华南陆块东北部构造特征、成矿作用与动力学机制——来自大地电磁测深的认识

Structural characteristics,mineralization and dynamic mechanism of the northeast South China Block: Understanding from magnetotelluric data

通常认为,华南陆块在新元古代由扬子克拉通和华夏地块沿江南造山带碰撞拼合而成,之后经历了陆内造山、洋壳俯冲等多期岩浆—构造活动。但因华南陆块所处的特殊地质构造环境,就目前华南陆块各块体之间的接触关系、江南造山带深部构造特征及区域动力学意义等诸多地质问题争议颇多。本文依托地质调查项目和“深部探测技术与实验研究(SinoProbe)”项目完成的8d和12g两条大地电磁测深剖面,经过精细的数据处理,使用非线性共轭梯度(NLCG)算法对TE+TM模式数据联合反演得到华南陆块东北部岩石圈尺度的二维电性剖面,并用ModEM三维反演代码对全阻抗张量数据反演获得了三维电性模型。对研究区内扬子地块东部、江南造山带以及华夏地块进行电性结构研究,发现研究区内的江南造山带西南段存在扬子地块和华夏地块碰撞镶嵌的构造表现,扬子地块已越过江南造山带,在江山—绍兴断裂位置与华夏地块挤压,形成江绍断裂等逆冲型深大断裂,从电性结构推测其现今仍然为活动断裂,但东北段块体之间的接触关系被上侵的地幔物质破坏,江绍、赣东北等断裂的深部结构已被剧烈改造,推测这种深部成矿热物质上涌是形成赣东北以金银矿种为主的岩浆热液型矿床的深部动力原因;研究区东部华夏地块电性特征为高阻的上地壳以及被岩浆底侵而破坏的中下地壳,发育其中的屯溪—鹰潭—安远和上虞—大浦—政和断裂切割深度超过了50 km,为深大断裂构造。结合前人地表侵入岩填图结果,认为由于中生代以来古太平洋板块向欧亚大陆的俯冲,华南陆块东部上地壳被严重破坏,从电性特征推断可能发生过大面积的板片重融,华夏地块东北部地壳相较于西南部厚度明显更小、后期改造严重,受到的太平洋板块俯冲导致的热扰动更剧烈。

Objectives: It is generally believed that the South China Block was formed by the collision of the Yangtze Craton and the Cathaysia Block along the Jiangnan Orogenic Belt in the Proterozoic,and then experienced multiple periods of complex magmatic—tectonic geological activities such as intracontinental orogeny and oceanic subduction. However,the geological features of the South China Block have not been well-explored so far due to the complex structurally geological environment,especially the spatial contact relationships of multiple blocks,deep mineralization characteristics,and regional geological dynamics.Methods: In our work,we performed 2D and 3D magnetotelluric inversions using the two MT profiles( i.e.,8d and 12g) collected from the programs of the crustal electrical structure of the Jiangnan Orogenic Belt and its adjacent areas,and SinoProbe project. Through the fine analysis and processing of the magnetotelluric sounding data,the lithospheric-scale 2-D MT profiles of the vicinity of the northeastern South China block were inverted using the TE-and TM-mode data with NonLinear Conjugate Gradient( NLCG) algorithm integrated in the WinGLink software. Besides,a 3-D electrical model of the study area is obtained from the inversion result of full impedance tensor data using the ModEM inversion program,which is also based on the NLCG algorithm.Results: According to the electrical structures of the eastern Yangtze Block,the northeastern section of the Jiangnan Orogenic Belt and the Cathaysia Block in the study area,we noticed the mosaic structure caused by the collision of the Yangtze Block and the Cathaysia Block in the southwest section of the Jiangnan Orogenic Belt. The Yangtze Block crossed the Jiangnan Orogenic Belt and squeezed the Cathaysia Block at the location of the Jiangshao fault,which results in deep thrusting faults( e.g.,Jiangshao fault). Although Jiangshao fault is an active fault,the contact relationship of the blocks was renewed by the invading mantle material in the northeast. The deep structures of faults,such as the Jiangshao and the Northeast Jiangxi,have been drastically modified. We propose that the upwelling of deep thermal materials is the primary reason for the deep dynamics of magmatic hydrothermal deposits,like gold and silver. According to the electrical characteristics,the Cathaysia Block is characterized by the highresistivity upper crust and magma intruded middle and lower crust. The Tunxi—Yingtan—Anyuan and the Shangyu—Dapu—Zhenghe faults in the Cathaysia Block are deep and large faults,extended a depth of more than50 km.Conclusions: Due to the subduction and retreat of the Paleo-Pacific Plate since the Mesozoic,the upper crust of the eastern South China Block was seriously damaged according to the result of surface intrusive rock mapping.Our resistivity models illustrate the existence of slab remelting in a large area. Also,the northeastern crust of the research area is thinned compared with the southwestern one,indicating it was modified severely by thermal disturbance caused by the subduction of the Paleo-Pacific Plate and post-modification.