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Título

IN SITU Rb–Sr GEOCHRONOLOGY OF SLICKENSIDES REVEALS REACTIVATION OF CRATONIC MARGINS POST–GONDWANA ASSEMBLY

Texto do resumo

The investigation of the deformation ages of orogenic structures and their evolution are essential, but presents some difficulties, especially in low to medium temperature shear zones. The Araçuaí Orogen, located in southeastern Brazil, is a component of a Neoproterozoic orogenic system that developed between the São Francisco and Congo cratons, inserted in the context of Western Gondwana. The limits between the craton and the external thrust faults of the Araçuaí Orogen occur in a tectonic contact between the rocks of the Espinhaço Supergroup, Macaúbas Group and the Bambuí Group, characterized as a foreland basin affected by a thin skin deformation of those thrust fronts and which records the first fossils of complex life, such as Cloudina sp. In this context, this research explores the structural relations between the Bambuí Group and the Araçuaí Orogen from localities of Lapinha da Serra and Serra do Cipó (MG), to understand the deformational phases and kinematics, in addition to the Rb–Sr geochronology from fault-related rocks. Here we chose six samples for in situ Rb–Sr geochronology of fine-grained muscovite and biotite from Bambuí Group and Macaúbas Group, collected in contexts of deformation bands, crenulation cleavage and slickensides. Structural analyzes in the Serra de Santa Helena, Sete Lagoas and Lagoa do Jacaré formations, the main outcropping units of the Bambuí Group, suggest a D1 folding, responsible for a sinistral transpression, with a top to the W. Five structures were identified: S0 sedimentary bedding, S1 foliation, generated by the interstratal sliding, S2 axial plane foliation, Lm mineral stretching lineation and bidirectional fractures N-S and E-W. The mineral phases (calcite, quartz, biotite, muscovite, chlorite and pyrite) are organized into a fine-grained domain (FGD) and a coarse-grained domain (CGD) in the samples and thin sections presented, controlling the geometry and rheology of the structures generated by D1. The micas, derived from structures related to D1, record isochronous ages between 500–450 Ma, interpreted as fault reactivation at the dawn of Gondwana assembly, caused by the gravitational collapse of the Araçuaí Orogen. This orogen presents metamorphic and anatexis peaks around 560 and 530 Ma, according to regional geochronological data. However, in agreement with U–Pb and Sm–Nd data from low-temperature minerals in the craton/orogen boundary zone and in the Quadrilátero Ferrífero, the action of a third metamorphic/deformational event was defined, in the aforementioned interval of 500–540 Ma, that indicates a thrust reactivation ca. 80-30 Ma younger than the last metamorphic peak in Araçuaí Orogen. The alternation between FGD and CGD aligned with structural control were able to record the progressive deformation by the Rb–Sr geochronological method, demonstrating that micas FGD can be a reliable geochronometer for mylonitization. The Rb–Sr ages define a minimum deposition age of ca. 500 Ma for the rocks of the Bambuí Group, in addition to demonstrating that the Neoproterozoic mountain belts that bordered this basin remained with high relief during the main moment of biogeochemical diversification in the planet Earth’s history.

This work is supported by Instituto Serrapilheira (Serra-1912-31510) and CNPq (Process 408815/2021–3).

Palavras Chave

Rb–Sr geochronology; Cratonic margins; Gondwana Orogen; Fault reactivation