Cordierite in the Shackleton Range, Antarctica: First Recorded Occurrence

An example of cordierite-bearing gneiss that is part of a high-grade gneiss-migmatite sequence is described from the Hatch Plain in the Read Mountains of the Shackleton Range, Antarctica, for the first time.The cordieritebearing rocks eonstitute the more melanosomic portions of the metatectic and migmatitic rocks that are associated with relict granulite facies rocks such as enderbitic granulite and enderbitic garnet granulite.The predominant mineral assemblage in the eordierite-bearing rocks is chemically homogeneous eordierite (X"" 0.61) and biotite (X"o 0.47), strongly zoned garnot (X"" 0.18-0.11),silli;llanite, K-feldspar (Or:'.94Ab5.,sAn06)'plagioclase (An28)~and quartz.Inclusions of sillimanite and biotite relics in both gamet and cordierite indicate that garnet and cordierite were produced by the coupled, discontinuous reaction biotite + sillimanite + quartz = cordierite + garnet + K-feldspar + H.O, Various garnet-biotite and garnet-cordierite geothermometers and sillimanitequartz-plagioclase-garnet-cordierite geobarometers yield a continuous clockwise path in the P-T diagram.The P-T conditions for equilibrium between garnet core and cordierite and between garnet core and biotite during peak metamorphism and migmatization were estimated to be 690°C at 5-6 kb.This was followed by cooling and unloading with continuously chan ging conditions down to 515 °C at 2-3 kb.This low-pressure re-equilibration correlates with the pressure conditions evaluated by SCHULZE (1989) for the widespread granitic gneisses of the Read Group in the Shackleton Range.The associated relict enderbitic granulites representing low-pressure type granulite (8 kb; 790°C) are comparable to similar low-pressure granulites from the East Antarctic craton.They were either formed by under-accretion processes after collision (Wnu.s1979, p. 217) or they are a product of remetamorphism at P-T conditions intermediate between granulite and amphibolite facies.A model of a multiple imbrication zone with crustal thickening (CUTHBERT et al. 1983) is discussed for the formation of the relict granulites of the central and eastern Read Mountains which show higher press ure conditions (8-12 kb, SCHULZE & OLESCH 1990), indicating a Proterozoic crustal thickness of at least 40 km.Zusammenfassung: Die Du Toit Nunataks in den Read Mountains der Shackleton Range, Antarktis, werden in ihrem metamorphen Anteil von hochgradigen Biotit-Granatgneisen, Amphiboliten und Migmatiten aufgebaut, welche lokal mit reliktisch erhaltenen enderbitischen Granuliten und enderbitischen Granatgranuliten verzahnt sind.Im Gebiet der Hatch Plain wurde darüber hinaus zum ersten Mal in der gesamten Shackleton Range Cordierit gefunden.Der Cordierit tritt vorwiegend im Melanosom der metatektisch ausgebildeten Sillimanit-Granat-Cordieritgneise auf.Die weitverbreitete Mineralparagenese ist hier Biotit (X"' 0,47), Cordierit (X", 0,61) und stark zonarer Granat (X", 0,18-0,11), neben Sillimanit, Kalifeldspat'(Or".94'Ab"."An O • 6 ) ' Plagioklas (An,,) und Quarz.Detailstudien an reliktischen Einschlüssen von Sillimanit und Biotit in Cordierit und in Granat belegen die Bildung von Granat und Cordierit über die gekoppelte, divariante Reaktion Biotit + Sillimanit + Quarz = Granat + Cordierit + Kalifeldspat + H 2 0 .P-T-Abschätzungen aufgrund verschiedener Granat-Biotit-und Granat-Cordierit-Thermometer, sowie von Sillimanit-Plagioklas-Quarz-