FLUID-ROCK INTERACTIONS WITHIN A QUARTZITE-SCHIST DETACHMENT DURING EXHUMATION: INSIGHTS FROM THE SNAKE RANGE CORE COMPLEX, NV, USA

Fluid inclusion (FI) and microstructural analyses as well as stable isotope data from fluids were obtained from rocks from traverses within the Snake Range detachment in order to reconstruct the fluid flow pathways and the evolution of fluid reservoirs during exhumation. Over the last decades several studies have demonstrated that surface derived fluids may penetrate down to depths of ductile shear zones during crustal extension. Meteoric fluids have also been recorded within the Snake Range detachment based on δD analyses of synkinematic muscovite.

High angle cataclasite zones, brittle faults and veins, potentially channelizing fluid flow, were compared to quartzite mylonite and schist layers from the detachment that have not been affected by late stage semi-brittle and brittle deformation. The study of FI distribution as a function of microstructure reveals that aqueous surface derived fluids were trapped within fluid inclusion planes that crosscut quartz grain boundaries at temperatures below 300°C. Significant amounts of aqueous-carbonic fluids are found within grain boundaries of dynamically recrystallized quartz grains and indicate trapping conditions above 300°C. Throughout the section δD values from fluid inclusions in quartzite mylonite and schist layers range from -40 to -60 ‰, whereas data from the high angle cataclasites and brittle fault zones are lower than -70‰. The latter indicate δD values close to isotopic equilibrium with calculated δD values of fluids based on muscovite δD data (-120 to -150 ‰). δD values of muscovite (-70 to -90 ‰) from the schist layers are also close to equilibrium with the extracted fluids. However, fluid inclusions within the dynamically recrystallized quartzite mylonite layers are in isotopic disequilibrium with the rock (δD fluid: -40 ‰ vs δD muscovite: -140 ‰ in the most extreme cases).

The recorded isotopic signatures of muscovite grains in the detachment are compatible with an early stage of pervasive influx of meteoric water within the quartzites. As the schist layers get involved during exhumation, δD values of FI are buffered by the metamorphic signatures of micas. Such differentiated meteoric fluids are considered to flow upwards within convection cells induced by an elevated geothermal gradient and deformation-enhanced permeability.