EM - Escola de Minas

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Towards an integrated tectonic model for the interaction between the Bambuí basin and the adjoining orogenic belts : evidences from the detrital zircon record of syn-orogenic units.
(2020) Amaral, Matheus Henrique Kuchenbecker do; Soares, Antônio Carlos Pedrosa; Babinski, Marly; Reis, Humberto Luís Siqueira; Atman, Dora; Costa, Ricardo Diniz da
The Sao ̃ Francisco craton and its surrounding orogens are some of the most important tectonic elements of South America, and key regions to understanding the assembly of Western Gondwana in the Proterozoic-Palaeozoic transition. During this major tectonic event, diachronic collisions between small continents developed an intricated orogenic system, where several orogens evolved in unique paths through time and space. In such collisional settings, there are some tectonic processes that promote subsidence, thus controlling the formation of orogenic-related sedimentary basins. Furthermore, the tectonic activity in collisional orogens generates dynamic landscapes that usually favor increased erosion and sediment generation to feed these basins, making them key places to seek for clues about the tectonic evolution of their surroundings. On the S ̃ ao Francisco craton, the Bambuí Group records a complex foreland system, which evolved in response to the lithospheric overload exerted by the uplift of both Brasília belt and Araçuaí orogen. In turn, both Salinas Formation and Ibia ́ Group comprise orogenic deposits resting within the Araçuaí orogen and Brasília belt, respectively, whose tectonic significance is still under debate. Here we present new U–Pb (LA-ICP-MS and SHRIMP) and Lu–Hf isotopic an- alyses on a great amount of detrital zircon grains extracted from the rocks of the Bambuí Group and Salinas Formation, together with a thorough data compilation from the literature. The two units present similar prov- enance patterns, sharing the major detrital zircon age peaks (550–650 Ma, 950–1050 Ma, 1750–2000 Ma, 2600–2800 Ma) and maximum depositional age in c. 550 Ma. The Ediacaran zircons recovered from the Bambuí Group show a wide range of the εHf(t), ranging from c. − 17 to +15, which suggest the existence of multiple late Neoproterozoic sources, some of them juvenile and some with a long crustal residence. The variation in detrital zircon age patterns and εHf(t) values from different units within the Bambuí Group provided additional clues of provenance changes occurred during the evolution of the basin. The new data obtained for the Salinas Formation constrain its deposition between 548 and 500 Ma, which have an important implication on its tectonic signifi- cance. We propose that in both Brasília belt and Araçuaí orogen sides, the early foredeep deposits of the Bambuí basin should have been incorporated to the orogenic domains, which could explain the apparent lack of deposits recording the climax of the Brasilia belt uplift (c. 630 Ma) within the cratonic area. In this same direction, we consider that both Salinas Formation and Ibi ́ a Group could represent remnants of these early foreland deposits related to the uplift of Brasília belt and Araçuaí orogen, respectively, incorporated to the orogenic wedges due to the advance of the deformational fronts. Therefore, what we know as Bambuí Group is in fact the remaining record of an advanced stage of the foreland system, when subsidence was already influenced by the two evolving orogens. Altogether, the analyses of the stratigraphic, structural and geochronological data converge towards an integrated tectonic model for the interaction between the Bambuí basin and the surrounding orogens during West Gondwana amalgamation.
Geochemistry and U–Pb zircon ages of the metamafic-ultramafic rocks of the Riacho dos Machados metavolcanosedimentary sequence : evidence of a late rhyacian back-arc basin during the assembly of São Francisco-Congo paleocontinent.
(2021) Leal, Victor Luiz Silva; Amaral, Matheus Henrique Kuchenbecker do; Barbuena, Danilo; Queiroga, Gláucia Nascimento; Pinheiro, Marco Aurélio Piacentini; Freimann, Marcelo de Almeida
The São Francisco paleocontinent comprises Archean nuclei and Paleoproterozoic magmatic arcs which were amalgamated during Siderian to Orosirian thermal-tectonic events (ca. 2.4–2.0 Ga). Located in the Porteirinha domain, one of these Archean segments, the Riacho dos Machados metavolcanosedimentary sequence encompasses metamafites and metaultramafites intercalated with metasedimentary rocks. The metamafites clustered in the Type I category are tholeiitic, present MORB-like affinities and flat REE patterns with enrichment in La, Rb, and Cs. The metamafites from Type II are calc-alkaline, showing arc signatures with enrichment in LILE (Cs, Ba, U, Rb, K) and LREE, and depletion in the HFSE. The associated metaultramafites were classified as high-Mg ultramafic rocks similar to Barberton-Type komatiites, with enrichment in LILE and LREE, Al2O3/TiO2< 16 and Gd/Yb(N) > 1. U–Pb (LA–SF–ICP-MS) zircon analyses were carried out on the Type II hornblendite, unraveling a concordant crystallization age of 2071 ± 9 Ma. Inherited zircons yield a discordant age of 2922 ± 22 Ma and a lower intercept at 473 ± 48 Ma. The hybrid magmatism of the studied metamafites coupled with the inherited zircon grains, indicate that this metavolcanosedimentary sequence was developed in an intracontinental back-arc basin. The rocks from Type I have been originated from MORB-like source with a slightly input of subduction-derived fluids. In its turn, Type II samples represent melts more influenced by the fluids from the dehydrated slab. This assumption implies that the São Francisco paleocontinent, in the region of the Porteirinha block and surroundings, were under an accretionary stage at the late Rhyacian. During this orogenic process the metamorphic PT conditions reached lower-amphibolite facies. Moreover, this thermaltectonic event in likely responsible for the auriferous fluid percolation of the Riacho dos Machados Gold Mine.

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