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The Southern Brasília Orogen is the result of Ediacaran continental collision between the active margin of the Paranapanema paleocontinent and the passive margin of the São Francisco paleocontinent. The active margin was deformed into the Socorro-Guaxupé Nappe and the associated Embu Domain. This paper presents new U–Pb zircon data and geothermobarometric estimates that allow to reconstruct the metamorphic evolution in these upper plate units. A pre-collisional metamorphism M1 with an age peak in metamorphic rims of zircon grains in the range 680–630 Ma is related to the development of the Socorro-Guaxupé magmatic arc with batholiths dated in a similar time span, 690 - 630 Ma. This metamorphism reached medium pressure granulite facies, now exposed by the tilted uplift and erosion due to the collision. Another peak is detected in metamorphic rims of zircon grains, in the range 630–590 Ma, labeled M2, related to the collision and corresponds to the main metamorphism in the lower plate and accretionary wedge-like nappe stack, underlying the Socorro-Guaxupé Nappe. This nappe stack was affected by greenschist facies metamorphism in the autochthonous underlying craton, grading to high pressure granulite facies in the upper nappe, with local occurrences of retroeclogite lenses. In the Socorro-Guaxupé Nappe and Embu Domain this main metamorphism (M2) is characterized by three metamorphic zones decreasing in grade with distance from the suture: a granulite facies orthopyroxene bearing zone; a high temperature amphibolite facies zone and a lower amphibolite facies zone. Geothermobarometric results in mineral associations related to M2 show temperatures decreasing from about 770 °C to ∼620 °C and pressures decreasing from about 9 kbar close to the suture to about 5 kbar in the southern and western parts of the studied region. On the upper Paranapanema plate, relative to the Socorro-Guaxupé Nappe and Embu Domain units, at the base of the plate, an older metamorphism (M1) is verified with temperatures between 960 and 890 °C and pressures between 11 and 9 kbar, reequilibrated at approximately 700 °C with a pressure of about 9 kbar (M2, continental collision). For this reason, it can be assumed that the rim of the Paranapanema plate was exhumed during the continental collision, since the temperature and pressure of the metamorphic peak determined during the continental collision are lower than those determined for the pre-collisional magmatic arc metamorphism. • Active margin in the Paranapanema Paleocontinent. • Metamorphism related to magmatic arc stage. • Exhumation of the upper plate during the Brasilia Meridional continental collision. • Arc metamorphism superimposed by collisional metamorphism of lesser degree. [ABSTRACT FROM AUTHOR]

The Passos Nappe is an important tectonic unit of the Southern Brasília Orogen with inverted metamorphism, grading from greenschist facies at the base to granulite facies in the upper part. In the central part, of amphibolite facies, mica schist layers contain garnet megaporphyroblasts (3–9 cm diameter) that contrast with the common-size garnet (0.2–1.5 cm diameter) present in other mica schist layers above and below. This paper intends to describe this textural feature in detail and to discuss its genesis. The megaporphyroblasts are ellipsoidal, with major axis parallel to the mineral and stretching lineation, associated with the main foliation (S 2). Inclusion trails analysis in forty-six megaporphyroblasts, demonstrate straight internal foliation (S i = S 1) in the core, progressively crenulated towards the rim by D 2. S i is discontinuous with the external foliation (S e = S 2), which wraps around the megaporphyroblasts, indicating early syn-D 2 growth. The angle between the axial planes of the included crenulations and S 2 in the matrix indicates that the D 2 deformation continued after porphyroblast growth. Asymmetric deformation shadows reveal non-coaxial D 2 deformation, associated with the emplacement of the nappe (top-to-the SE). The megaporphyroblasts are almandine-rich (73–82%) and spessartine-poor (0.1–2.5%) exhibiting prograde growth zonation. The common-size garnet porphyroblasts of the adjacent micaschists show similar inclusion patterns and chemical zonation indicating a similar tectonometamorphic evolution. A possible origin of the megaporphyroblasts by introduction of fluids along shear zones is discarded because the megaporphyroblasts are distributed along S 0 //S 1 planes that make an angle with S 2. Since the porphyroblasts grew early syn-D 2 , these shear zones should be parallel to S 2 which does not match the observations. The chemical data indicate that megaporphyroblast formation is related to a high-Al protolith composition, associated with scarce nucleation sites. • A first description of garnet megaporphyroblasts from the Passos Nappe. • Developed in the context of a greenschist to granulite inverse metamorphic gradient. • The main control for the large crystal growth was the low garnet nucleation density. • Megaporphyroblasts and common-size garnets share the same tectono-thermal evolution. • Megaporphyroblasts growth-rate was 20 times greater than common-size garnets. [ABSTRACT FROM AUTHOR]