Geocronologia U-PB em zircões detríticos e caracterização litoestratigráfica das rochas metassedimentadas da Unidade Catarina Mendes- implicações quanto a evolução geotectônica do Quadrilátero Ferrífero

Abstract

The Catarina Mendes unit (CMU) is a greywacke-argillite metasedimentary association of the Rio das Velhas greenstone belt (RVGB), in the Quadrilátero Ferrífero province (QF), southern São Francisco craton. This unit is stratigraphically positioned between the granite-gneiss terrains that constitute the crystalline basement of the QF and the paleoproterozoic metasedimentary sequence of the Minas Supergroup. The RVGB rocks are constrained in the homonymous Supergroup, which comprises three groups (from bottom to top): i) Quebra-Osso Group (metamafic-ultramafic complex); ii) Nova Lima Group (metavulcano-sedimentary sequence); and iii) Maquiné Group (siliciclastic metasedimentary rocks). To southeast of the QF, in the core area of the Mariana anticline, i.e. between the flanks of this regional fold, topographically represented by the Ouro Preto and Antônio Pereira ridges, the upper Nova Lima Group is subdivided into three informal units, named Catarina Mendes, Córrego da Paina and Fazenda Velha. These units comprise turbidite-gravel-clay associations that show transitional contacts with each other and common intercalations of amphibolites, tourmalinites and carbonate-rich rocks, besides metaconglomerates. The metasedimentary rocks in these upper units of the Nova Lima Group show a compositional banding marked by rhythmic intercalation of centimetric layers and an upward coarsening pattern. These rocks register amphibolite facies metamorphic conditions containing staurolite, biotite and garnet assemblages, which later retrogressed to greenschist facies conditions. In addition, this work presents new geochronogical data for CMU and new insights of its source area and environmental setting through detailed lithostratigraphy at its section type area, besides petrographic and lithogeochemical investigation. Two samples were analyzed (U-Pb LA-ICP-MS in detrital zircon grains) to constrain maximum depositional age, that yielded of 2481 ± 12 Ma. The U-Pb isotopic results also presented Cryogenian and Rhyacian records, respectively at 678 ± 4 Ma and 2114 ± 5 Ma, considered as tectono-metamorphic overprints. These data disclose new elements on the QF architecture and basins development and unravel how CMU relates to the Rio da Velhas and Minas Supergroups.