长江中下游深部构造及其中生代成矿动力学模式

长江中下游地区是中国重要的成矿带之一。本文利用地震、大地电磁数据以及野外地质调查,并结合前人研究的地球物理和岩石地球化学资料,明确了长江中下游地区现今深部构造,系统分析了其成矿动力学演化机制。本区发育有三大断裂体系:大别-苏鲁前陆断裂系、江南-雪峰断裂系和中国东部NE-NNE向走滑断裂系。大别-苏鲁前陆断裂系为一自北向南的叠瓦式逆冲推覆构造,而江南-雪峰断裂系为一自南向北的多级逆冲推滑构造,它们沿来安-望江-阳新-天门一线形成强烈的挤压对冲构造样式。中国东部NE-NNE向走滑断裂系早期主要表现为左行平移走滑并侧向挤压,参与了对冲构造形成过程,只是部分切割其它两个逆冲体系。这三大断裂体系均经历了印支-燕山期穿时递进的构造变形。152~135Ma,古太平洋板块向欧亚大陆俯冲时,板片可能沿着转换断层撕裂并产生底侵体。下地壳在底侵体的烘烤作用下熔融并受到混染,其岩浆在多级逆冲推覆和滑脱构造背景下充分结晶分异形成低镁埃达克岩,于断隆或隆坳过渡带生成铜矿。135~127Ma,长江中下游成矿带深部地幔开始上隆,诱发加厚岩石圈沿着郯庐断裂带局部拆沉,并引发富集地幔上升流。其与残留地壳交代反应,在郯庐断裂带两侧形成高镁埃达克岩。古太平洋板块继续向南西俯冲并发生逆时针旋转,长江中下游地区大多数NNE向断裂已转变为右行走滑,形成右行右阶的走滑拉分盆地。上隆地幔的基性岩浆沿着深切地壳的走滑断裂上升到盆地中,快速冷却形成橄榄玄粗岩岩系,从而在接触带或潜火山岩体顶部分异产生铁矿。

Deep-seated structural styles and Mesozoic metallogenic dynamic model in the Middle-Lower Yangtze Region, China

The Middle-Lower Yangtze region is one of the important metallogenic belts in China. Based on a series of MT and seismic interpretation and field geological survey, combined with previous geophysical and geochemical data, the deep-seated structures in the Middle-Lower Yangtze River area are analyzed, and its metallogenic dynamics are systematically discussed in this paper. In this area, three fault systems well developed, including the Dabei-Sulu Fault System, the Jiangnan-Xuefeng Fault System and the NE-NNE-trending Strike-slip Fault System in East China. The Dabei-Sulu Fault System underwent the south-directed imbricate thrusting, and the Jiangnan-Xuefeng Fault System underwent the north-directed multilayer nappe-detachment. Along the Laian-Wangjiang-Yangxin-Tianmen, a strongly opposite thrust styles developed. The NE-NNE trending Strike-slip Fault System in East China underwent left-lateral strike-slip shearing and transpression, being involved in the opposite thrusting. Some strike-slip faults offset the thrust faults. All of the three fault systems experienced diachronous and progressive propagation during the Indosinian-Yanshanian movements. During 152~135Ma, the paleo-Pacific Plate subducted under the Eurasian Plate and the subducted slabs were torn along the transform fault owing to subduction angle variations, which triggered the melting and magma underplating of the subducting lithosphere. The lower crust was partially melted as well by the baking and contaminated some mantle materials. The contaminated magma experienced a full crystal fractionation in the multilayer nappe-detachment structures, forming the low-Mg adakites. Meanwhile, Cu deposits well developed at the fault-uplift or uplift-depression transitional zones. During 135~127Ma, with the paleo-Pacific Plate changed to SW directed subduction and ongoing underplating, the upper mantle under the Middle-Lower Yangtze metallogenic belt started to uplift, triggering the delamination of the thickened lithosphere and the upwelling of the enriched mantle along the Tan-Lu Fault Zone. The mantle metasomatism proceeded with residual crust and the high-Mg adakites were generated on both sides of the Tan-Lu Fault System. At this point, many NNE-trending faults had switched into the right-lateral strike-slip faults, creating the pull-apart basins with right-stepping alignment. Mafic magma from the upwelling mantle erupted into basins along the strike-slip faults which deeply cut the crust, and, then the magma rapidly cooled and crystallized as shoshonites. The Fe deposits developed at the contact zone or the top of the subvolcanic bodies.