古地幔柱作用“遗迹”的深部地球物理探测——以峨眉山大火成岩省为例

Geophysical Signature of the Ancient Mantle Plume Activities:A Case Study of the Emeishan Large Igneous Province

大火成岩省成因是国际地学界的研究热点,涉及地球内部运行机制和过程、资源和生物环境效应等多个地学前沿研究领域。峨眉山大火成岩省不仅是中国境内第一个获得国际学术界广泛认可的大火成岩省,也是全球范围内研究程度最高的大陆溢流玄武岩省之一。前人基于沉积地层学、岩石地球化学等系列证据提出了该大火成岩省的"地幔柱头熔融"成因模型。深部是否存在"地幔柱头熔融"假说所预示的大规模岩浆作用"遗迹",有待地球物理探测的进一步检验。地球物理方法是探测现代地幔柱的常用手段。本项研究集成利用人工源地震、天然源宽频带地震探测,以及密集重力/地磁剖面测量等综合地球物理手段探测"古地幔柱"作用,在峨眉山大火成岩省研究中取得了系列成果,其研究思路与方法对其他古地幔柱作用"遗迹"的识别具有重要的借鉴意义。通过几何结构-物性结构-动力学属性参数之间的联合约束,以及与地球化学、地质学研究成果的有机结合,可靠地揭示出与古地幔柱作用有关的大规模岩浆活动导致的地壳结构和组分变化以及深浅响应过程等,显示了综合地球物理方法、不同学科交叉融合在重建古老重大地质事件深部过程方面的巨大潜力。大规模岩浆作用"底侵"位置的确定、"透明"上地壳特征的发现等,对于该区未来的深部找矿具有重要的指导意义。

Formation of Large Igneous Provinces（ LIPs） is one of the global multidisciplinary research fronts in geosciences,which involves the working mechanism and process of Earth＇s interior,and its effects on resources and environment through geologic time. The Emeishan LIP is not only the first LIP widely recognized by the international geosciences community in China,but also one of the most intensely studied LIPs in the world. Based on the sedimentary and geochemistry evidences,a model of ＂melting of the mantle plume head＂was proposed to explain the formation of the Emeishan LIP. It will give this hypothesis a critical verification whether there exists the compositional and structural records related to plume-lithosphere interaction within the lithosphere of the LIP. In this study,we used comprehensive geophysical investi-gations,including active deep seismic soundings,passive broadband seismic arrays,and dense gravity/geomagnetism measurements,to identify the signature of ancient mantle plume activities. The structural geometry,physical properties,and dynamic characteristics recorded in the crust and lithospheric mantle from geophysical investigations compose a joint constraint of ancient plume activities. The crustal properties collectively suggest that a mafic layer of 15 ~ 20 km thickness and 150 ~ 180 km width exists at the base of the crust of the Inner Zone. Such a mafic layer reflects a vertical crustal growth through underplating of large-scale magmatism which was originated at least from the deep mantle and induced kilometer-scale domal uplift at the surface. This study provides a successful example to identify the geophysical signature of ancient mantle plume activities. The location of crustal magmatic underplating,and discovery of the overriding transparent upper crust modified by the subsequent magmatic intraplating could provide useful insights into the deep mineral exploration in this area.