Mantle origin of the Emeishan large igneous province (South China) from the analysis of residual gravity anomalies

摘要

Despite numerous geologic and geochemical studies conducted on the Emeishan Large Igneous Province (UP) in SW China, the deep origin of this LIP is still poorly constrained. Here we investigate the residual gravity anomaly in South China, and its relationship to the Emeishan LIP, in conjunction with deep seismic sounding profiles, deep seismic reflection surveys and a variety of broadband seismic observations performed in South China during the past few decades. Our analysis includes the removal of related gravitational effects due to: (1) the sediments, (2) the crystalline basement, undulations of (3) the upper crust, (4) the Moho and (5) the mantle lid. The resultant residual gravity anomaly in the Emeishan LIP and surrounding region reaches a maximum value of +150 mGal and decreases gradually with distance from this inner zone. With the conjugate gradient method, we develop a lithospheric model consisting of a cylindrical-shaped positive density anomaly that provides a good fit to the observed residual gravity anomaly. The inverted density anomaly of the Emeishan LIP is + 0.06 g/cm~3 in the inner zone and decreases to about + 0.03 g/cm~3 in the outer zone. The observed positive residual gravity and the corresponding high density can be attributed to mafk/ultramafic rocks and cooled surrounding rocks generated by large scale magmatic intrusion. Hence, taking account of the Permian Emeishan LIP, our residual gravity and density model provide evidence for the formation by an upwelling of a mantle plume.