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Abstract The Rio dos Remédios Group comprises a supracrustal sequence that occupies the base of the Espinhaço Supergroup, São Francisco Craton, Brazil. Its basal formation, Novo Horizonte, crops out in the Paramirim region mainly as metavolcanic rocks that represent one of the fewer occurrences of blue quartz phenocrysts in South America. Their mineralogy consists of quartz and K-feldspar phenocrysts, whereas biotite, muscovite, fluorite, allanite, chlorite, sericite, zircon, and opaque phases occur immersed in a quartz-feldspar-rich groundmass. Such heterogeneous composition is also supported by x-ray diffraction and chemical data. Electron probe microanalysis in some samples revealed the presence of two distinct groups of biotite (magmatic and neoformed), in addition to the presence of iron-rich white mica and almost pure orthoclase feldspar. Our data suggest that the studied metavolcanic rocks have maintained their magmatic characteristics, which were progressively overprinted by hydrothermal fluids and ductile-to-brittle deformation. The magmatic mineralogy is akin to strongly peraluminous and alkaline magmas, common in anorogenic settings – a fertile site for the origin of blue quartz-bearing rocks worldwide.

Rochas metapelíticas são importantes marcadores petrocronólogicos, não apenas pelas variadas e sensíveis paragêneses minerais e presença de fases datáveis, mas também por sua ampla e contínua distribuição ao longo de terrenos, permitindo, assim, estudos integrados e detalhados do metamorfismo e eventos em diferentes regiões. O presente trabalho visou sintetizar aspectos relevantes à caracterização petrológica e geocronológica de metapelitos para regimes de pressão média. Considerando o sistema químico KFMASH (K2O, FeO, MgO, Al2O3, SiO2, H2O), minerais como clorita, muscovita, cloritoide, biotita, estaurolita, granada, cordierita, andaluzita, cianita, sillimanita e feldspato potássico são típicos da paragênese de metapelitos para baixas ou médias pressões, desde que haja disponibilidade química e condições P-T para sua formação. De forma geral, tem-se como distintivas, com o aumento das condições P-T e o aparecimento dos respectivos minerais-índice, as zonas metamórficas da clorita, biotita, granada, estaurolita, cianita, sillimanita e sillimanita + ortoclásio. Para a determinação petrogenética e das condições do metamorfismo desses litotipos, estudos macro- e microestruturais, associados com análises de química mineral e de rocha total, propiciam a aplicação de métodos termobarométricos diversos, desde os convencionais, passando pelos otimizados e diagramas isoquímicos de fases e chegando nos termômetros monoelementares, cada qual com suas especificidades e aplicações. De modo a promover um estudo petrocronológico dos litotipos, fases minerais como zircão, granada, monazita e rutilo, em seus respectivos sistemas isotópicos, possibilitam atribuir idades a esses eventos metamórficos e, com a integração e interpretação dos dados obtidos, construir a trajetória P-T-t-d de formação dessas rochas e dos eventos/estágios dos processos envolvidos. Uma abordagem sistemática, de acordo com as particularidades da rocha, deve ser empregada, garantindo, assim, a acurácia dos resultados obtidos.

Abstract The Moeda Formation (Caraça Group, base of Minas Supergroup) registers the initial rift stages of the Minas Basin in the Quadrilátero Ferrífero, Southern São Francisco craton, SE-Brazil. We present stratigraphic and U-Pb detrital zircon dating analyses of the Moeda Formation that contribute to the comprehension of its sedimentary evolution in the southernmost Gandarela and Ouro Fino synclines (Central-Eastern Quadrilátero Ferrifero). The Moeda Formation sequence consists of a basal proximal alluvial fan with clast-supported conglomerates, grading upward into an alluvial floodplain with quartz arenites, and upper massive sandstones from distal fluvial fans. Because the Moeda Formation is normally limited by similar lithologies at both its base (Nova Lima Group, Rio das Velhas Supergroup) and top (Batatal Formation, uppermost Caraça Group) and stratigraphic inversion has occurred in the study area, U-Pb detrital zircon dating was also extended to these units to differentiate them stratigraphically. The youngest clusters of detrital zircon ages in the Nova Lima Group and Moeda and Batatal formations were 2716, 2777 and 2786 Ma, respectively. Based on our data and relevant literature, the depositional age of the Moeda Formation was interpreted as between 2716 and 2520 Ma with the Mesoarchean continental crust, the Rio das Velhas Supergroup and the Archean TTG complexes as its main source areas.

ABSTRACT In the Southeastern Quadrilátero Ferrífero, a package of metapelitic rocks previously attributed to the Archean Rio das Velhas Supergroup crops out in Piranga locality. This study presents the mineral chemistry and U-Pb-Hf zircon geochronology on foliated staurolite-garnet mica schists. Garnet and staurolite index minerals are syn- to post-kinematic towards the main schistosity. Garnet porphyroblasts display well-developed compositional zoning of Mg-Fe-Mn-Ca, with increase of almandine and pyrope and decrease of spessartine towards the rim, implying in prograde metamorphic pattern. Estimates of P-T values for the metamorphic peak resulted in temperatures between 630 to 650ºC and pressure around 7 kbar. Pseudosections show well-defined stability fields in amphibolite facies, with a metamorphic path displaying progressive increase in P-T conditions. Maximum depositional age of 1,875 ± 51 Ma is established for the Piranga mica schists pointing to a depositional history that is younger than those previously described. Metamorphic Cambrian ages characterize the strong influence of deformational processes related to the final stages of Brasiliano Orogeny in the Southeastern Quadrilátero Ferrífero.