Deep Structure Of Magmatic Systems Constrained By Regional Magnetic Anomalies

Large-scale tectono-magmatic events have significant influence on the evolution of the earth system.They not only change the composition and physical properties of the crust,but also control the formation and distribution of ore deposits,and cause great impacts on the environment and the species on the earth.Regional magnetic anomalies are generally related to strongly magnetized magmatic intrusions,reflecting a synthesized effect of all the magnetic rocks above the Curie depth in the lithosphere,but there are few cases applying magnetic anomalies quantitatively to regional tectonics in China.In this study,we selected three regional magnetic anomalies to detect the deep structure of the corresponding magmatic systems.The three regional magnetic anomalies are in different tectonic settings and have distinct magnetic anomaly characteristics,which are located in the Yarlung-Zangbo suture zone,Emeishan large igneous province and Sichuan Basin.First,we took upward continuation and reduction to the pole of the magnetic anomalies to analyze their source characteristics;then,appropriate inversion method was chosen to quantitatively study the nature and structure of the magnetic sources;finally,we discussed the tectonic implications of the magnetic anomalies compared to other related geodata,and explored the deep structure of magmatic systems revealed by the magnetic sources.There are two subparallel E-W trending linear aeromagnetic anomalies along the central Yarlung-Zangbo suture zone,southern Tibet.Yarlung-Zangbo suture zone represents the boundary of Asian and Indian plate,recording the subduction of the NeoTethyan oceanic slab and the subsequent continental collision.The two magnetic lineaments are spatially associated Gangdese batholith and Xigaze ophiolites,separated by Xigaze fore-arc basin filled with non-magnetic sedimentary sequences.By analysis of our inverted results and further magnetic modelling,we propose that Gangdese batholith has a magmatic root with a higher magnetization than the widespread exposed granitoids,indicating the deeper part of Gangdese batholith may contain more mafic constituents,which might be related to the multi-stage magma mixing/mingling in Gangdese magmatic belt.The Emeishan large igneous province is located in the southwest of China.At ~260 Ma in the late Permian period,it erupted massive continental flood basalts.The magnetic anomalies in this area consist of a series of discrete ones with obvious remanence.These underground discrete magnetic sources may indicate the maficultramafic intrusions beneath the Emeishan large igneous province.These intrusions might be formed during underplating or intraplating in the period of Emeishan large igneous province,which might be magma chambers or magma transport channels filled with mafic magma.After cooling,these mafic-ultramafic intrusions would have produced magnetic anomalies observed now.The Sichuan Basin magnetic anomaly is the most striking regional magnetic anomaly in South China.It is characterized by a continuous large-scale magnetic anomaly and contains long-wavelength constituents.Its spatial distribution is related to the range of the rigid basement beneath Sichuan Basin,reflecting the magnetization difference of its Precambrian basement.By analysis of our magnetization model with other geodata,we suggest that the Sichuan Basin magnetic anomaly may be related to a Neoproterozoic tectono-magmatic event in craton scale,with massive mantle-derived magma underplating around Moho and intruding into the crust,forming maficultramafic intrusions in the middle-lower crust and generating the magnetic anomaly after cooling.This large-scale tectono-magmatic event may strengthen the basement of Sichuan Basin,and make Sichuan Basin become a stable cratonic basin since then.Based on the above case studies,most regional magnetic anomalies in different tectonic settings are related to the mafic-ultramafic magmatic intrusions,indicating that mafic magmas played an important role in large-scale tectonic-magmatic events in the earth history and had great influence on the evolution of the earth's crust.