Fluid‐rock interaction during early contact metamorphism of the Reynolds Range Group, central Australia

Abstract

Sediments of the Reynolds Range Group, central Australia, experienced mid‐Proterozoic contact metamorphism around granitoids during M21‐D21, that was overprinted by low pressure‐high temperature regional metamorphism (M22‐D22). M22 greenschist facies marbles of the Upper Calc‐silicate Unit contain coarse‐grained porphyroblasts of intermediate scapolite (EqAn49–59) that pre‐date D22 fabrics, and which are partially pseudomorphed by peak‐M22 greenschist facies mineral assemblages. The stability of EqAn59 scapolite requires a minimum temperature of 540°C prior to M22 chlorite zone greenschist‐facies metamorphism (T 400–450°C). The marbles also display a range of stable isotope values (δ18O = 15.8 to 19.7%O; δ13C = ‐1.4 to 0.7%o). Early high‐temperature scapolitization and lowering of marble oxygen isotope values are interpreted as resulting from contact metamorphism around a large M21 granite, the Conniston Schist. Early contact metamorphism of siliceous marls of the Lower Calc‐silicate Unit is indicated by the occurrence of massive grandite (grossular‐andradite) garnet‐rich skarns that occur close to the contact with the Napperby Gneiss, an M21 granite. These grandite‐rich layers pre‐date regional deformation and crop out across the M22 amphibolite facies to granulite facies transition zone. The mineral assemblages of the grandite‐rich layers are consistent with their formation through fluid infiltration (Xco2 < 0.1 to 0.3) at T 550–600°C. Stable isotope ratios (calcite: δ13C = ‐4.20 to‐0.8%o; δ18O = 10.5 to 14.0%o; bulk silicate fractions: δ18O = 6.1 to 10.8%o) indicate that the fluids were magmatic, and it is suggested that these fluids were derived from the Napperby Gneiss (WR δ18O = 6–9%o) during M21 contact metamorphism. Recognition of early contact metamorphic assemblages in these calc‐silicate rocks have broader implications for fluid flow and P‐T‐t studies of regional low pressure‐high temperature metamorphism.