型花岗岩,形成于818±7Ma,是由前存年轻(中元古代末-新元古代初)岛弧地壳物质部分熔融形成的,并混染了少量古老地壳物质.石棉花岗岩形成于扬子地块西缘由会聚挤压向陆内伸展的转折时期,其'岛弧地球化学特征'是继承了源岩的地球化学特征的结果,不代表其形成时的构造环境.

Neoproterozoic magmatic rocks are widespread in western margin of the Yangtze block, and their genesis and tectonic setting have been an issue in hot debate at the present time. SHRIMP U-Pb zircon age, petrology, geochemical and Nd isotopic data are reported for the Neoproterozoic Shimian granite in the Kangdian rift of western Sichuan. This pluton is of metaluminous high-K calc-alkalie I-type granite and emplaced at 818±7 Ma. Petrology, geochemical and Nd isotopic characters suggest that the pluton was generated by partial melting of pre-existing, young （Late Mesoproterozoic to Early Neoproterozoic） island arc crust, with contamination of old crust materials during magma ascending and emplacement. The Shimian granite is the product of the tectonic transition from compression to introplate extension in western margin of the Yangtze block. Their arclike geochemical features （such as Nb-Ta depletion） should have been inherited from the protoliths, rather than reflection of their tectonic setting when the pluton formed.