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

Abstract

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 CO 2 À H 2 O ± CH 4 ± N 2 -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 (δ 18 O) of quartz measured in situ by secondary ion mass spectrometry and bulk by the BrF 5 method show limited variations in individual quartz veins, but δ 18 O quartz values vary from 12.07 to 18.16‰ (V-SMOW) among veins. The corresponding δ 18 O fluid values range from 7.71 to 13.80‰, based on equilibrium temperatures obtained from fluid inclusions. From the footwall to the detachment zone, δ 18 O fluid 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. K E Y W O R D S fluid circulation, fluid inclusions, gneiss dome, mixing processes, oxygen isotopes, quartz veins, South Tibetan Detachment System 1 | INTRODUCTION Stable isotopes in fluids and minerals have been widely utilized to fingerprint the origins and fluxes of fluids over a wide range of timescales and geological settings, such as in basins, thrust fault zones,

Thermal pulse induced by emplacement of Ramba leucogranites in southern Tibet

“…The pressure corresponding to the core composition depends on the interpretation of the nucleation kinetics (see above). Overall, the background pressure is equivalent to a depth of ~10 km (2.5–3 kbar), consistent with the depths recorded by fluid inclusions in quartz veins (8–10 km; Li et al, 2022). The subsequent 0.5–1 kbar compression, if due to lithostatic overload, corresponds to 2–3 km burial.…”