Geocronologia U-PB, análises isotópicas LU-HF e litoquímica da Região de Ribeirão da Folha, Nordeste de Minas Gerais: implicações tectonossedimentares para a zona de sutura do Orógeno Araçuaí

Abstract

The Araçuaí Orogen, located in southeastern Brazil, is part of a Neoproterozoic orogenic system developed between the São Francisco and Congo cratons in the Western Gondwana setting. In the vicinity of this orogen's suture zone, the Ribeirão da Folha region and its homonymous ophiolitic complex occur. This work aims to present a synthesis of this complex, and address particular attention to its upper section related to the oceanic turbidite sedimentation of distal passive margin with investigations concerning its sedimentary origins. Three samples (LC-004, LC-019 and LC-037) were selected for detrital zircons analyzes with combined isotopic data (U-Pb and Lu-Hf), and ten samples for whole rock geochemistry analyzes. Sample LC-037, representative of Unit A (predominantly metapelitic), and sample LC-019, representative of Unit B (predominantly metapsamitic), exhibit maximum sedimentation ages of 741 Ma and 673 Ma, respectively. Both present great similarity in their age spectra and in their Hf isotopic signatures, which are compatible with the Ribeirão da Folha Formation, top unit of the Macaúbas Basin, and precursor to the Araçuaí Orogen. Sample LC-004 exhibits detrital zircons of Ediacaran and Cryogenian ages (599 Ma to 648 Ma), with Hf (t) of -7.2 to -6.3, derived the Rio Doce Magmatic Arc, and consistent with the characteristic contributions of the syn-orogenic Salinas Basin. The whole rock geochemistry reveals that most of the analyzed metapelitic rocks show enrichment of LREE in relation to HREE ([La/Yb = 11.9 5.3]N), negative Eu anomaly ([Eu/Eu* = 0.8 0.4]N), and a relatively flat distribution pattern for the HREE ([Gd/Yb = 1.8 1.3]N), though that different sedimentary contributions are also part of the of setting, with contrasting patterns to the others. Samples with similar geochemical characteristics exhibit Th/U (4.3 to 7.8) and Rb/Sr (>0.5) ratios which indicate an important history of sediment recycling and weathering processes, and Th/Sc (0.5 to 1.2) and Zr/Sc (5.6 to 8.2) ratios suggest that sedimentary contributions are derived mainly from evolved igneous rocks. Multidimensional geochemical diagrams indicate that the sedimentary contributions are mainly derived from collisional geotectonic environments, with minor participation of magmatic arc and continental rift environments. By means of microstructural analyzes in the metapelites, three deformation phases were recognized, with the progressive metamorphic peak in the amphibolite facies, recognized in the mineral association of its main deformation phase (Dn). The data and structural interpretations of Dn suggest a top to SSW-SW shear, dominated by dextral transpression. Given the large lithological variability in a reduced area, with the involvement of several evolutionary stages of the Araçuaí Orogen (from the precursor basin to the syn-orogenic basin, with presence of ophiolitic bodies and late to post-collisional intrusive granites), as well as its complex depositional and deformational history, it is possible to point out the ophiolitic complex of Ribeirão da Folha as the possible tectonic mélange region of the Araçuaí Orogen.