新疆西准噶尔花岗岩类的时代及其成因

A GEOCHRONOLOGY AND GENESES OF THE WESTERN JUNGGAR GRANITOIDS, XINJIANG, CHINA

在西准噶尔地区存在两期不同成因的花岗岩类,一期为与弧后盆地封闭有关的海西中期(305—320Ma)、以小岩体产出的花岗闪长岩-石英闪长岩;另一期为后造山的海西晚期(240—280Ma)的以巨大岩基形式产出的碱长花岗岩。

There are two genetically different granitoids in the western Junggar area, Xinjiang. The middle-Hercynian granitoids in forms of small bodies consist mainly of granodiorite-quartz diorite. Zircon from five sampling sites in body I gives an inconsistent U-Pb age of 305 Ma, and seven samples from body V give Rb-Sr age of 322 Ma. Another large-scale granitoid intrusion developed in late-Hercynian is syenite-granite and alkali granite. Four fractions of zircon from Akbastaw batholith give slightly consistent U-Pb age of 256 Ma on average and ziroon from Hongshan batholith gives U-Pb age of 245 Ma. Moreover five major batholithes give their 40Ar/ 56Ar-40K/MAr age of 281Ma.The quartz diorite emplaced during middle-Hercynian period was probably derived from the lower crust-upper mantle. During the conrraition and closure of ancient oceanic basin the quartz-dioritic magma was squeezed upward and emplaced. The granite batholith emplaced during late-Hercynian period came from partial melting of the upper crust. Initial Pb isotope data from potassium feldspar of granites indicate the granites of mantle characterstics, they might be formed by recent intracontinental convergeuce which lad partial melting of the sedimentary-volcanic series in back-arc basin along the Karamay fault.