Stable isotopic signatures of superposed fluid events in granulite facies marbles of the Rauer Group, East Antarctica

Abstract

ABSTRACTThe role of volatiles in the stabilization of the lower (granulite facies) crust is contentious. Opposing models invoke infiltration of CO2‐rich fluids or generally vapour‐absent conditions during granulite facies metamorphism. Stable isotope and petrological studies of granulite facies metacarbonates can provide constraints on these models. In this study data are presented from metre‐scale forsteritic marble boudins within Archaean intermediate to felsic orthogneisses from the Rauer Group, East Antarctica.Forsteritic marble layers and associated calcsilicates preserve a range of 13C‐ and 18O‐depleted calcite isotope values (δ13C= ‐9.9 to ‐3.0% PDB, δ18O = 4.0 to 12.1% SMOW). A coupled trend of 13C and 18O depletion (∼2%, ∼5%, respectively) from core to rim across one marble layer is inconsistent with pervasive CO2 infiltration during granulite facies metamorphism, but does indicate localized fluid‐rock interaction. At another locality, more pervasive fluid infiltration has resulted in calcite having uniformly low, carbonatite‐like δ18O and δ13C values. A favoured mechanism for the low δ18O and δ13C values of the marbles is infiltration by fluids that were derived from, or equilibrated with, a magmatic source. It is likely that this fluid‐rock interaction occurred prior to high‐grade metamorphism; other fluid‐rock histories are not, however, ruled out by the available data. Coupled trends of 13C and 18O depletion are modified to even lower values by the superposed development of small‐scale metasomatic reaction zones between marbles and internally folded mafic (?) interlayers. The timing of development of these layers is uncertain, but may be related to Archaean high‐temperature (>1000d̀C) granulite facies metamorphism.