滇西北休瓦促Mo-W矿区印支晚期和燕山晚期岩浆活动与成矿作用:来自锆石U-Pb年代学和地球化学的证据

Geochronology and geochemistry of Xiuwacu intrusions,NW Yunnan: Evidences for two-period magmatic activity and mineralization

本文首次在格咱岛弧休瓦促Mo-W矿区识别出印支晚期似斑状黑云母花岗岩,并确定其结晶年龄为200.93±0.65Ma,同时获得燕山晚期二长花岗岩结晶年龄83.57±0.32Ma;即首次在休瓦促Mo-W矿区内厘定出印支晚期和燕山晚期两期花岗岩浆叠加活动,而Mo-W成矿作用与燕山晚期二长花岗岩具有成因关系。岩石地球化学显示燕山晚期二长花岗岩具有较高的SiO_2和全碱含量及较低的Fe、Mg、Ca和P含量,呈准铝质-弱过铝质;富集Rb、Th、U、Nb、Zr和轻稀土元素,亏损Ba、Sr、P、Eu,具有高分异I型花岗岩特征;其形成于与拉萨-羌塘板块碰撞相关的陆内伸展环境,主要来自中-基性下地壳物质的部分熔融,为Mo-W成矿作用提供了重要的物质基础。相对于二长花岗岩,印支晚期似斑状黑云母花岗岩具有较低的SiO_2、Na_2O+K_2O含量和A/CNK比值,较高的Mg、Ca和P含量;富集Th、U、Rb和轻稀土元素,强烈亏损Nb、Ta、Zr、Hf等高场强元素,为准铝质高钾钙碱性具有岛弧岩浆性质的花岗岩,可能形成于甘孜-理塘洋壳俯冲作用结束后,松潘-甘孜地块和义敦岛弧碰撞后伸展环境,为俯冲期改造后形成的下地壳部分熔融的产物。

Two-period igneous intrusions have been distinguished by zircon LA-ICP-MS U-Pb dating in the Xiuwacu Mo-W deposit that the earlier period is Late Indosinian （200.93±0.65Ma） porphyritic biotite granites and the later period is Late Yanshanian （83.57±0.32Ma） monzonitic granites. The Mo-W mineralizations are genetically related to the Late Yanshanian monzonitic granite. The Late Yanshanian monzonitic granites are metaluminous or slightly peraluminous granites, with high SiO2, K2O＋Na2O, and Ga/Al, and relatively low CaO, MgO, and P2O5. The monzonitic granites are enriched in light rare earth elements （LREEs）, Rb, Th, U, Nb and Zr but depleted in Ba, Sr, and P, with significantly negative Eu anomalies, and show highly fractionated I-type granites affinities. These characteristics indicate that the monzonitic granites were generated under a late-or post-collision environment related to the Lhasa-Qiangtang collision during the Late Cretaceous. In addition, the monzonitic granites origin predominately through partial melting of mafic-intermediate lower continental crust, which provide ore metals for Mo-W mineralization. Comparing to the Late Yanshanian monzonitic granites, the Late Indosinian porphyritic biotite granites present lower SiO2, K2O＋Na2O, and A/CNK, and relatively higher CaO, MgO, and P2O5. The porphyritic biotite granites are enriched in light rare earth elements （LREEs）, Rb, Th and U, but depleted in Nb, Ta, Zr, and Hf, and shown calc-alkaline arc-type granites affinities. The porphyritic biotite granites were generated under a late- or post-collision environment related to the Yidun Arc and Songpan-Ganzi Belt collision, and derived from partial melting of subduction-modified lower crust.