STUDY OF A FOSSIL HYDROTHERMAL SYSTEM IN THE NORTHERN SNAKE RANGE USING STABLE ISOTOPE, MICROSTRUCTURAL AND 40Ar/39Ar DATA AND ITS USE AS A PALEOALTIMETER

The northern Snake Range in east-central Nevada, is a classic example of normal fault-bounded metamorphic core complex that developed as a consequence of Miocene extension of the Basin and Range Province. This dome experienced a history of rapid exhumation and extensional faulting and exposes a N-S striking detachment zone characterized by muscovite-bearing quartzite mylonites separated by micaschists. This top-east sense of displacement, low angle detachment is overlain by the Miocene Sacramento Pass basin. We present a multi-disciplinary study combining structural and microstructural observations to hydrogen stable isotope and ⁴⁰Ar/³⁹Ar thermochronology data to better understand the spatial and temporal relationships among microstructure development, localization of strain, fluid-rock interaction and surface processes. Synkinematic muscovites collected systematically over 300 m of section within the detachment zone reveal a pattern of ⁴⁰Ar/³⁹Ar ages that become increasingly younger from the top to the bottom (26.9 Ma to 21.3 Ma). The same minerals show δD _muscovite values as low as -150 ‰ at the top of the detachment zone and attain progressively higher values of up to -72 ‰ at the bottom of the section. This hydrogen stable isotope trend suggests that meteoric water percolated 10s to 100s of meters into the detachment zone during normal faulting, can be correlated with quartz microstructures that are characterized by a very fine grain size and intense dynamic recrystallization at the top and a coarser quartz fabric showing grain boundary migration towards the bottom. Together with data from basin records, these hydrogen stable isotope data reflect the composition of precipitation that infiltrated the detachment during extension and can be used to evaluate climate change and also to understand evaporative effects on meteoric waters prior to mineral formation at or below the Earth’s surface. The hydrogen isotopic composition of detachment muscovite probably records the isotopic composition of surface waters that fed synextensional lake basins on top of the evolving core complex. Indeed the oxygen isotope record in carbonates from the Sacramento Pass basin reveals the combined effects of changes in topography and lake water evaporation.