Fluid—Rock Interaction during Low-Pressure Polymetamorphism of the Reynolds Range Group, Central Australia

Abstract

Marbles and metapelites from the Reynolds Range Group (central Australia) were regionally metamorphosed at low pressure during M^ at ~/-6~ Go. M^ ranged in grade from gnenschist to granulite fades along the length of the Reynolds Range, and overprinted ~l-78 Ga granites and their contact aureoles in the Reynolds Range Group metasediments. At all M.2 grades the marbles and metapelites have highly variable oxygen isotope ratios [marbles: 5I8O(carb) ~14~20%o; metapelites: b180 ~ 6-14%o]. Similarly, ~l-78 Ga granites have highly variable oxygen isotope ratios (8I8O ~ 5-13%o), 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-Mi2 contact metamorphism and fluid—rock interaction around the ~l-78 Ga granites. In contrast, mineral assemblages in the marbles define a trend of increasing Xco, with increasing grade from <0-05 (gnenschist fades) to ~0-71 -0 (granulite fades). 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. KEY WORD& law pressure; polymetamorphism; fluids; stable isotopes; petrology