Fluid circulation in the South Tibetan Detachment System: Evidence from fluid inclusions and oxygen isotope data of quartz veins in the Ramba Dome, North Himalayan Gneiss Domes

Detachment faults are sites of intensive fluid–rock interactions. Here, we report fluid inclusion and oxygen isotope data for quartz veins in the Ramba Dome in the North Himalayan Gneiss Domes, with an aim to constrain the origin and circulation of crustal fluids associated with the South Tibetan Detachment System (STDS). Microthermometric data for fluid inclusions in quartz indicate that the fluids were aqueous and CO2 − H2O ± CH4 ± N2‐bearing with low to moderate salinities (0.60–11.80 wt% eq. NaCl). The entrapment conditions are 295–410°C and 98–135 Mpa, indicating a forming‐depth of 8–10 km. Oxygen isotopic compositions (δ18O) of quartz measured in situ by secondary ion mass spectrometry and bulk by the BrF5 method show limited variations in individual quartz veins, but δ18Oquartz values vary from 12.07 to 18.16‰ (V‐SMOW) among veins. The corresponding δ18Ofluid values range from 7.71 to 13.80‰, based on equilibrium temperatures obtained from fluid inclusions. From the footwall to the detachment zone, δ18Ofluid values exhibit a broadly decreasing trend and indicate that the STDS dominated the fluid flux pathway in the crust, with more contributions of meteoric water in the detachment zone. We further quantified the contribution of meteoric fluids to 8–27% using a binary end‐member mixing model. These data imply that the fluids were predominantly metamorphic/magmatic in origin, and were mixed with infiltrating, isotopically light, meteoric water during extensional detachment shearing of the STDS. The meteoric water can infiltrate from the surface to 8–10 km depth. The meteoric water infiltrated from the surface to 8–10 km depth and interacted with metamorphic and magmatic fluids in the STDS. The maximum contribution of meteoric water occurred close to the detachment zone, indicating that the STDS dominated fluid flux pathway in the crust.