The magma plumbing system of the potentially hazardous Laoguipo volcano in the Tengchong Volcanic Field, SW China

Abstract

The Late Pleistocene-Holocene Laoguipo volcano in the Tengchong Volcanic Field (TVF), southwestern China, displays significant geochemical and geophysical anomalies characteristics. Here we present petrographic observations, mineral chemistry, bulk rock geochemistry, thermobarometry, and thermodynamic simulation to evaluate the crystallization conditions and pre-eruptive magmatic processes occurring within the magma plumbing system. This study reveals the existence of two magma reservoirs beneath the Laoguipo volcano. The deep magma reservoir is composed of basaltic trachyandesite (SiO₂ = 54–57 wt%), which is located at 15–19 km depths with 1087–1160 °C, 1.5–2 wt% H₂O content, oxygen fugacity of ΔNNO+1 (Ni-NiO buffer), melt viscosity of 10^1.7–10^2.6 Pa·s, and density of 2.5–2.6 g/cm³. The formation of the deep magma reservoir is attributed to the 31% mass fractional crystallization of primitive basalt in the TVF. The shallow magma reservoir is composed of trachyte (SiO₂ = 63–64 wt%), which is located at 6–11 km depths with 780–825 °C, 5.9–6.5 wt% H₂O content, oxygen fugacity of ΔNNO+1 (Ni–NiO buffer), melt viscosity of 10^3.9–10^4.8 Pa·s, and density of 2.2–2.3 g/cm³. The shallow magma reservoir formed after the basaltic trachyandesite had assimilated 19% mass of the upper crustal material and fractionated 41% mass of the crystals. This study suggests that the shallow trachyte magma reservoir is being heated by the ascending deep basaltic trachyandesite magma, resulting in crystal dissolution, remobilization of crystal mush, and magma convection, which may be the main factors responsible for the geochemical and geophysical anomalies characteristics. The Laoguipo volcano is forming a mature magma plumbing system, which is of significance for forecasting future volcanic eruptions.