Neoproterozoic tectonothermal overprint on the Southern São Francisco Craton basement, Brazil

Abstract

The Archean domain of the Southern São Francisco Craton (SFC) acted as a foreland for the docking of the Mineiro Belt (2.47–2.05 Ga) through soft-collision processes during the Paleoproterozoic. Much later, during the Neoproterozoic, the newly formed Archean-Paleoproterozoic protocraton again served as a foreland for the development of thrust belts that contributed to the formation of the Gondwana supercontinent. From the Paleoproterozoic to the Neoproterozoic, pressure–temperature (P-T) conditions in the SFC crust varied from prograde amphibolite to retrograde amphibolite-greenschist metamorphic facies. In this paper, we reinterpret past ⁴⁰K/⁴⁰Ar data and present new ⁴⁰Ar/³⁹Ar dates in biotite and amphibole—evaluating the P-T conditions affecting metagranitoids, orthogneisses, and metamafic rocks of the southern SFC basement—to reconstruct its tectonothermal history during the assembly of the Gondwana supercontinent. Recalculated ⁴⁰K/⁴⁰Ar* and new ’total fusion’ ⁴⁰Ar/³⁹Ar dates facilitated the creation of a contour map with a 900 Ma isochron, roughly corresponding to the tectonic boundary between the Archean and Paleoproterozoic domains, with the Neoproterozoic tectonothermal imprint recorded south of the 900 Ma line. The younger ⁴⁰Ar/³⁹Ar plateau dates from 651 to 526 Ma obtained on biotite crystals delineate a 600 Ma isochron within the Mineiro Belt area, approximately coinciding with the Lenheiro Shear Zone (LSZ). This comprehensive dataset, encompassing dates from 900 to 526 Ma, underscores a polycyclic tectonothermal reactivation that either delimits the Mineiro Belt area or occurs along the LSZ. This prominent structure within the belt differentiates deeper plutonic rocks from shallower (sub)volcanic rocks to its north and south regions, respectively. These relatively deeper to shallower crustal positions suggest that Neoproterozoic collisional fronts developed progressively from northwest (earlier collisions) to southeast (later collisions), corresponding to older and younger dates, respectively. Additionally, Discordia lower-intercepts in Wetherill diagrams from 977 to 488 Ma, obtained in previous U-Pb investigations within the Mineiro Belt, align with our new ⁴⁰Ar/³⁹Ar plateau and ’total fusion’ dates, providing further evidence for reheating and partial to complete resetting of magmatic biotite-amphibole crystals and neoformed metamorphic biotite. Our results confirm that the Mineiro Belt did not behave as a stable continental landmass during the Neoproterozoic Brasiliano Orogeny, instead experiencing significant tectonothermal overprinting.