EM - Escola de Minas

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Andean fingerprint on placer sands from the southern Brazilian coast.
(2022) Rizzi, Monique Aparecida Marchese; Dillenburg, Sérgio Rebello; Takehara, Lucy; Girelli, Tiago Jonatan; Wust, Cássia Fatima; Lana, Cristiano de Carvalho; Chemale Júnior, Farid
On the passive margin of southern Brazil, where the availability of sediments and coastal system conditions were adequate for forming a Ti–Zr-bearing placer in the Holocene, could there be a source of detrital contribution other than the Precambrian South American cratons? To answer that, detrital zircon U–Pb–Hf analyses were performed using the LA-ICP-MS method. The U–Pb age pattern distribution of 866 zircon grains showed that 35.3% of the grains belong to the Neoproterozoic, which covers the Brasiliano orogenic belts. However, 9.1% of the total zircons analyzed (up to 12% in some samples) correspond to grains younger than 50 Ma, restricted to the Andean orogeny in southern South America. The Hf signature of the zircons stands out by pointing to five significant and distinct groups interpreted as coming from the Andean (0–50 Ma), Gondwanides (230–380 Ma), Famatinian (380–500 Ma), Brasiliano (850–541 Ma), and Grenville (900–1300 Ma) orogens, confirming an Andean fingerprint as source rocks in addition to the sediments originating from the craton. This Andean source is related to the distal sediment contribution from the La Plata River system and Argentina rivers that discharge in the Atlantic Coast. Based on a statistical and Hf isotope approach, we defined that around 50% of the zircons grains were transported from the Argentina coast and La Plata River by the littoral drift for more than 1000 km to their final sink in the southern Brazilian coast.
U–Pb ages and Hf-isotope data of detrital zircons from the late Neoarchean-Paleoproterozoic Minas Basin, SE Brazil.
(2017) Martinez Dopico, Carmen Irene; Lana, Cristiano de Carvalho; Moreira, Hugo Souza; Cassino, Lucas F.; Alkmim, Fernando Flecha de
Because of its world-class iron ore deposits and promising Au and U mineralizations, the late Neoarchean to Paleoproterozoic Minas Basin (Minas Supergroup, SE of Brazil) is one of the best-studied basins in South America. However, the lack of datable interlayered volcanic rocks prevented discourse over ages of the strata, the sources and the nature of its ore deposits. In this paper, we present detrital zircon U– Pb age patterns coupled with Lu–Hf data for 18 samples, representing different stages of the Minas Basin evolution ( 2000 analyzed zircons). Age spectra for the main basal unit (Moeda Formation) show a classic rift-related detrital zircon pattern, characterized by multiple autochthonous sources, which in turn are much older than the age of deposition. Maximum age for the rifting event is constrained at ca. 2600 Ma. Detritus accumulated at the base of the Minas Supergroup were derived from Archean source rocks and their sedimentation was marked by differential uplift of the Archean crust, shortly after the 2730–2600 Ma high-K calc-alkaline magmatism (Mamona Event). The age of the BIF deposits is younger than 2600 Ma, most likely coinciding with the great oxygenation event between 2400 and 2200 Ma and the precipitation of banded iron deposits worldwide. Detrital zircons from the topmost units of the Minas strata suggest that tectonic inversion and closure of the basin took place at ca. 2120 Ma with the deposition of the synorogenic Sabará Group. Rhyacian zircon supply showing juvenile Hf signatures gives evidence of a late Rhyacian amalgamation between the Mineiro Belt and the craton. The eHf signatures support the hypothesis that the Archean crystalline crust of the craton was mostly built by crust–mantle mixing processes, with a successive decrease of eHf values in zircons crystallized after 3250 Ma and minor mantle-like additions after Paleoarchean times. Regionally, our dataset supports previous interpretations of a long-lived evolution of the southern São Francisco Craton comprising a succession of convergent island arcs, small microplate collisions, and development
The detrital zircon record of an Archaean convergent basin in the Southern São Francisco Craton, Brazil.
(2015) Moreira, Hugo Souza; Lana, Cristiano de Carvalho; Nalini Júnior, Hermínio Arias
Detrital zircon ages are an importanttoolto 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 ofthe greenstone belt basin and the stabilization ofthe 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 Hf(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.

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