Pressure–temperature evolution of metapelitic granulites in a polymetamorphic terrane: the Rauer Group, East Antarctica

• Published in: Journal of Metamorphic Geology
• Main Authors: HARLEY, S. L., FITZSIMONS, I. C. W.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1991
• Subjects: Antarc*; Antarctica; East Antarctica
• Online Access: http://dx.doi.org/10.1111/j.1525-1314.1991.tb00519.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1525-1314.1991.tb00519.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1525-1314.1991.tb00519.x

Distinctive lithological associations and geological relationships, and initial geochronological results indicate the presence of an areally extensive region of reworked Archaean basement containing polymetamorphic granulites in the Rauer Group, East Antarctica. Structurally early metapelites from within this reworked region preserve complex and varied metamorphic histories which largely pre‐date and bear no relation to a Late Proterozoic metamorphism generally recognized in this part of East Antarctica. In particular, magnesian metapelite rafts from Long Point record extreme peak P–T conditions of 10–12 kbar and 100–1050°C, and an initial decompression to 8 kbar at temperatures of greater than 900°C. Initial garnet–orthopyroxene–sillimanite assemblages contain the most magnesian (and pyrope‐rich) garnets ( X Mg = 0.71) yet found in granulite facies rocks. A high‐temperature decompressional P–T history is consistent with reaction textures in which the phase assemblages produced through garnet breakdown vary systematically with the initial garnet X Mg composition, reflecting the intersection of different divariant reactions in rocks of varied composition as pressures decreased. This history is thought to relate to Archaean events, whereas a lower‐temperature ( c. 750–800°C) decompression to 5 kbar reflects Late Proterozoic reworking of these relict assemblages. The major Late Proterozoic ( c. 1000 Ma) granulite facies metamorphism is recorded in a suite of younger Fe‐rich metapelites and associated paragneisses in which syn‐ to post‐deformational decompression, through 2–4 kbar from maximum recorded P–T conditions of 7–9 kbar and 800–850°C, is constrained by geothermobarometry and reaction textures. This P–T evolution is thought to reflect rapid tectonic collapse of crust previously thickened through collision.