A further discussion on geophysical characteristics of Mazhan magma pocket in Tengchong volcano-tectonic zone

The existence of magma pocket in the depth of Tengchong volcanic basin of Yunnan Province is a topic which has aroused much attention. Extensive studies have been conducted from many aspects but there still exist different opinions. It was previously inferred by the authors based on low-resistivity anomalies that, underneath the craters of Xiaokong Mountain, Dakong Mountain and Heikong Mountain, there is a low-resistivity magma pocket with a depth range of 13-30 km and sizes of 25 km (from east to west) by 30 km (from south to north), on the east of which is Dayingjiang Fault. Based on seismic reflection survey, gravity and aeromagnetic survey data, this paper further discusses other geophysical characteristics of the magma pocket. Because of volcanic eruption, not only local material depletion is present near the craters, but the magma pocket itself depletes magma material as well; after the filtering of local negative gravity anomaly of surficial volcano, the gravity upward continuation map shows a negative gravity anomaly corresponding to the low-resistivity body on the north of Dakong Mountain, which is closely related to the depletion of aqueous low-density material from magma pocket in molten state at high temperatures. The entire negative gravity anomaly corroborates the existence of low-resistivity magma pocket. The Tengchong Mazhan magma pocket has temperatures of 397-651℃, with a mean value of 527℃, so magnetic body is impossibly present at the main central place of the pocket. After processing of the aeromagnetic data to eliminate the influence of surficial volcanic rock, large strong magnetic anomaly was still found, close to low-resistivity body in size, and it is presumed that the magnetic anomaly is at the top of the magma pocket at a depth above 15 km; as magma pocket temperature decreases gradually, new magnetic mineral will be formed below 480℃; as a result, there appears relatively strong magnetic body at the top of the pocket with a similar range to that of the pocket. Only seismic reflection at shallow depths reveals clearly that the relatively continuous reflected wave interface is a reflection of the interface among shallow volcanic rock, aquifer and granite. The reflected wave velocity map exhibits a number of velocity zones relatively continuous in vertical direction, the reflection interface towards deep formation is very discontinuous, presumably because of multiple times of magma upwelling, variable eruption times and different magma components, so the inhomogeneity of magma pocket material gives a novel geophysical model, indicating that magma pocket components are uneven; although aqueous mineralized melt constitutes a generality of low resistivity, uneven temperature shows variation in components of magma at different periods; in the pathway of magma pocket going towards ground surface, the eastern granite is underlain by high-velocity low-temperature material; and between two large concealed granite plutons in the east and west of the Dayingjiang River fault zone deeply underneath Xiaokong Mountain, Dakong Mountain and Heikong Mountain, is the newest volcanic eruption pathway, which is a low-velocity pathway.