Characteristics of Multi-Scale Gravity Field and Deep Structures in Nanling Metallogenic Belt

In this paper, the multi-scale wavelet decomposition method was applied to separate the Bouguer gravity anomalies of Nanling region to obtain the local anomalies of granites at different depth scales. The granite distribution is delineated at 5 km, 15 km and 25 km respectively by 3D density mapping technique. We discuss some hot issues of Nanling granitic intrusions, including their geological and geophysical model, induced heat source and genetic model, by integrating and analyzing the seismic imaging and magnetotelluric data. The results show that the gravity anomalies and granitic intrusions have apparent partition features marked by Chaling-Chenzhou fault. The local gravity anomalies in southeast region with lower amplitude indicate shallow emplacement depth of granitic intrusions, and the magma intrude into the near-surface along the small channel to form rock cover, whereas the local gravity anomalies in northwest region with higher amplitude suggest deeper emplacement depth. Most of granites' emplacement depths are no more than 25 km. The large-scale low-density and low-speed region in Zhuguangshan and Maoerling area exits near the depth range of 15-25 km. The low-speed and low-resistance region also appears at the top of mantle in Zhuguangshan area, suggesting the upwelling channel of high-temperature asthenosphere lava. Therefore, the basaltic magma underplating may cause the protolith partial melting in middle-lower crust, which also provides the heat and material sources for the large-scale mineralization in Nanling region.