Partial melting in the upper crust in southern Tibet: Evidence from active geothermal fluid system

During the process of carrying out a series of geophysical probes, a suite of seismic bright spots were observed along the Yaclong-Gulou rift valley zone. Concerning the nature of these bright spots, two interpretations have been advanced: 1) saliferous super-critical fluid and 2) magma bodies on the top of partially melted layer in the upper crust. Due to lack of direct means and methods of investigation, their characteristics and distribution remain speculative. Based on research of violent hydrothermal activity of the South Tibet, this paper makes an attempt to discuss the characteristics of these bright spots from another perspective. Helium isotope composition of geothermal gas, geochemistry of geothermal water and modelling of temperature field suggest that these bright spots consist of silicate magma, providing new evidence for the existence of partial melting in southern Tibet.