Transcrustal Magmatic Systems In Longgang Volcanic Field Revealed By 3D Magnetotelluric Results

The northeast China,centering on the Songliao Basin,is widely distributed with intraplate volcanoes formed in the late Cenozoic.The genesis and evolution of these intraplate volcanoes cannot be simply explained by the traditional plate tectonic theory or the mantle column hypothesis.Among the Cenozoic volcanoes located in northeast China,Longgang volcanic field(LVF)has the most number and density of volcanic cones,and it is also one of the most typical areas in China monogenetic volcanism occurred since the Quaternary.The existing petrological,geochemical and geophysical results suggest the LVF and the Changbaishan volcanic field(CBVF)have a common source in the mantle depth,but the Longgang volcanic area is a simple trachybasalt area.petrological evidence also shows the magma comes from the uncontaminated mantle-derived magma,and does not experience the crustal magma chamber stage.What different evolution mechanisms have undergone between the LVF and the CBVF which caused different states still lack support of geophysics.In order to study the distribution and source of magma,we carried out magnetotelluric(MT)observations and obtained an3 D MT network containing 41 stations within a coverage of 124.5?129E,41.5?43.5N.The phase tensor analysis shows the subsurface is strongly 3D,so 3D MT inversion method was used.The 3D resistivity result shows a low-resistivity body in the middle and lower crust between the Dunhua-Mishan Fault(DMF)and the Hunjiang Fault(HJF),and is distributed in a north-east strip,with the main body of the low-resistivity anomaly extending about 60 km from east to west,30 km from north to south,17 to 45 km in depth,and 5 to 40 ohm meter in resistivity.The anomaly gradually joins with other lowresistivity bodies around the DMF after 35 km to form a larger low-resistivity body.Combined with the results of volcanic rocks study,it is speculated that the lowresistivity anomaly in the LVF comes from a deep source,and it has a good corresponding relationship with the DMF.The DMF is a deep fault at the lithosphere scale.Therefore,it is speculated that the partial melting and other thermal substances from the mantle migrate from the mantle depth to the lower part of the volcanic field with the DMF as the channel,which constitutes the main source of volcanic substances and causes volcanic eruption in the Cenozoic.In addition,the northeast of volcanic area is also a seismic activity area,seismic activities are mainly controlled by the northeast part of HJF,our results show there is a low resistivity anomaly of 5 ? 20 ohmm at the depth of 10 km,and the anomaly is a branch of the main anomaly in the volcanic area.Therefore,it is considered the seismic activity in this area is an important evidence of magmatic activity.Compared with the results of earth density,velocity,temperature and leveling,it is found that there is a more active magmatic activity below the LVF.