THERMAL REWORKING OF HIGH-GRADE GNEISSES – EVIDENCE FROM U-PB RUTILE AGES IN THE EAST ANTARCTIC SHIELD

In recent years the focus of much research in east Antarctica has been a better characterization of Pan African collisional belts that are interpreted to have formed during final amalgamation of Gondwana. In areas such as Prydz Bay and the Prince Charles Mountains, ~550-490 Ma granulite facies metamorphism and partial melting are pervasive, inferred to be related to collision of the “Mawson Continent” with the “Indo-Antarctic Craton” to form East Gondwana. Despite being widespread in this region, only rare and ambiguous evidence for these events has been found further east in the Rayner Complex, an area that would have been inboard of the collisional margin near Prydz Bay.

This contribution will present new data from the Oygarden Islands, Rayner Complex, east Antarctica, that show evidence for a thermal overprint that lacked substantial reworking of older (~930-890 Ma) structures and fabrics. In the Oygarden Islands, orthogneisses and paragneisses were metamorphosed to HT/UHT conditions at ~930-890 Ma. Later events are limited to retrograde (amphibolite facies) shear zones that occur as <5cm mylonites or are focused along post-UHT pegmatites. The sense of shear across these zones is broadly south over north thrusting. Recent U-Pb dating of rutile from metapelitic gneisses has revealed pervasive ~500 Ma ages without any preservation of older cores. These ages correlate with ~500 Ma rims locally formed on monazite through fluid-mediated recrystallization. All samples investigated occurred away from late shear zones, and did not contain macroscopic evidence for recrystallization of high-grade assemblages or the development of new fabrics. Combined, these new data suggest that far-field effects of Gondwana collision were experienced well inboard of the Prydz Bay margin, and involved heating above the closure temperature for Pb diffusion in rutile.