11:45 AM - 12:00 PM
[SMP22-11] Ultrahigh temperature metamorphism at Sør Rondane Mountains, East Antarctica: Evidence from calcite-graphite carbon isotope thermometry
Keywords:UHT metamorphism, C-isotope thermometry, Sør Rondane Mountains
Estimation of accurate peak metamorphic P-T condition is essential in formulating tectonic models of crustal evolution. High-grade metamorphic terrains in many cases cannot preserve peak metamorphic chemical equilibrium in matrix mineral assemblages because of widespread overprinting of retrograde processes during exhumation. Sør Rondane Mountains in East Antarctica is one such terrain where extensive retrograde hydration has erased the earlier granulite facies metamorphic imprints, especially in the matrix of the most rock types. Here we present results on the determination of peak metamorphic temperature condition using calcite-graphite carbon isotope thermometry in metacarbonate rocks from the Sør Rondane Mountains.
High-grade marbles are common lithological units in 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 garnet-biotite and hornblende-biotite gneisses. Mineralogically the marbles are composed of alternating pure calcitic/dolomitic layers and impure forsterite + spinel + diopside + phlogopite ± clinohumite ± apatite-bearing marble layers. For this study, we have carefully selected pure metacarbonate rocks, which contain graphite. Furthermore, carbon and oxygen isotopic composition of calcite and dolomite were used for selecting marbles unaffected by rehydration process, because these isotopes will shift their values if external fluids had infiltrated the layers (Otsuji et al., 2013). Some layers showed large carbon and oxygen isotope shifts relating to re-hydration process (delta 18O values decrease up to 13 permil from unaltered values of 24 - 25 permil). Well-formed polygonal graphite crystals in calcite-rich and dolomite-rich marbles were selected for carbon isotope measurements because earlier studies have suggested that irregular and dull surfaced graphite preserve later stage overgrowth (e.g. Satish-Kumar et al., 2002). Coexisting calcite/dolomite and graphite gave carbon isotope fractionation between 2.2 to 3.8, suggesting a range values for Sør Rondane marbles. These values correspond to peak metamorphic temperatures between 1000 and 680 C. We discuss the results comparing with the recent estimates of peak metamorphic temperature condition (e.g. Higashino and Kawakami, 2022). The results indicate that some of the regions in the Sør Rondane Mountains have experienced ultra-high temperature metamorphism.
References:
Higashino and Kawakami, 2022, Journal of Mineralogical and Petrological Sciences
Otsuji et al., 2013, Precambrian Research
Satish-Kumar, M. et al., 2002, Journal of Metamorphic Geology
High-grade marbles are common lithological units in 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 garnet-biotite and hornblende-biotite gneisses. Mineralogically the marbles are composed of alternating pure calcitic/dolomitic layers and impure forsterite + spinel + diopside + phlogopite ± clinohumite ± apatite-bearing marble layers. For this study, we have carefully selected pure metacarbonate rocks, which contain graphite. Furthermore, carbon and oxygen isotopic composition of calcite and dolomite were used for selecting marbles unaffected by rehydration process, because these isotopes will shift their values if external fluids had infiltrated the layers (Otsuji et al., 2013). Some layers showed large carbon and oxygen isotope shifts relating to re-hydration process (delta 18O values decrease up to 13 permil from unaltered values of 24 - 25 permil). Well-formed polygonal graphite crystals in calcite-rich and dolomite-rich marbles were selected for carbon isotope measurements because earlier studies have suggested that irregular and dull surfaced graphite preserve later stage overgrowth (e.g. Satish-Kumar et al., 2002). Coexisting calcite/dolomite and graphite gave carbon isotope fractionation between 2.2 to 3.8, suggesting a range values for Sør Rondane marbles. These values correspond to peak metamorphic temperatures between 1000 and 680 C. We discuss the results comparing with the recent estimates of peak metamorphic temperature condition (e.g. Higashino and Kawakami, 2022). The results indicate that some of the regions in the Sør Rondane Mountains have experienced ultra-high temperature metamorphism.
References:
Higashino and Kawakami, 2022, Journal of Mineralogical and Petrological Sciences
Otsuji et al., 2013, Precambrian Research
Satish-Kumar, M. et al., 2002, Journal of Metamorphic Geology