Fluid-rock interaction during low-pressure polymetamorphism of the Reynolds Range Group, central Australia

Marbles and metapelites from the Reynolds Range Group (central Australia) were regionally metamorphosed at low pressure during M₂ at ∼1·6 Ga. M₂ ranged in grade from greenschist to granulite facies along the length of the Reynolds Range, and overprinted ∼1·78 Ga granites and their contact aureoles in the Reynolds Range Group metasediments. At all M₂ grades the marbles and metapelites have highly variable oxygen isotope ratios [marbles: δ¹⁸O(carb) ∼14-20‰; metapelites: δ¹⁸O ∼ 6-14‰]. Similarly, ∼ 1·78 Ga granites have highly variable oxygen isotope ratios (δ¹⁸O ∼ 5-13‰), with the lowest values occurring at the granite margins. In all rock types, the lowest oxygen isotope values are consistent with the infiltration of channelled magmatic and/or meteoric fluids. The variable lowering of oxygen isotope values resulted from pre-M2 contact metamorphism and fluid-rock interaction around the ∼1·78 Ga granites. In contrast, mineral assemblages in the marbles define a trend of increasing X_CO2 with increasing grade from <0·05 (greenschist fades ) to ∼0·7-1·0 (granulite facies). This, together with the lack of regionally systematic resetting of oxygen isotope ratios, implies that there was little fluid-rock interaction during prograde regional metamorphism.