藏北布若错超浅成花岗岩体地球化学特征及成因机制探讨

Geochemical characteristicss and genetic mechanism of the Buruocuo super-shallow granite body in Northern Tibet

藏北布若错岩体是一个主要由花岗斑岩、流纹质碎斑熔岩组成的花岗岩体。岩体具有高硅,富钾,全碱含量较高,A/KNC值大于1.10的特征,属于强过铝质花岗岩类。CaO/Na2O>0.3,同时大离子亲石元素(LILE)K、Rb、Ba、Th及REE富集,高场强元素(HFSE)Ta、Nb、Zr、Hf相对亏损,中等Eu亏损,反映为地壳部分熔融形成的后碰撞花岗岩。岩体侵入侏罗系,全岩K-Ar法同位素年龄值94.8 Ma,是晚侏罗世末-早白垩世早期羌南陆块与羌北陆块之间发生陆内碰撞造山作用,地壳深部发生局部融熔形成岩浆,并沿断裂上涌至地壳上部超浅成环境侵位的花岗岩体。

Buruocuo granite consisted mostly of granite porphyry and rhyolitic cataclastlava has high silica, potassium, higher total alkalis, A/KNC value is greater than 1.10, and it＇s belongs to peraluminous granites. CaO/Na20 ratio is greater than 0.3, and enrichment of the LILEs K, Rb, Ba, Th and REE, depletion of high field-strength elements Ta, Nb, Zr, Hf, coupled with slightly negative anomalies, moderate depletion of Eu-anomalies, which indicated that the post-collisional granite was formed by partial melting of the crust. Whole-rock K-Ar isotopic ages suggest that the Granites were formed at 94.8 Ma by intracontinental collision between the landmass of Qiangnan and Qiangbei in Late Jurassic to Early Cretaceous. The genetic mechanism is partial melting of deep crust and then the formation of magma, and the influx of upper crust of ultra-shallow granite emplacement environment along the fault.