摘要
岩石圈结构和深部过程对理解成矿带和大型矿集区的形成十分重要。岩石圈尺度的地球动力学过程将在地壳中留下各种结构的或物质的"痕迹",这些"痕迹"可以通过地球物理的手段去探测。为深入理解长江中下游成矿带形成的深部动力学过程,作者在国家深部探测专项(SinoProbe)和国家自然科学基金重点项目支持下,在长江中下游成矿带开展了综合地球物理探测。方法包括宽频地震、深地震反射、广角反射/折射和大地电磁测深。数据处理和反演结果取得一系列新发现:(1)成矿带上地幔顶部存在低速体,在中心深度300km处有一向SW倾斜的高速体;(2)S波接收函数证实成矿带岩石圈较薄,只有50~70km;横波分裂结果显示,成矿带上地幔各向异性方向和强度与邻区有较大区别,显示平行成矿带(NE-SW向)的上地幔变形和流动;(3)深反射地震揭示成矿带上地壳曾发生强烈挤压变形,以紧闭褶皱、逆冲和推覆为特征;在宁芜火山岩盆地、长江断裂带和郯庐断裂之下出现"鳄鱼嘴"构造,指示上下地壳在挤压变形过程中解耦;深反射地震证实发生过陆内俯冲和叠瓦,并认为是岩石圈增厚和拆沉的主导机制;(4)广角反射和大地电磁反演给出了跨成矿带地壳剖面的速度和电性结构,速度和电阻率分布总体上与构造单元相吻合。本文分析和解释了这些发现的地质意义,并结合近年在长江中下游地区的地球化学研究进展,提出了成矿带地球动力学模型。该模型认为:中、晚侏罗世陆内俯冲、岩石圈拆沉、幔源岩浆底侵和MASH过程造就了长江中下游世界级成矿带的形成。
The lithosphere structure and deep processes are keys to understanding the formation of ore-district and metallogenic province. Lithosphere-scale process could create big footprints or signatures which can be observed by geophysics methods. To better understand the deep dynamic of the major metallogenic province,the authors has conducted an integrated deep exploration across Middle and Lower Reaches of Yangtze Metallogenic Belt( YMB) in eastern China,these included broadband seismic,deep reflection seismic,wide-angle reflection seismic and magnetotelluric sounding. The findings based on the data processing and inversion of these integrated geophysical exploration include:( 1) Lower velocity body at the top of upper mantle and a SW dipping high velocity body with center depth at 300km,were imaged by teleseismic tomography beneath YMB;( 2) S-wave receive function evidence the thinning of the lithosphere beneath the YMB to 50 ~ 70km; and SKS,SKKS shear wave splitting results show NE parallel fast-wave polarization direction which parallel with the regional tectonic lineament;( 3) The reflection seismic data reveals strong NW-SE upper crustal contraction deformation,characterized as tight fold,thrust fault and nappe. The"crocodile"reflection structures are found beneath the Ningwu volcanic basin,Changjiang deep fault and Tan-Lu fault,indicating the decoupled deformation process of the upper and lower crust. Overall,the seismic data show evidence for an introcontinental orogen with lower crust and upper mantle subduction and imbrication;( 4) Refraction seismic and MT data provide velocity and resistivity distribution across the YMB,and show a general agreement with the tectonic units. Based on the results of the integrated geophysical exploration, combining with the recent geochemistry results,a geodynamic model is proposed for the YMB. The model suggests that Middle and Later Jurassic introcontinental subduction,lithosphere delamination,mantle-derived magmatic underplating and MASH process are responsible for the formation of this world-class metallogenic belt.