Identification of Multiple Thermal Events in High-Grade Metacarbonate Rocks Using Carbon Isotope Thermometry: An Example From the Sør Rondane Mountains, East Antarctica

Abstract

Nine metacarbonate layers from the regionally metamorphosed terrane of the Sør Rondane Mountains in the Eastern Dronning Maud Land in East Antarctica were examined in detail for constraining the thermal events using carbon isotope exchange between dolomite/calcite and graphite. Equilibrium carbon isotope fractionation between dolomite and graphite suggested peak metamorphic temperature conditions reaching up to 802°C ± 29°C were estimated at the Balchenfjella locality, where multiple samples from six thick layers of metacarbonate rocks were examined. However, some of the samples exhibit lower carbon isotope fractionation reflecting the possibility of ultrahigh-temperature metamorphic conditions, which is consistent with recent reports. Furthermore, several metacarbonate rock samples display large variations in δ¹³C_VPDB values for graphite grains, despite dolomite and calcite showing homogeneous carbon and oxygen isotopic composition indicating signatures of retrograde metamorphism and fluid infiltration events. Detailed textural observation suggested alteration of δ¹³C_VPDB values of graphite during retrograde metamorphism might have resulted due to the overgrowth of graphite crystals by the infiltration of low δ¹³C_VPDB-bearing fluids, the extent of alteration being a direct function of the fluid–rock ratio. Field evidence indicates the presence of carbonate veins cutting across the metacarbonate rocks suggesting that carbon isotope thermometry can also be utilised to understand the effect of external fluid infiltration. At Perlebandet locality the metamorphic temperature conditions were estimated to be around 915°C, whereas those from Tanngarden and Menipa gave lower temperature estimates. Detailed textural analysis of graphite in combination with isotopic composition provided clear evidence for retrograde events. Thus, our results provide tight constraints of peak and post-peak metamorphic temperature conditions and a regional thermal structure for the Sør Rondane Mountains and further testify the usefulness of carbon isotope thermometry in polymetamorphic terrains.