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Detrital zircon ages are an important tool to study the provenance and evolution of sedimentary rocks, and currently there is a general lack of such studies in greenstone belt basins. In this paper, we combine field observations and in situ U–Pb/Lu–Hf detrital zircon analyses to constrain the evolution of a large, ancient convergent basin in SE Brazil. The Maquiné Group (the uppermost clastic sequence of the Rio das Velhas Greenstone Belt) was deposited during a fundamental shift in the evolution of the southern São Francisco Craton that ended with closure of the greenstone belt basin and the stabilization of the continental crust at ca. 2730–2700 Ma. We show that detritus accumulated in the Maquiné basin derived from uplifted nearby source rocks and that sedimentation was marked by drastic upward lithological changes (from flysch to molasse-type sedimentation). The restricted distributary provenance (marked by a strong uni-modal age spectra at 2770–2780 Ma) requires that sedimentation was concomitant with exhumation of a proximal 2770–2780 Ma TTG magmatic arc. The entire detrital zircon spectra show that maximum deposition ages are very close to the timing of deposition, and confirm that deposition occurred during or immediately after tectonic convergence. Variations in ɛHf (t) values support the idea that the Archaean crystalline crust of the craton was built by crust–mantle mixing processes with a successive decrease of values in zircons crystallized after 3100 Ma. In a regional context, our dataset supports previous interpretations of a long-lived evolution of the southern São Francisco Craton comprising a succession of magmatic arcs and Archaean convergent basins. [ABSTRACT FROM AUTHOR]

Offshore the emissions of dihydrogen are highlighted by the smokers along the oceanic ridges. Onshore in situ measurements in ophiolitic contexts and in old cratons have also proven the existence of numerous H2 emissive areas. When H2 emanations affect the soils, small depressions and vegetation gaps are observed. These depressions, called fairy circles, have similarities with the pockmark and vent structures recognized for long time in the sea floor when natural gas escapes but also differences. In this paper we present a statistic approach of the density, size, and shape of the fairy circles in various basins. New data from Brazil and Australia are compared to the existing database already gathered in Russia, USA, and again Brazil. The comparison suggests that Australia could be one of the most promising areas for H2 exploration, de facto a couple of wells already found H2, whereas they were drilled to look for hydrocarbons. The sum of areas from where H2 is seeping overpasses 45 km2 in Kangaroo Island as in the Yorke Peninsula. The size of the emitting structures, expressed in average diameter, varies from few meters to kilometers and the footprint expressed in % of the ground within the structures varies from 1 to 17%. However, globally the sets of fairy circles in the various basins are rather similar and one may consider that their characteristics are homogeneous and may help to characterize these H2 emitting zones. Two kinds of size repartitions are observed, one with two maxima (25 m and between 220 m ± 25%) one with a simple Gaussian shape with a single maximum around 175 m ± 20%. Various geomorphological characteristics allow us to differentiate depressions of the ground due to gas emissions from karstic dolines. The more relevant ones are their slope and the ratio diameter vs. depth. At the opposite of the pockmark structures observed on the seafloor for which exclusion zones have been described, the H2 emitting structures may intersect and they often growth by coalescence. These H2 emitting structures are always observed, up to now, above Archean or Neoproterozoic cratons; it suggests that anoxia at the time the sedimentation and iron content play a key role in the H2 sourcing. [ABSTRACT FROM AUTHOR]

In a recent volume of Precambrian Research (vol. 323, pp. 82–101), Kuribara et al. (2019) published a paper on the Eoarchean to Neoproterozoic evolution of the Mantiqueira and Juiz de Fora Complexes of SE Brazil. In this work, Kuribara et al. (2019) suggest that the complexes were previously indicated to be part of the Neoproterozoic to Cambrian Brasiliano Orogeny and that the results of their work instead show that these complexes are fragments of Archaean crust that accreted during the 590–560 Ma Brasiliano orogeny. Kuribara et al. (2019) also present conventional thermobarometry for both complexes that they interpret represents the single metamorphic event that they obtain zircon U-Pb age data for from the Juiz de Fora complex. No metamorphic ages were obtained from the Mantiqueira Complex, instead they have used the metamorphic ages that many workers have obtained from the Mantiqueira Province. In this comment we show that the Archaean crustal input in the Mantiqueira complex has long been known, and the Juiz de Fora Complex has long been considered a Palaeoproterozoic ocean arc. The Mantiqueira Complex records a polymetamorphic history with high-grade metamorphism occurring at ca. 2020–1940 Ma and a later amphibolite facies Brasiliano overprint at 580–540 Ma. The Mantiqueira Province and Mantiqueira Complex are different geological entities, and while the Mantiqueira Complex is affected by the Brasiliano event and occurs within the Mantiqueira Province, the use of metamorphic ages from the Mantiqueira Province is not appropriate for the Mantiqueira Complex. Finally we show that the data presented by Kuribara et al. (2019) actually fits in with the most recent version of the model for the formation of the two complexes. [ABSTRACT FROM AUTHOR]