High-temperature Retrogression of Granulite-facies Marbles from the Reynolds Range Group, Central Australia: Phase Equilibria, Isotopic Resetting and Fluid Fluxes

Abstract

Reynolds Range Group rocks underwent granulite-facies metamorphism (M2) at ∼1.6 Ga ( 5 kbar, 750–800°C) and were subsequently retrogressed in narrow strike-parallel zones at 1.59–1.57 Ga. Within these zones, metacarbonates that initially equilibrated at XCO2 >0.8 during M2 were mineralogically reset by the infiltration of water-rich fluids (XCO2≤0.02–0.3) at 650–700°C and 3–4 kbar. δ18O(Carb) values of the retrogressed metacarbonates were variably reset during fluid infiltration, with the lowest values (10–13‰) suggesting that the fluids that caused retrogression were exsolved from segregated partial melts, themselves derived from the underlying granulite-facies metapelites. Mineralogical and isotopic resetting were locally accompanied by silica metasomatism. The mineralogically reset marbles record time-integrated fluid fluxes of typically 101–104m3/m2. For upward flow of high-temperature fluids through the marbles over a distance of 200 m in 18 Ma, the observed mineralogical and isotopic resetting, and metasomatism require intrinsic permeabilities between 10−22 and 10−19 m2 that vary across strike on a centimetre to metre scale, indicating that fluid flow was strongly channelled.