西藏冈底斯西部江巴、邦巴和雄巴岩体的锆石U-Pb年代学与Hf同位素地球化学

Zircon U-Pb geochronology and Hf isotopic geochemistry of Jiangba,Bangba,and Xiongba granitoids in western Gangdese,Tibet

对青藏高原冈底斯带西部中生代花岗岩的研究依然十分有限。本文选择青藏高原冈底斯带西部狮泉河-革吉-雄巴地区的三个花岗岩基进行了锆石SHRIMP U-Pb定年和锆石Hf同位素分析,并探讨了中冈底斯带中侏罗世-早白垩世花岗岩的分布特征及其揭示的地壳基底的属性。定年结果表明,江巴岩体花岗闪长岩年龄为170±3Ma,雄巴岩体碱长花岗岩年龄为149±3Ma,它们形成于中晚侏罗世;邦巴岩体寄主岩石正长花岗岩和其中的石英二长闪长岩包体年龄分别为144±3Ma和133±3Ma,形成于早白垩世。锆石Hf同位素和地壳模式年龄结果表明,中晚侏罗世的江巴岩体(εHf(t)为-16.8～-13.4,Hf同位素地壳模式年龄为2.1～2.3Ga)与雄巴碱长花岗岩(-11.3～-6.2和1.6～2.0Ga)具有富集的Hf同位素特征,显示了新元古界的地壳基底年龄。邦巴正长花岗岩(-4～-0.8和1.2～1.5Ga)与其中的闪长质包体(-2.8～+0.6和1.2～1.4Ga)具有一致的Hf同位素特征,显示了岩浆混合作用。本文花岗岩定年与Hf同位素结果进一步揭示出中冈底斯带存在未出露地表的古元古代地壳基底。

The study on the Mesozoic granitoids in the western Gangdese belt is still limited.This paper presents the zircon SHRIMP U-Pb dating and Hf isotopic results on the Jiangba,Bangba,and Xiongba granitoids.The results are also applied to the discussion of the granitoids distribution as well as the nature of the crustal basement from which the granites originated.The ages of the Middle-Late Jurassic Jiangba granodiorite and Xiongba alkali granite are 170±3Ma and 149±3Ma,respectively.The ages of the Bangba orthogranite and its quartz monzodiorite enclaves are 144±3Ma and 133±3Ma,respectively,suggesting the Early Cretaceous magmatism.The Hf isotopes(εHf(t))and Hf crustal model age of the Middle-Late Jurassic Jiangba(-16.8^-13.4 and 2.1~2.3Ga)and Xiongba(-11.3^-6.2 and 1.6~2.0)granitoids are similar,which indicates an enriched,Paleoproterrozoic crustal basement under the study areas.The Early Cretaceous Bangba intrusive(εHf(t)=-4^-0.8,model age=1.2~1.5Ga)and the dioritic enclave(εHf(t)=-2.8^+0.6,model age=1.2~1.4Ga)have similar features,suggesting the magma mixing event during their forming processes.The result in this paper further implies that the Paleoproterrozoic crustal basement has involved in the reworking of the Mesozoic magmatism,although they have not been revealed on the surface.