Modeling of mineral parageneses and thermobarometry of metavolcanic rocks of the Ruker group in the Southern Prince Charles Mountains, East Antarctica
Gulbin Yu. L., Mikhalsky E. V.
Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2019. V. 148. N 5. P. 24-44
There are diverse geological complexes that expose at Prince Charles Mountains and form the crystalline basement of the East Antarctic Platform. One of them is the Ruker Metasedimentary Group that is a part of the Palaeproterozoic-Neoproterozoic suprastructure of the Ruker Terrane and is divided into two sequences of metasedimentary and metavolcanic rocks, highly deformed and metamorphosed in greenschist facies. In order to estimate metamorphic conditions in this group, mineral and major element compositions of metavolcanic rocks were studied and isochemical phase diagram were calculated with the Theriak/Domino program. Mafic schists contain the mineral assemblage Chl—Ep—Ab—Qz—Ttn ± Act ± Bt ± Ms ± Cal that is typical of metabasites, and which varies with the bulk rock composition and ratio of components in a water-carbon dioxide fluid involved in phase reactions. The calculated mole fraction of CO2 in the fluid equilibrated with carbonate-bearing parageneses lies in a range between 0.13 and 0.27. The chloritoid schist is essentially composed of Cld, Chl, Ms, and Rt. It is debated whether this rock is genetically related to metamorphism of laterites derived from the basalts. Results of forward thermodynamic modeling, supplemented by chlorite-phengite thermobarometry, indicate that metamorphism of the Ruker Group occurred under conditions of high-P part of the greenschist facies (temperature 300—450 °C, pressure up to 7—8 kbar). These conditions significantly higher than the stable continental geotherm and is close to temperatures and pressures corresponding to those along the slow subduction geotherm. According to available geological data, the same geodynamic setting may have taken place in a course of the evolution of the Neoproterozoic intraplate sedimentary basin in connection with sinking of parts of the basement at large depth as a consequence of tectonic aggregation during the closure of the basin.
Key words: mafic schist, chloritoid schist, isochemical phase diagram, Theriak/Domino, chlorite-phengite thermobarometry, Ruker Group, Palaeoproterozoic, East Antarctica