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Monazite and zircon petrochronology of granulites and migmatites as remnants of a magmatic arc on the West Gondwana margin (Socorro Nappe, south-eastern Brazil)

In the Southern Brasília Orogen (south-eastern Brazil), a nappe system that represents the roots of a magmatic arc records HT-UHT metamorphic conditions in lower to mid-crustal rocks. It is divided into two segments by a major shear zone, of which the northern nappe hosts the most extreme metamorphism and has been targeted for most petrochronological studies. These rocks carry insights into the stages of orogeny, as well as the first direct evidence of the paleo-active margin basement, and time-constraint (1) a metamorphism related to the magmatic arc consolidation on the active margin at 670-640 Ma and (2) an enduring UHT event related to collision and decompression at 630-590 Ma. The southern nappe (Socorro Nappe) hosts felsic and mafic granulites, and migmatites that apparently describe a less extreme pressure-temperature setting and relatively younger ages. We found distinctive patterns pertaining the inner nappe and its outward boundaries (Embu Terrane and São Roque Domain). We present new U-Th-PbC monazite data and LA-ICPMS U-Pb and Lu-Hf systematics in zircon retrieved from granulites, migmatites and paragneisses, and partial results on conventional thermobarometry and thermodynamic modelling. Our data plot within the P-T range of upper amphibolite to granulite facies and describe mostly post-peak retrograde clockwise trajectories, with lesser conditions at the nappe boundaries. Temperature, textural context and anatexis are major controls on the preservation of monazite versus zircon records. The outer nappe tends to host prominent 750 Ma-old monazite, whereas in the inner nappe such ages are scarce and most prevalent in the zircon records.


Mikaella Balis1, Bernhard Schulz1, Mario da Costa Campos Neto2
1TU Bergakademie Freiberg, Germany; 2University of São Paulo, Brazil
GeoBerlin 2023