RESOLVING OVERPRINTING EVENTS IN POLYCYCLIC METAMORPHIC TERRAINS – AN EXAMPLE FROM PRYDZ BAY, EAST ANTARCTICA

Placing tight constraints on pressure-temperature-time histories in poly-metamorphic terrains is a complex task where unambiguous interpretation of mineral assemblages and textures is difficult. The relative and absolute timing of deformation events in such terrains are commonly achieved by the bracketing of cross-cutting veins, dikes or other intrusions. However, this does not eliminate the uncertainty surrounding textures that may or may not have developed on a single P-T path or as a result of overprinting thermal events. One approach to this problem is through an integration of petrologically constrained geochronology with complementary mineral trace element geochemistry. This approach is being applied to help unravel the complex event history that is revealing itself in the Brattstrand Bluffs, east Antarctica.

The target rocks for this study are structurally constrained pegmatites and granitic dikes from the Brattstrand Bluffs in east Antarctica. U-Pb zircon and monazite ages from migmatites and voluminous granitic magmatism in the Prydz Bay area were previously interpreted to suggest that partial melting in this region occurred during a single metamorphic episode between ~530-500 Ma. However, preliminary results from recently acquired age data suggest that the dominant partial melting event occurred earlier than expected, and was overprinted by a separate high-grade event. To place tighter constraints on the absolute timing of events, in particular the relationship of high-grade structures to each tectonic episode, 4 generations of structurally constrained felsic intrusions are being studied in detail. To make more robust interpretations from zircon and monazite age data in these rocks, an integrated petrologic and trace element geochemical approach using in situ analysis is being applied.

This approach to deciphering complex overprinting events aims to provide a more complete picture of the complex, poly-cyclic history of the Brattstrand Bluffs, and therefore the Prydz Bay region. In addition, integrated geochronology-petrology studies of such rocks will help us better understand the nature and significance of partial melting in high-grade terrains.