青藏高原碰撞造山带:Ⅱ.晚碰撞转换成矿作用

Metallogenesis in Tibetan collisional orogenic belt:Ⅱ. Mineralization in late-collisional transformation setting

许多古老造山带的碰撞造山过程，因从晚碰撞向后碰撞的转换，既不清楚，又难以界定，常被分为碰撞和后碰撞2个阶段。文章对青藏高原碰撞造山过程进行了分析，发现其具有明显的3段性，由此将碰撞造山过程分为主碰撞（65-41Ma）、晚碰撞（40-26Ma）和后碰撞（25-0Ma）3个阶段。其中，晚碰撞造山作用发生于印度与亚洲大陆的持续汇聚和SN向挤压背景之下，以陆内俯冲、大规模逆冲推覆、走滑断裂系统的发育为特征，导致了区域尺度的地壳缩短及藏东富碱斑岩和碳酸岩一正长岩、藏北钾质-超钾质火山岩的大规模产出。晚碰撞期成矿作用强烈发育，主要集中于高原东缘的构造转换带，成矿高峰期集中于（35±5）Ma。现已识别出4个重要的成矿事件：①与大规模走滑断裂系统有关的斑岩型Cu-Mo（Au）成矿事件，形成著名的玉龙斑岩铜矿带（40～36Ma）；②与碳酸岩-正长岩杂岩有关的REE成矿事件，在二叠纪攀西古裂谷带内发育勉宁-德昌喜马拉雅期REE成矿带（41～27Ma）；③与逆冲推覆构造系统有关的热卤水型Pb-Zn-Ag-Cu成矿事件，集中产出于兰坪盆地，形成大型Pb-Zn-Ag矿集区（40-30Ma）；④与大规模剪切系统有关的剪切带型Au成矿事件，形成著名的哀牢山大型Au矿带（63～28Ma）。晚碰撞成矿作用主要发育于陆内转换造山环境，受大规模走滑-推覆-剪切作用控制，受控于统一的深部作用过程，与软流圈上涌导致的幔源或壳很混源岩浆活动密切相关。在综合研究基础上，初步建立了晚碰撞转换成矿模型。

The collisional processes established many paleo-mountain belts are commonly divided by many geologists into two simple, but distinct stages, i.e., syn-collisional and post-collisional periods, as the transition from lateto post-collisional is not clear and the definition of post- and late-collisions also is blurring. However, the tectonic evolution of the Tibetan orogen underwent a complicate history, which is divided into （1） syn-collisional （65- 41 Ma）, （2） late-collisional（40-26 Ma）; and （3） post-collisional （25-0 Ma） periods. The orogeny during the late-collisional period generally happened in a compressional regime resulted from successional convergence between Indian and Asian continents, and is characterized by intra-continental subduction （underthrust）, crustal shortening, large-scale thrusting and strike-slip faulting systems developed in the margins of the Tibetan plateau, which resulted in the development of potassic porphyry stocks and carbonatite-alkalic complexes in the eastern Tibet and potassic-ultrapotassic volcanic rocks in the northern Tibet. At least four significant metallogenic events, associated with the late-collsional orogeny, have been recognized in the eastern margin of the Tibetan plateau, a tectonic transformation belt. They are： （ i ） the porphyry Cu-Mo（-Au） mineralization related to large-scale strike-slip faults system, which controlled the localization of the Yulong porphyry Cu belt （40- 36 Ma） in east Tibet, （2） the REE mineralization associated with carbonatite-alkalic complexes, formed the Mianning-Dechang REE belt （40-27 Ma） in the western Sichuan, （3） the Pb-Zn-Ag mineralization controlled by thrust faulting systems, formed at least two large-sized Pb-Zn-Ag deposits （40-30 Ma） in the Lanping strikeslip pull-apart basin, western Yunnan, and （4） the Au mineralization related to large-scale shearing, which produced the Ailaoshan Au belts composed of numerous shear-type or orogenic-type Au deposits in the western Sichuan and Yunnan. These data indicate that the late-collisional metallogensis in the Tibetan orogen were mainly developed in an intra-continental transformation orogenic sdetting, characterized by a series of large-scale strike-slip faulting, thrusting, and shearing systems developed in the late-collisional period. Available data suggest that the metallogensis is probably controlled by a shared deep lithospheric process, most likely related to upwelling of the asthenosphere triggered by face to face subduction of the Yangtze and Lhasa blocks （continental slabs） in the eastern margin of the Tibetan plateau during the late-collisional period.