Abstract
Prograde, peak and retrograde metamorphic P-T condition is essential in formulating tectonic models for orogenic belts. The higher the peak metamorphic temperature conditions, the more difficult it is to estimate the temperature accurately. In many cases the estimated temperatures may not represent peak metamorphic chemical equilibrium because of widespread overprinting of retrograde processes during exhumation. Extensive retrograde hydration has been reported from the Sør Rondane Mountains in East Antarctica. We present results on the metamorphic temperature estimates using calcite-graphite carbon isotope thermometry in a suite of metacarbonate rocks from the Sør Rondane Mountains and discuss the various conditions prevailed during the orogenesis. Metacarbonate rocks are common lithological units in the Sør Rondane Mountains, which occur as layers up to few tens of meters in thickness and extending several kilometers in strike length. They are associated with metapelitic and metaigneous rocks. Mineralogically the marbles are composed of alternating pure calcitic/dolomitic layers and impure layers comprising of forsterite + spinel + diopside + phlogopite +/- clinohumite +/- apatite with varying amounts of carbonate minerals. Pure metacarbonate rocks that contain graphite were selected for this study. In addition, the carbon and oxygen isotopic composition of calcite and dolomite were used for selecting metacarbonate rocks which are unaffected by rehydration process, because these isotopes will shift their values if external fluids had infiltrated the layers (e.g. Otsuji et al., 2013). Well-formed polygonal graphite crystals in calcite-rich and dolomite-rich marbles are in equilibrium, where overgrown and dull textured graphite show values departing from isotopic equilibrium. Coexisting calcite/dolomite and graphite gave carbon isotope fractionation between 2.2 to 3.8 permil, suggesting a range of values for Sør Rondane marbles. These values correspond to peak metamorphic temperatures between 1000 and 680⁰C. We discuss the results comparing them with the recent reports of UHT peak metamorphic temperature condition (e.g. Higashino and Kawakami, 2022). Our results indicate that some of the regions in the Sør Rondane Mountains have experienced ultra-high temperature metamorphism, whereas some effects of retrograde metamorphism can be retrieved from metacarbonate rocks, if a careful textural observations are carried out on graphite crystals.
References: Higashino and Kawakami, 2022, Journal of Mineralogical and Petrological Sciences; Otsuji et al., 2013, Precambrian Research.
