Contrasting provenance and timing of metamorphism from paragneisses of the Araçuaí-Ribeira orogenic system, Brazil: Hints for Western Gondwana assembly

Abstract

The Araçuaí orogen and the Ribeira belt make up a complex Neoproterozoic-Cambrian orogenic system, the Araçuaí-Ribeira orogenic system (AROS) located from the eastern to southeastern Brazil. Along the AROS, the Ediacaran Rio Doce magmatic arc represents a geotectonic connection between the Araçuaí and the Ribeira orogenic domains. Although the nature and evolution of the Rio Doce plutonic rocks is regionally well established, it lacks detailed studies on the paragneisses found along the western and central regions of this magmatic arc. Besides information on the nature and provenance of their sedimentary protoliths, the paragneisses provide data to unravel the palaeogeographic scenario from the precursor to arc-related basins. Six samples of Al-rich gneisses covering a large AROS region were selected for electron microprobe (EMP) mineral analyses in order to obtain geothermobarometric data and monazite ages, as well as for Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) isotopic analyses on zircon (U-Pb, Lu-Hf) and monazite (U-Pb). The different age spectra from detrital zircon grains and contrasting Hf isotopic signatures suggest a complex sedimentary history. Located in the western sector of the study region, the samples RC-02 and RC-34, with an 80% age peak of detrital zircon grains from 2158Ma to 1830Ma, εHf(t) from −2.2 to −22.7, and Hf TDM model ages from 3530Ma to 2440Ma, suggest sediment sources located in the São Francisco craton basement. The samples RC-03, also from the western sector, and RC-46 from the southern sector, have a more complex assemblage of detrital zircon grains with an 87% age peak from 987Ma to 592Ma, εHf(t) from +14.9 to −2.9, and Hf TDM model ages from 2220Ma to 720Ma, indicating provenance from mainly juvenile sources of distinct ages. Candidates to be juvenile sources for RC-03 and RC-46 sedimentary protoliths are the Rhyacian Juiz de Fora and Pocrane complexes in the basement of the Rio Doce arc, the Neoproterozoic Rio Negro arc system of the Ribeira belt, and AROS ophiolite complexes. Samples RC-30 and RC-38 from the eastern sector of the study region, with most detrital zircon ages between 650Ma and 552Ma and very negative εHf(t) (−25.3 to −16.5), suggest main sediment sources in the Rio Doce arc. By extending U-Pb analyses on metamorphic zircon and monazite, we have identified a complex timing of metamorphism, represented by metamorphic ages ranging from 621Ma to 480Ma, with the main collisional activity between 580Ma and 540Ma. Geothermobarometric studies on garnet porphyroblasts, syn-kinematic to the D2 regional foliation, show a retrograde metamorphic path typical of continental collision belts, starting with P-T conditions of Tmax=733°C and Pmax=6.43kbar. Our data also suggest: i) the studied paragneisses represent distinct Neoproterozoic basin stages, shifting from passive to active margin settings; ii) if the Rio Negro arc system really provided sediments for the basin stage represented by the RC-03 and RC-46 paragneisses, it would have amalgamated with the AROS before 614Ma; iii) the final amalgamation of Western Gondwana took place around 540Ma in the focused region; iv) an important re-heating period (520–480Ma) can be related to the AROS gravitational collapse, after Western Gondwana assembly.