西藏加多捕勒铁铜矿成矿岩体时代与成因：锆石U-Pb年龄、Hf同位素与稀土元素证据

摘要

西藏加多捕勒矽卡岩型铁铜矿床位于拉萨地块的中部,矿体产于中粗粒黑云母二长花岗岩与中二叠统下拉组灰岩的接触带.黑云母二长花岗岩内的锆石具典型岩浆锆石的特征,可分为4期,其第Ⅳ期锆石的SHRIMP U-Pb年龄为(50.9±1.8)Ma,代表黑云母二长花岗岩的结晶年龄,表明岩体形成于印度-亚洲大陆碰撞造山的主碰撞期.第Ⅳ期锆石的176Hf/177Hf=0.282 523～0.282 864,εHf(t)=-7.72～+4.29,平均地壳模式年龄TCDM=851～1 618 Ma,∑REE=351～2 198μg/g,(Yb/Nd)CN=11～506,δCe=1.21～65.58,δEu=0.09～0.33,表明岩浆源区具有壳幔混源的特征,其中的壳源岩浆是中元古代上地壳部分熔融的产物.上述结果显示,在印度-亚洲大陆碰撞造山的主碰撞期,于印度陆壳随同特提斯洋壳向拉萨地块陡深俯冲之后,发生了印度陆壳与特提斯洋壳断离,导致印度陆壳折返和软流圈地幔流体上涌,引发了上覆地壳的侧向伸展和部分熔融,发生了软流圈地幔流体与壳源岩浆的混合,壳幔混源岩浆经岩浆房结晶分异后上侵而形成了黑云母二长花岗岩.%Located in the central part of the Lhasa terrane, the Jiaduobule skarn Fe-Cu deposit occurs in the contact zone between a biotite monzogranitic body and the marble of Permian Xiala Formation. The zircons from biotite monzogranite are typical magmatic zircons, which can be divided into four groups based on their characteristics. SHRIMP U-Pb dating of the fourth group zircons yields the age of (50.9 + 1.8) Ma, which represents the crys-tallization age of biotite monzogranite, showing that the rock was formed in the main collisional period of Indo-Asia continental collision process. The fourth group zircons have intermediate 176Hf/177Hf of 0.282 523 ~ 0.282 864, εHf(O of -7.72～+4.29, model age of crust TDMC from 851 to 1 618 Ma, high 2REE of 351~ 2 198 (μg/g, varying (Yb/Nd)cN from 11 to 506 and Sce of 1.21-65.58, and low Seu (0.09-0.33), indicating that the rock was formed from a mixed magma between the partial melting materials of the Mesoprotero-zoic upper crust of Indian continent and asthenospheric mantle. These data suggest that the magma was derived from a mixing process between the asthenospheric mantle fluid and the partial melting material of the upper crust, which was motivated by the asthenospheric mantle upwelling through the break-off of the Tethyan oceanic slab from the Indian continental crust after the steep and deep subduction of Indian continental crust with Tethyan oceanic slab into Lhasa terrane in the main collisional orogenic epoch of the Indo-Asia continental collision. The mixed magma produced fractional crystallization in the magma chamber and then intruded upward to form the biotite monzogranite.