摘要
本文以INDEPTH项目对印度大陆与欧亚大陆碰撞带深部成像结果为基础,从构造演化角度探讨藏南陆-陆碰撞带冈底斯斑岩铜矿带的成矿作用问题。深部探测给出的碰撞带深部结构与侯增谦等地质学家提出的深部结构有较大的异同,如何协调起来以深化对藏南陆-陆碰撞条件下成矿作用的认识,这是本文讨论的中心。藏南碰撞带成矿实际上是在新特提斯大洋岩石圈俯冲形成的冈底斯岩浆弧成矿作用的基础上,再经过陆-陆碰撞挤压强烈改造后的再成矿。碰撞带的深部结构构造演化的特点是:(1)新特提斯大洋岩石圈板块向北连续俯冲了约120 Ma,形成的冈底斯陆缘火山岩浆弧带,这导致了陆缘带地壳增厚并含有大量的地幔岩浆流体物质(如南美安第斯成矿带那样);(2)在印度大陆与冈底斯陆缘弧接近碰撞时,在对挤中新特提斯大洋洋壳与大洋岩石圈地幔发生向上挤出与向下拆沉,并使部分洋壳残片和大洋岩石圈物质保存在中上地壳内;(3)两大陆岩石圈碰撞对接后,印度岩石圈地幔加深达70~80 km并沿地壳底部向北推进,并将加厚地壳内大量的成矿物质、钙碱性岩浆,洋壳及新生的下地壳,以及部分地幔物质从地壳底部将其围限起来,成为后期再成矿的物质基础;(4)查明了碰撞带深部壳/幔间产生了一层中间速度层(相当于MASH层),在中上地壳部位出现一层巨大的部分熔融层;(5)在碰撞挤压下冈底斯带内产生多组断裂构造,大型逆冲断裂系与背冲断裂,并引发了含矿岩浆的再活动,并在浮力(下地壳内)和挤压力作用下多次活动上升生成斑岩型铜矿床;(6)成矿后地表遭受过强烈的风化剥蚀作用,使矿床出露地表。
Based on deep imaging results of India and Eurasia collision belt by INDEPTH project, this paper deals with the metallogenic problem of the Gangdise porphyry copper belt in south Tibet continent-continent collision belt from the perspective of structural evolution. Gangdise continental margin was modified by the subduction of Neo-Tethys Ocean lithosphere and then remodified by the continent-continent collision. Deep structure of the collision belt shows the following characteristics:(1) Neo-Tethys Oceanic plate subducted continuously at about 120 Ma, and formed the Gangdise epicontinental volcanic arc, which resulted in the thickening of the crust and brought a large amount of mantle fluid material into thickened crust.(2) Some residual oceanic crust, juvenile lower crust and probably mantle material were existent beneath the collision zone.(3) The Indian lithospheric mantle extended northward under and across the collision zone without any break off.(4) A large partial melting layer(or magma chamber) at the depth of 20 km existed across the Gangdise block, after the initiating of Gangdise thrust at about 10 Ma that, which began to supply the magma with copper; in addition, a layer of middle velocity existed between the crust and the mantle. In the upper crust there were large thrust systems and a backthrust.(5) The formation of multiple sets of thrusts and faults with the collision compression in the Gangdise belt triggered the ore magma activity again, which rose to generate porphyry copper deposits.(6) The surface suffered strong weathering denudation and made the deposit exposed on the surface.