初论陆-陆碰撞与成矿作用——以青藏高原造山带为例

青藏高原碰撞造山带以其成矿规模大、形成时代新、矿床类型多、保存条件好诸特征而被誉为研究大陆成矿作用的天然实验室。文章基于青藏高原已有的矿产勘查与研究成果,概述了大陆碰撞过程中的主要成矿作用及其成矿带的时空分布,初步分析了陆一陆碰撞所造就的成矿背景和成矿环境以及控制成矿作用的关键地质过程．并草拟了可供今后研究的工作模型。初步研究认为,始于60Ma的印度大陆与亚洲大陆碰撞至少形成了3个重要的控矿构造单元,即雅鲁藏布江以北的主碰撞变形带,雅鲁藏布江以南的藏南拆离-逆冲带和高原东缘的藏东构造转换带。主碰撞变形带以巨大规模的地壳缩短、双倍地壳加厚、大规模逆冲系和SN向正断层系统发育为特征,控制了冈底斯斑岩铜矿带(含浅成低温热液金矿)、安多锑矿化带和风火山铜矿化带及腾冲锡矿带的形成及分布;藏南拆离一逆冲带由藏南拆离系(STDS6)和一系列北倾的叠瓦状逆冲断裂带构成,控制了藏南变质核杂岩型金矿化、热液脉型金锑矿化和蚀变破碎带型金锑矿化的形成;藏东构造转换带以发育大规模走滑断裂系统、大型剪切带、富碱斑岩带和走滑拉分盆地为特征,控制了玉龙斑岩铜矿带、哀牢山和锦屏山金矿带及兰坪盆地银多金属矿带的分布。按成矿系统的基本思想,初步将青藏高原碰撞造山带的成矿作用划分为3个成矿巨系统：大陆俯冲碰撞成矿巨系统、陆内走滑一剪切成矿巨系统和碰撞后伸展成矿巨系统。在大陆俯冲碰撞阶段,主要发育与流体迁移汇聚和排泄有关的锑金铜热液成矿系统和碰撞期花岗岩岩浆．流体锡稀有金属成矿系统;伴随陆．陆碰撞而发生的陆内走滑．剪切作用,主要导致了走滑拉分盆地银多金属热液成矿系统、斑岩型铜钼金成矿系统和剪切带型金成矿系统的形成;在碰撞后伸展阶段,主要发育受SN向正断层系统控制的斑岩铜矿成矿系统、浅成低温热液金矿成矿系统和热水沉积铯锂硼金属成矿系统。在此基础上,初步提出了碰撞造山带成矿作用的构造控制模型。

Continental Collision and Related Metallogeny: A Case Study of Mineralization in Tibetan Orogen

Metallogeny in the collisional orogen is an important research frontier for economic geologists both in China and abroad. The Himalayan_Tibetan orogen, which is characterized by the occurrence of large_scale, intense and young mineralizations as well as various kinds of large_size and weakly reformed deposits, is regarded as an ideal field for understanding ore_forming processes in collisional orogens. Based on data available and the authors' preliminary research results, this paper describes geological features and temporal_spatial distribution of important ore deposits in the Tibetan orogen, discusses geodynamic settings, mineral environments, and key geological processes governing the formation of these deposits, and proposes a possible working model for metallogeny in the Tibetan orogen. It seems that the Indo_Asian continent collision since 60 Ma resulted at least in the formation of three significant tectonic units controlling the development of ore deposits in the Tibetan orogen. They are: (1) the main collisional deformation zone north of IYS, (2) the southern Tibet detachment_thrust zone south of IYS, and (3) the eastern Tibet tectonic transformation zone. In the main collisional deformation zone characterized by the development of large_scale crustal shortening, thickening and thrusting as well as SN_striking normal faulting systems across the zone, there have occurred since Paleocene the Gangdese porphyry (_epithermal) Cu_ (Au) belt, the Ando Sb mineralization belt and the Fenghuoshan sandstone Cu mineralization belt in Tibet, and the Tengchong Sn belt in western Yunnan. In the southern Tibet detachment_thrust zone, the STDS and a series of thrusting systems control the Au_Sb hydrothermal mineralizations related to metamorphic core_complexes and tectonic_controlled fracture zones. Characterized by the development of large_sized strike_slip faulting systems constraining the localization of ore_bearing potassic porphyry intrusion groups, large_sized shear belts and strike_slip pull_apart basins in the transpressional regime, the eastern Tibet tectonic transformation zone has controlled the Yulong porphyry Cu belt, the Ailaoshan and Jinpingshan gold belts, and the Lanping Ag polymetallic belt. According to the concept of the ore_forming system, the authors have divided the mineralizations in the Tibetan orogen into three ore_forming super_systems related respectively to (1) continental collision and subduction since Paleocene, (2) strike_slip and shearing associated with collision, and (3) post_collisional extension at mid_Miocene. In the first super_system, the Au_Sb_Cu system was formed by fluid migration and hydrothermal discharging in the pressure regime, accompanied by continental convergence, whereas the Sn and rare metal system was produced by the emplacement of syn_collisional granite and magmatic_hydrothermal activity. In the second system, stress relaxation during the formation of strike_slip pull_apart basins resulted in the escaping of hydrothermal fluids from porous strata in basins or porphyry magmatic chambers and thus formed the Ag_Pb_Zn_Cu system and the porphyry Cu_Au system, whereas the shearing process led to the formation of the Au system related to the shear_zone. In the third super_system, post_collisional crustal extension and normal fault systems controlled the localization of Cu_bearing porphyry intrusions, which drove convection circulation of hydrothermal fluids and formed the epithermal and hot_spring Au_Sb system. On the basis of the above analysis, a preliminary working model is proposed for the tectonic constraint over the ore_forming processes in the Tibetan orogen.