Journal of Petrology

Fluid/Rock Interaction During Low-Pressure Polymetamorphism of the Reynolds Range Group, Central Australia.

Ian S. Buick, Ian Cartwright

ABSTRACT

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: d18O(carb) ≈14 to 20%o; metapelites: d18O ≈ 6 to 14%o). Similarly, ~1.78 Ga granites have highly variable oxygen isotope ratios (d18O ≈ 5 to 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-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 XCO2 with increasing grade from < 0.05 (greenschist facies) 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.

Key Words

low pressure; polymetamorphism; fluids; stable isotopes; petrology

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