西藏拉萨岩体黑云母二长花岗岩：锆石U-Pb年龄、地球化学特征及其地质意义

Zircon U-Pb age, geochemistry and geological significances of the biotite monzogranite in Lhasa rock mass, Tibet

关键词:  古新世花岗岩  地球化学  锆石U-Pb定年  岩石成因  拉萨岩体

Key words:  Paleocene granite  geochemistry  zircon U-Pb dating  petrogenesis  Lhasa rock mass

摘要:

拉萨岩体位于冈底斯铜、铁、铅锌多金属成矿带东段，是一个典型的复式岩体。通过对拉萨岩体开展详细的野外地质调查和岩相学观察，结合锆石LA-ICP-MS U-Pb定年、岩石地球化学分析等研究，认为拉萨岩体中代表性的黑云母二长花岗岩具有高SiO₂（含量为69.94%～76.62%，平均为72.18%）、高碱（Na₂O+K₂O=6.95%～8.44%，平均为7.44%）、较高CaO（含量为0.68%～2.53%，平均2.00%）的特点，相对富集Ba、Rb、Th，亏损Nb、Ta、P、Ti，LREE富集、HREE亏损，轻、重稀土元素分馏强烈，Eu负异常明显，是典型的钙碱性I型花岗岩。3件黑云母二长花岗岩成岩的锆石LA-ICP-MS U-Pb年龄为57±0.5Ma、58±1Ma、59±1Ma，表明其为古新世岩浆活动产物。综合研究认为，拉萨岩体黑云母二长花岗岩形成于印度与亚洲大陆主碰撞阶段（65～45Ma），为底侵作用产生的大量壳源熔体与少量幔源熔体混合并发生一定程度分离结晶后的产物。陆内碰撞环境下的岩浆活动及元素的迁移和富集，为冈底斯成矿带内铜、金等矿产提供了良好的成矿条件。

Abstract:

The Lhasa rock mass is a compound rock mass, locating in the eastern section of the Gangdise belt. Based on detailed geological survey, zircon U-Pb dating and geochemical studies on the biotite monzogranite in the Lhasa rock mass, it is believed that the Lhasa rock mass was formed during the main collision stage of the Indian and Asian continents. The U-Pb dating of zircons from the biotite monzogranite yields three ages, 57±0.5Ma,58±1Ma, and 59±1Ma, suggesting that the Lhasa rock mass is the product of paleocene magmatic activity. Geochemically, the representative biotite monzogranite in the Lhasa rock mass is of high SiO₂ (69.94%~76.62%), high alkali (6.95%~8.44%), and high CaO(0.68%~2.53%) compositions. Relatively, the biotite monzogranite in the Lhasa rock mass is rich in LREE and LILE such as Ba、Rb、Th and poor in HREE and HFSE such as Nb, Ta, P, and Ti, with an obvious Eu negative anomaly. All geochemical characteristics indicate that the Lhasa rock mass is a typical calc-alkaline I-type granite rock mass, which is favorable for skarn-typed metallogenic processes in the Gangdise belt.