Ultrahigh temperature granulite metamorphism (1050 °C, 12 kbar) and decompression in garnet (Mg70)–orthopyroxene–sillimanite gneisses from the Rauer Group, East Antarctica

• Published in: Journal of Metamorphic Geology
• Main Author: HARLEY, S. L.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1998
• Subjects: Geochemistry and Petrology; Geology; East Antarctica; Mather Peninsula; Rauer Group; Antarc*; Antarctica
• Online Access: http://dx.doi.org/10.1111/j.1525-1314.1998.00155.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1525-1314.1998.00155.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1525-1314.1998.00155.x

Highly magnesian and aluminous migmatitic gneisses from Mather Peninsula in the Rauer Group, Eastern Antarctica, preserve ultrahigh temperature (UHT) metamorphic assemblages that include orthopyroxene+sillimanite±quartz, garnet+sillimanite±quartz and garnet+orthopyroxene±sillimanite. Garnet that ranges up to X Mg of 71.5 coexists with aluminous orthopyroxene that shows zoning from cores with 7.5–8.5 wt% Al 2 O 3 to rims with up to 10.6 wt% Al 2 O 3 adjacent to garnet. Peak P – T conditions of 1050 °C and 12 kbar are retrieved from Fe–Mg–Al thermobarometry involving garnet and orthopyroxene, in very good agreement with independent constraints from petrogenetic grids in FeO–MgO–Al 2 O 3 –SiO 2 and related chemical systems. Sapphirine, orthopyroxene and cordierite form extensive symplectites and coronas on the early phases. The specific reaction textures and assemblages involving these secondary phases correlate with initial garnet X Mg , with apparent higher‐pressure reaction products occurring on the more magnesian garnet, and are interpreted to result from an initial phase of ultrahigh temperature near‐isothermal decompression (UHT‐ITD) from 12 to 8 kbar at temperatures in excess of 950 °C. Later textures that involved biotite formation and then partial breakdown, along with garnet relics, to symplectites of orthopyroxene+cordierite or cordierite+spinel may reflect hydration through back‐reaction with crystallizing melts on cooling below 900–850 °C, followed by ITD from 7 to 8 kbar to c . 5 kbar at temperatures of 750–850 °C. The tectonic significance of this P–T history is ambiguous as the Rauer Group records the effects of Archean tectonothermal events as well as high‐grade events at 1000 and 530 Ma. Late‐stage biotite formation and subsequent ITD can be correlated with the P–T history preserved in the Proterozoic components of the Rauer Group and hence with either 1000 or 530 Ma collisional orogenesis. However, whether the preceding UHT‐ITD history reflects a temporally unrelated event (e.g. Archean) or is ...