CAMBRIAN GRANULITE-FACIES METAMORPHISM OF THE LARSEMANN HILLS, PRYDZ BAY, ANTARCTICA: USING THERMODYNAMIC MODELING TO INFORM MONAZITE PETROCHRONOLOGY

Rocks exposed along the southeast coast of Prydz Bay were affected by granulite-facies metamorphism, deformation, and plutonism associated with continental collisions at 900-1000 Ma and 500-530 Ma. Pseudosection modeling was used in conjunction with petrography, trace element mapping, and in-situ (U-Th)/Pb dating of monazite by electron microprobe to characterize the P-T-t history of Cambrian metamorphism in the Larsemann Hills. Age populations were identified by grouping domains based on petrographic context, and U, Th, Y, and Pb concentrations; these were linked to the P-T path by comparison with our modeling results. Two garnet-cordierite bearing metapelites and one garnet-orthopyroxene bearing metapsammite were selected for Perple_X modeling in the Na₂O-CaO-K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O-TiO₂±Fe₂O₃ system. Peak metamorphic assemblages are predicted to occur at 5-7 kbar and 780-840 °C in all models, similar to past estimates of 6.5-7.5 kbar and 800-860 ° C. Less than 8 vol. % melt is stable at peak P-T, and both garnet and melt modes increase along possible prograde trajectories. Previous studies estimate two stages of retrogression: decompression to 3.0-4.5 kbar and 700-750 °C, followed by isobaric cooling at 3-3.5 kbar and 550-600 °C. Garnet is predicted to break down between 5.5 and 3 kbar, whereas cordierite begins to form and hercynite mode increases. These predictions reconcile early retrograde P-T estimates with corderite-quartz symplectite coronas on garnet and hercynite grains enclosing garnet and sillimanite. Low-moderate Y and Th monazite corresponds to garnet growth during prograde metamorphism from 561-527 Ma. High Th populations at 522-528 Ma correspond to melting at peak conditions. Retrograde monazite dated at 522-500 Ma is divided into high and low Y populations. Low-Y monazite is found as inclusions in non-symplectic cordierite, whereas high Y monazite is associated with cordierite symplectites rimming garnet. None of the monazite can be attributed to late andalusite-grade retrogression. In summary, Cambrian metamorphism followed a clockwise P-T path and lasted at least 40-60 Ma, i.e., synchronous with the assembly of Gondwana at c. 530 Ma. These results illustrate how pseudosection modeling may be used to inform the assignment of monazite age populations to reactions in P-T space.