摘要
The eastern Himalayan syntaxis is the easternmost end of southern Tibet and undergoing the continental Indian-Eurasian collision. The occurrence of large amounts of geothermal waters with high temperatures along the regional active faults is a clear manifestation of an abnormal thermal structure of the crust. In order to further complement the distribution characteristics of hydrothermal activity in southern Tibet, geothermal waters were collected and the hydrochemical characteristics and genetic mechanisms were analyzed. In the eastern Himalayan syntaxis geothermal field, geothermal waters show different compositional features. The major ions are Na+, Ca2+ and HCO3−, SO42−, which are mainly controlled by water-rock interaction. Two peculiar hydrochemical characteristics of geothermal waters are proposed: ⅰ) the Cl− concentration of three water samples is abnormally high due to the likely contribution of magmatic water; ⅱ) the other three have a lower Cl− concentration but higher temperature because of flashing steam heating. The positive correlations between B, Li and Cl concentrations indicate that the geothermal waters originate in a parent geothermal liquid. The enthalpy-chloride diagram shows that the Cl− concentration and the enthalpy of the parent geothermal liquid are 283 mg/L and 1732 J/g (matching a temperature of 413.74 °C), respectively. Four cooling processes for the geothermal waters which ascend from reservoirs to surface were suggested, i.e., conductive cooling, adiabatic cooling, mixing with cold groundwater and the combination of these processes. The partial melting derived from crustal anatexis or crust-mantle mixing is the heat source for this geothermal field. The oxygen and hydrogen isotopic ratios indicate that the geothermal waters are meteorically-fed. The meteoric waters infiltrate via brittle tensional small-scale faults. According to the results of hydrochemical analysis, the evolution patterns of geothermal waters have been further summarized into seven patterns. Except for Pattern g, Patterns a to f are associated with the parent geothermal liquid."