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Metallogenesis in Tibetan collisional orogenic belt:Ⅲ. Mineralization in post-collisional extension setting

HOU ZengQian~1, QU XiaoMing~2, YANG ZhuSen~2, MENG XiangJin~2, LI ZhenQing~~2, YANG ZhiMing~1, ZHENG MianPing~2, ZHENG YouYe~3, NIE FengJun~2, GAO YongFeng~4, JIANG SiHong~2 and LI GuangMing~5 (1 Institute of Geology, CAGS, Beijing 100037, China; 2 Institute of Mineral Resources, CAGS, Beijing 100037, China; 3 China University of Geosciences, Wuhan 430074, Hubei, China; 4 Shijiazhuang University of Economy, Shijiazhuang 050031, Hebei, China; 5 Institute of Geology and Mineral Resources, CGS, Chengdu 610082, Sichuan, China)  
As a significant and late stage process in the collisional orogeny characterized by a variety of geological features indicating tectonic evolution and large-scale, high-intense mineralization, post-collision has aroused much interest among geologists. However, numerous geological issues, such as post-collisional structural features and tectonic evolution, magmatic sequences and tectonic-magmatic associations, and metallogensis and mineralization systems in the post-collisional settings, have not yet been fully understood. This paper studied and summarized the major features of post-collisional orogeny and related metallogensis, and proposed a tectonic model for metallogensis in the post-collisional setting in Tibet. The available data indicate that there have been at least two stages of tectonic evolution since Miocene in the Tibetan plateau, which is now tectonically in a post-collisional stage. The low-crustal flow and upper-crustal shortening took place in an early post-collisional stage (18 Ma), which led to the southward extrusion of the low-crustal materials, producing the EW-tending south Tibet an detachment system (D) and the High Himalayan block to the south, and the EW-striking thrust faulting systems and the thrust nappe structures along the Gangdese range in the Lhasa terrane, respectively . The crust extension and rifting occurring in the late post-collisional stage (18 Ma) formed a series of NS-striking normal faults and associated rifting basins (≤13.5 Ma) across the Tibetan plateau. The post-collsional magmatism in Tibet is characterized by the mid-Miocene ultra-potassic and potassic volcanic rocks, adakitic intrusives, and calc-alkaline granites developed along the Gangdese batholiths, and the south Tibetan lecuogranites related to D. Mineralization during the post-collisional periods produced a variety of significant mid-Miocene deposits, including the porphyry Cu deposits and the associated skarn Ag-Pb-Zn deposits along the Gangdese batholiths, the epithermal Sb-Au deposits in southern Tibet, the hydrothermal vein-type and skarn-type Ag-Pb-Zn deposits to the north of the Gangdese porphyry Cu belt, and modern Cs-Au deposits related to hot-spring activity in Tibetan plateau. The porphyry Cu deposits, occurring in the post-collisional setting, are associated with felsic stocks that show geochemical affinity with adakites, which are regarded as products of partial melting of newly-formed underplated basaltic lower-crustal source beneath the Tibet. The Sb-Au vein deposits , tectonically located in the Ds and controlled by the metamorphic nuclear complexes (thermal domes) and NS-striking normal faults, are related to the epithermal systems driven by the mid-Miocene leucogranitic bodies. The vein-type Ag-Pb-Zn deposits occur within a thrusting nappe structural zone in the Lhasa terrane, and are related to the fluid flows discharged and moved along a northward gently-dipping detachment fault zone related to upper-crust shortening. In general, three kinds of structural systems in the upper-crust level, i.e., the NS-striking normal faulting system and associated rifting basins, the EW-tending thrusting faults and associated nappe structures, and the EW-tending Ds, constrained the metallogensis during the post-collisional periods. Nevertheless, the flow and extrusion of the mid-lower crust and the breaking-off of the subducted Indian continental slab are considered to be the principal deep dynamic processes leading to the formation of these distinct deposits in the post-collisional setting in Tibet.
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