科研之友
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摘要
The Middle and Lower Reach of Yangtze River(MLYR) metallogenic belt is a juncture among the Dabie orogen, the MLYR depression and the Jiangnan uplift, and an important Cu-Fe-Mo-Au polymetallic metallogenic belt in eastern China. The study of the lithospheric structure in this region is important to reconstruct the geodynamic processes controlling metallogenesis and understand the genetic mechanism of the metallogenic belt. With advantages of high lateral resolution and deep investigation depth, magnetotelluric sounding can provide electrical constraints for layered structure of continental lithosphere and location of tectonic boundaries as well as properties of continental lithospheric mantle. Broad-band magnetotelluric (BBMT) data at a total of 150 sites were acquired along two approximately northwest-southeast trending 300-km-long profiles across the middle corridor of the MLYR metallogenic belt. Modern processing techniques were applied to these data to ensure that the accurate and realistic MT response curves were produced to the longest period possible for each site. These techniques included robust estimate, processing using robust remote referencing codes for the BBMT data, and analysis of geoelectric strike direction as well as dimensionality by phase tensor decomposition. The electrical structure down-to 50 km depths was finally imaged by two-dimensional TE +/- TM mode continuum medium inversion. The deep conductivity structure revealed mainly includes the crust-mantle transition, fault zones which separate tectonic units, as well as conductibility of the mantle. (1) The electric crust-mantle boundary is at the depth of 30 km, separating the conductive lower crustal granulite above from the more resistive underlying lithospheric mantle, except the conductive mantle beneath the Yangtze River. Another boundary at the depth of 10 km separates the resistive upper crustal granitoids from the lower crust. (2) The lateral electrical gradients at the Tan-Lu fault zone and Jiangnan fault zone separate the resistive crust of Dabie orogen, the relatively conductive crust of MLYR depression and the resistive crust of Jiangnan uplift, respectively. (3) The mantle conductivity model shows three kinds of mantle with a conductive mantle beneath the Yangtze River in the middle of the profile, and both resistive mantles in the north-western and southeastern, respectively. It can be inferred from the present stable electric crust-mantle boundary that MLYR metallogenic belt experienced a lithospheric re-balance (an adjustment of lithosphere) after the Yanshanian intracontinental tectono-magmatic and metallogenic processes. The distinct electrical gradients at the boundaries of deep fault zones (e.g. Tan-Lu fault zone, concealed Yangtze River fault zone and Jiangnan fault zone) imply their extensional property in post-Indosinian and playing a key role of controlling the Yanshanian tectonic-magmatic activity. The high conductive mantle's localized existence reflects mantle beneath different geotectonic units that experienced different kinds of transformation(deformation and alteration) during the geodynamic process and formed the geological polarity at depth.
- 出版日期2015-12
- 单位中南大学; 中国科学院地质与地球物理研究所兰州油气资源研究中心; 中国地质科学院矿产资源研究所; 中国科学院地质与地球物理研究所
