Deformation conditions and quartz c-axis fabric development along nappe boundaries: The Andrelândia Nappe System, Southern Brasília Orogen (Brazil)

Abstract

The Liberdade and Andrelândia nappes, SE Brazil, record the western Gondwana amalgamation in the Southern Brasília Orogen during the Ediacaran. The Liberdade Shear Zone acted as the boundary between these two overlapping nappes. Quartz c-axis fabrics and strain data from quartzite samples and petrological data from hosting metapelite rocks were analyzed to investigate the tectonic processes involved in nappe development. Further constraints were achieved by conducting petrological modeling and geothermobarometry using P–T isochemical phase diagram approaches. Quartz axis plots show consistency with different kinematic criteria verified in the field in meso and macroscale. The fabrics are associated to both coaxial deformation and non-coaxial deformation. Quartz microstructures, metamorphic assemblages and c-axis concentrations primarily around Y and X structural directions suggest that deformation was accommodated by grain boundary migration recrystallization under temperature above 500 °C. Disregarding pressure effects the opening angles of quartz c-axis fabrics suggest temperatures between 480 and 640 °C (muscovite quartzite) and 650–720 °C (sillimanite quartzite). Considering pressure data, the opening angles indicate conditions of 550–650 °C and 7–10 kbar (muscovite quartzite) and 600–730 °C and 4–7 kbar (sillimanite quartzite), reflecting primarily distinct baric regimes. Sillimanite-bearing samples estimatives are supported by ubiquitous presence of chessboard subgrain patterns in quartz and by modeled isochemical phase diagrams that constrain conditions of 660–730 °C and 6–7 kbar for garnet-sillimanite-biotite schist samples. Altogether, these results indicate progressive tectonic evolution linked to oblique convergence between tectonic units represented by the Andrelândia and Liberdade nappes and the Paleoproterozoic basement. Besides recording distinct thermal and baric regimes, the nappes also present different structural styles, suggesting that pressure and temperature are important boundary conditions for the nappe structural style.