Zircon U-Pb and Hf petrochronology of mafic high-grade rocks of a Neoproterozoic nappe system in West Gondwana (Socorro Nappe, SE Brazil)

Abstract

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, amphibolites and migmatites with intricate occurrences and complex pressure-temperature-time histories that may preserve distinct age populations in the inner nappe and its outward boundaries (Embu Terrane and São Roque Domain). The mafic lower to mid-crustal rocks of the Socorro Nappe lack detailed comprehensive studies of their P-T-t evolution. We present new preliminary LA-ICPMS U-Pb and Lu-Hf systematics in zircon retrieved from metamafic rocks such as granulites, amphibolites and orthogneisses, and partial results on conventional thermobarometry and thermodynamic modelling. We investigate the significance of a wide timespan from ca. 750 to 570 Ma where granulites tend to preserve older ages in contrast to amphibolites. However, Hf signatures are complex and also show a wide range of ε values from 0 to weakly radiogenic, and strongly radiogenic that are not straightforwardly related to a clear time evolution. Herein we discuss preliminary insights into the zircon petrochronology and P-T-t evolution of metamafic high-grade rocks as a tool to unravel the tectonic evolution of the southernmost segment of the Southern Brasília Orogen in relation to the adjacent São Roque Domain and Embu Terrane, in the context of the Western Gondwana amalgamation.