Magnetotelluric evidence for crustal decoupling: Insights into tectonic controls on the magmatic mineral system in the Nanling–Xuancheng area, SE China

Abstract

Abstract Despite extensive efforts to understand the proliferation of metallogenic deposits in the Middle to Lower Reaches of the Yangtze River (MLYR) important knowledge gaps remain. Deep seismic reflection and geochemical studies suggest large-scale lithospheric thinning and basaltic magma underplating associated with Mesozoic magmatism and ore-formation, but the magmatic mineral system that caused the mineralization and its relationship to the regional tectonic evolution remains unclear. Three geodynamic models, i.e., collision deformation, subduction deformation, and tectonic regime conversion, have been used to explain the relationship between mineralization and underplating. However, none of these models can reconcile all the regional geological and geophysical data. For the first time, we present a 3D geo-electrical model, created by 3D inversion of magnetotelluric data, which reveals the electrical structure beneath the Nanling-Xuancheng ore district in the MLYR; an emerging new ore district in the MLYR metallogenic belt. Conductivity variations have distinctly different orientations in the upper and lower crust. The trend is northeast-southwest in the upper part of the crust and east–west in the deeper crust. This is interpreted as evidence for decoupling between the upper-to-mid and mid-to-lower crust. Two major conductive zones revealed in the crust are used to interpret the relationship between magmatism and deep underplating. According to the analysis of rheology parameters calculated from the electrical model, we propose a new mechanism, referred to as “crustal decoupling” to explain the tectonic-magmatic history of the Nanling – Xuancheng area. The upwelling fluids created a weak zone at ductile–brittle transition, and provided the conditions for crustal decoupling caused by the NW directed compression of Paleo-Pacific plate. Fluids migrated through NE-SW directed channels and interacted with the upper crustal rocks. The Cu-Au mineralization related to cryptoexplosive breccia developed in a subsequent process involving rapid release of the fluids due to a change in pressure. This process can be attributed to the extensional decoupling of the crust in changing the stress regime.