西北天山－西准噶尔陆下岩石圈地幔属性及其对斑岩型铜金矿床成矿的动力学意义

Cu-Au porphyry ore deposits are typically formed in oceanic or continental arcs above subduction zones and in back-arc extension setting. A recently proposed model suggests that a fertile or refertilized subcontinental lithospheric mantle (SCLM), trans-lithospheric faults, and multi-phase triggered tecto-thermal events are three key factors for formation of giant Cu-Au porphyry ore deposits. Therefore, major challenge rises in developing techniques to unravel time evolution and spatial variation of SCLM in trans-lithospheric structural corridors, including to determine the present lithospheric architecture and composition, to reconstruct its ancient tectonic setting, and to trace its deformation history since the formation of porphyry ore deposit. We propose to use northwestern Tianshan and western Junggar in the Central Asian Orogeny Belt as a field laboratory for its well-known superior Cu-Au accumulation in the Late Paleozoic. We will use local and teleseismic earthquake body-wave travel-time tomography, earthquake and noise-based surface waves tomography, P- and S-wave receiver function imaging, and magnetotelluric three-dimensional isotropic inversion and anisotropic modeling, to determine lithospheric architecture and physical properties. We will use the obtained geometry of lithospheric mantle and anisotropies of seismic velocity and conductivity to reveal possible frozen-in lithospheric deformations, distribution of predominant weak and conductive features such as trans-lithospheric shear-zones and magmas. The Late Paleozoic geological evolution of SCLM will be reconstructed by comprehensive analysis of mantle xenoliths, including lithofacies inspection, geochronological dating, and geochemical tracing techniques. Based on these data, we will build a regional model consisting of several key snapshots of SCLM to describe its geodynamic process in relation to the formation of the porphyry Cu-Au ore deposits in the area.

饱满或重新饱满化的岩石圈地幔、超岩石圈断层、构造-热事件触发多期次岩浆上升被认为是形成Cu-Au超常富集的三个关键因素，洋内弧或陆缘火山弧、浅俯冲带内及弧后伸展环境是斑岩型Cu-Au矿床最典型成矿区域。因此，如何在超岩石圈断裂构造走廊带探测岩石圈地幔的结构、构造及物质组分并追踪其变形历史，成为寻找斑岩型Cu-Au巨矿床面临的主要技术挑战。鉴于中亚造山带在晚古生代的Cu-Au超常富集，选择其西南缘的西北天山-西准噶尔地区作为野外实验室，以地震体波走时层析成像、面波频散层析成像、P和S波接收函数成像及大地电磁三维反演等方法构建岩石圈结构、构造和物性模型；采用多种方法提取波速和电导率各向异性及岩石圈地幔厚度的三维变化，追溯岩石圈地幔的变形历史；结合地幔岩捕虏体的岩相学和年代学研究及岩石地球化学示踪结果，重构该区晚古生代岩石圈地幔的环境演化及其与斑岩型Cu-Au矿床形成相关联的动力学模型。

研究区定型于晚古生代的古亚洲洋闭合阶段，是我国重要的金属成矿带。本项目采用先进的地球物理成像技术和岩石探针方法，联合解析该区岩石圈地幔属性，力图为解决已有争议提供新的证据。项目主要研究内容包括：建立岛弧带岩石圈地幔属性与Cu-Au成矿省之间的关系；研究探测岩石圈地幔的地球物理方法技术和岩石探针方法；岩石圈地幔组分和状态的数据融合分析。取得的主要成果如下：.1.确认西准噶尔晚石炭世大地构造背景是古亚洲洋俯冲背景下的洋内弧。根据Vs速度确定了达尔布特带地壳主体为大洋地壳残片(Wu et al. 2018)；三维电阻率模型揭示西准噶尔岩石圈地幔内存在横向延伸超过300 km的巨型地幔高导体 (Xu et al.2020)，推测由含水的俯冲大洋岩石圈经后期岩浆/流体交代作用和变质作用改造而形成。根据角闪辉石岩中锆石的U-Pb定年结果，该区残余洋盆持续到了晚石炭世，比前人结果晚~20 Ma(苏炳秀等,2019)。.2.岩石圈地幔结构和属性研究结果揭示北天山西部和伊犁盆地下方的岩石圈地幔与西准噶尔同源。根据100km深度以下的巨型地幔高导体横穿西北天山直达那拉提山(Xu et al.2020)、且准噶尔盆地西部和伊犁盆地下伏均为饱满型岩石圈地幔(Zhang et al. 2019)的事实，揭示北天山西部和伊犁盆地下方的岩石圈地幔与西准噶尔同源，而且浅部强烈变形与深部解耦。.3.西准噶尔北部大地电磁各向异性三维模拟为洋脊俯冲提供了新的地球物理证据(Liu et al. 2019)。该区域上地壳最小电阻率的近东西指向，代表了310-321 Ma之间洋脊俯冲时洋脊的走向；而下地壳最小电阻率NE20°的指向，代表了俯冲夭折时(~300 Ma) 继承自古亚洲洋板块的固有各向异性。该成果显示了电阻率各向异性信息的重要科学价值。.4.通过解剖包谷图中型斑岩型铜金矿床和哈图大型低温热液金矿床的深部地球物理结构，揭示了弧上深部地壳中金属预富集的主要控制因素。该研究将岩浆/流体作用、金属元素的来源及成矿的动力学环境结合起来，论证了地球物理结构在揭示成矿的深部过程中的重要价值。