Ultrahigh-Temperature Mafic Granulites in the Rauer Group, East Antarctica: Evidence from Conventional Thermobarometry, Phase Equilibria Modeling, and Rare Earth Element Thermometry

• Published in: Journal of Petrology
• Main Authors: Chen, Longyao, Liu, Xiaochun, Wang, Wei-(RZ), Liu, Jian
• Format: Article in Journal/Newspaper
• Language: English
• Published: Oxford University Press (OUP) 2023
• Subjects: East Antarctica; Rauer Group; Antarc*; Antarctica
• Online Access: http://dx.doi.org/10.1093/petrology/egad014; https://academic.oup.com/petrology/advance-article-pdf/doi/10.1093/petrology/egad014/49447774/egad014.pdf; https://academic.oup.com/petrology/article-pdf/64/4/egad014/50266096/egad014.pdf

Abstract The Rauer Group in East Antarctica is a typical high- to ultrahigh-temperature (HT–UHT) granulite-facies terrane. As UHT metamorphism has been recognized only in Mg–Al-rich pelitic granulites from the Mather Paragneiss, the regional extent of UHT metamorphism remains uncertain, which has hindered our understanding of the genesis and tectonic setting of UHT metamorphism in the Rauer Group. In this study, representative samples of mafic granulite were selected from Archean crustal domains to constrain the peak metamorphic conditions and P–T path and to assess the regional extent of UHT metamorphism in the Rauer Group. Integrated results from mineral reaction histories, thermobarometry, and phase equilibria modeling indicate a multi-stage clockwise P–T evolution for mafic granulites involving pre-peak compression, heating to UHT peak conditions, post-peak near-isothermal decompression under UHT conditions, and subsequent decompressional cooling. The pre-peak prograde history is based mainly on the inclusion assemblage of clinopyroxene + plagioclase + amphibole + quartz + ilmenite ± orthopyroxene ± k-feldspar within porphyroblastic garnet and clinopyroxene and records the transformation from a quartz-present to quartz-absent system. The UHT peak conditions are well constrained at 930°C–1030°C and 10.6–12.8 kbar on the basis of the stability field of the observed peak assemblage of (orthopyroxene–quartz)-free garnet + clinopyroxene + plagioclase + amphibole + ilmenite + melt, as well as measured mineral compositions, including the high Ti content in amphibole (Ti = 0.38–0.42 p.f.u.), the anorthite content of coarse-grained plagioclase cores (XAn = 0.35–0.42), and the grossular content in garnet (XGrs = ~0.21) in P–T pseudosections. The peak T conditions are consistent with thermometric estimates in the range of 930°C–1030°C obtained from garnet–clinopyroxene, garnet–orthopyroxene, and Ti-in-amphibole thermometers, and are slightly lower than estimates (1020°C–1120°C) obtained from thermometers based on rare ...