长江中下游成矿带岩浆-成矿系统深部背景和过程的电性结构约束

Electrical constraints on the deep setting and process of magma-mineral system in the Middle-Lower Reaches of Yangtze Metallogenic Belt

长江中下游成矿带是我国重要的铁铜金属“仓库”,然而,关于矿体密集分布成因的深部背景研究仍存较多争议,前人相继提出了碰撞变形、俯冲变形、构造机制转换等多种解释模型。本文基于成矿系统概念和前人对成矿系统各组成要素的研究成果,分析长江中下游成矿带内宁芜、庐枞、南陵-宣城和安庆-贵池矿集区的深部电性结构,并发现成矿带地壳深部普遍存在的相对统一的高导结构。探测结果显示:(1)长江中下游成矿带下地壳高导层指示着曾经发生的部分熔融以及壳幔物质混染的空间;(2)成矿带的浅部垂向高导异常是控矿通道的反映,并与深部高导层相连。电性结构具有一定程度的同源性,是研究区铁铜成矿系统的共性特征之一。基于电性结构和以往地球物理、地球化学研究成果,本文提出下地壳与上地幔混染的熔融物质不仅为区域广泛分布的中酸性岩浆岩提供了岩浆热液,同时“扮演”着铁铜成矿系统中的深部金属物源要素的角色,而与源区相连的通道在应力转换过程中控制着浅部成岩-成矿作用。基于通道宽度估算的应力体制转换过程的发育时间约为10Myr,期间控制着铜铁成矿系统的两个主要阶段。对于铜成矿系统,应力转换初期含铜热液迁移路径较长,混染了更多的地壳物质;而后期的铁成矿系统中含铁热液的上升通道较宽,二次壳幔混染较弱。

The Middle-Lower Reaches of Yangtze Metallogenic Belt is an important“warehouse”of iron and copper metals in China.However,there are still disputes over the deep setting of genesis of ore bodies and its dense distribution,including collision deformation,subduction deformation and tectonic mechanism transformation.Considering about the previous points of the mineral system,we analyze the deep electrical structure of Ningwu,Luzong,Nanling-Xuancheng and Anqing-Guichi ore districts in the study region,and find a widespread conductive system in the crust.From the observations,two associated structures can be inferred with magma-mineral system:(1)The large-scale conductive layer in the lower crust indicates the space of partial melting of crustal material and underplating material mixing from the upper mantle;(2)The vertically conductive belts represent the ore controlling tunnels connected with the deep conductive layer.The structures indicate a homologous electrical feature for the iron-copper mineral system in our study area.Based on the geo-electrical models and the previous geophysical and geochemical study results,we suggest that the mixed material of the lower crust and upper mantle not only provided magmatic hydrothermal solutions for the widely distributed medium-acid magmatic rocks in the region,but also served as the material source of metals for this mineral system.Moreover,the tunnels controlled the diagenesis and mineralization in the upper crust,following the regional stress conversion.We then estimated the development time of these tunnels,and suggest the transformation of regional tectonic system has experienced ca.10Myr,including two stages.In the early stage,the migration path of copper bearing hydrothermal solution was long and narrow,leading to mixing with more crustal materials.In the late stage,the migration channel of iron bearing hydrothermal solution was wider,lacking of secondary crust-mantle interaction.