西藏冈底斯成矿带深部构造背景研究

The Gangdese belt located in the leading edge of the India-Eurasia collision zone, is the key area to investigate the tectonic process relevant to the continental collision. The deep structure and process has long been the focus of international society of geo-sciences. The Gangdese is also an important metallogenic belt, where a large number of world-class deposits(eg. the 17-13Ma Qulong porphyry copper deposit) with an extremely rare intensity have been discovered in the region. It is a important region for the mineral deposit research. The knowledge about the deep structure is the cornerstone for the establishment and development of continental mineralization theory. Previous studies for the deep-seated structure beneath the Gangdese belt were not taking these tremendous metallogenic districts as target area. Although we have deployed tens of broadband seismic stations to investigate the deep structure around the Qulong-Jiama metallogenic districts in the region since 2011, these stations are still too sparse and not enough to unveil the tectonic process to a satisfatory degree. Therefore, some more deployments of the broadband stations are still indispensable. This application will take the key mineralization zone (Qulong-Jiama ore district) as the target region, deploy 50 new broadband seismic stations, synthesize the acquired data with those one obtained by previous studies in the region, then reveal the deep-seated structure by seismic tomography, receiver function imaging and other means. As a result, our research will provide insights into the deep tectonics, and the ore-forming process in the region.

冈底斯带位于印度与欧亚碰撞的前缘，是研究大陆碰撞过程的关键地区，其深部过程一直是国际地学关注的焦点，冈底斯带同时又是重要的成矿带，目前已经发现了以驱龙斑岩铜矿(17-13Ma)为代表的一大批世界级大矿床，其密集程度极为罕见，它也是矿床地质研究关注的焦点，深部过程研究是研究成矿作用过程、建立和发展大陆成矿理论的基石。 前人对冈底斯带深部过程探测均未以这些关键成矿区为中心研究对象，申请者2011年以来在冈底斯驱龙-甲玛成矿区布置了数十个天然地震台站，然而由于台站数量和观测时间的限制，台站覆盖程度还不够理想，还需要做进一步野外补充测量。 本次申请将以冈底斯带关键成矿地段（驱龙-甲玛成矿区）为研究对象，在深化前人资料的基础上，部署50个宽频带天然地震观测台站并与前期台站一起形成对研究区的二维覆盖，通过地震层析/接收函数成像等手段，揭示其深部过程，为大陆动力学和成矿作用研究提供深部构造背景资料。

冈底斯带位于印度与欧亚大陆碰撞的前缘，是研究大陆碰撞过程的关键地带，其深部结构和过程一直是国际地学关注的焦点。冈底斯带同时又是重要的成矿带，目前已发现13-17Ma与大陆碰撞过程有关的世界级大矿。为了深入研究和理解大陆碰撞与成矿过程，研究者通过收集、整理现有天然地震探测数据，并在穿过冈底斯成矿带的重要地段补充了数条天然地震探测剖面，获得了海量高品质宽频带天然地震探测数据，通过采用层析成像、接收函数成像和各向异性分析等方法对数据进行处理研究，获得了冈底斯碰撞带碰撞结构从地壳到上地幔的完整结构图像, 发现在驱龙-甲玛矿集区(大约91.5E)下方印度岩石圈地幔存在由西向东从底垫(underplating)到俯冲(subduction)的转换现象，在该转换带以西，印度岩石圈地幔以低缓的角度(大约7左右倾角)底垫到了拉萨地体的中部大约100km左右深度，印度下地壳厚度达25km左右，而在转换带以东，印度岩石圈地幔以较陡的角度(最大达18左右倾角)，在雅鲁藏布江缝合线以南大约60km处就已完全隐没到了软流圈地幔之中，其北侧存在软流圈地幔楔，在软流圈地幔楔(或软流圈窗口)的上方观察到了明显的莫霍面缺失和印度下地壳减薄(厚度仅10km左右)现象。我们结果表明印度克拉通型(亏损型)的岩石圈地幔在软流圈上涌和下地壳拆沉的作用下可以发生与洋壳类似的俯冲，颠覆了克拉通型岩石圈只能底垫不能俯冲的传统认识。我们结果显示，冈底斯下面的下地壳高速层可以一直追踪到喜马拉雅地区，是底垫的印度下地壳，不是前人所认为的与新生代成矿作用有关的新生下地壳，驱龙-甲玛矿集区的形成应该与印度岩石圈地幔回转(roll-back)、软流圈地幔物质上涌及底垫在冈底斯下面的印度下地壳的拆沉有关。本次研究获得的数据、资料与结果对深入揭示和理解印度与欧亚大陆碰撞与成矿的深部过程，发展大陆动力学与成矿理论具有极其重要的意义。