Contrasting provenance and timing of metamorphism fromparagneisses of the Araçuaí-Ribeira orogenic system, Brazil : hints for Western Gondwana assembly.
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2017
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Abstract
The Araçuaí orogen and the Ribeira belt make up a complex Neoproterozoic-Cambrian orogenic system, the
Araçuaí-Ribeira orogenic system(AROS) located fromthe 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 fromthe 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 fromdetrital zircon grains and contrasting Hf isotopic signatures suggest a complex sedimentary history.
Located in thewestern sector of the study region, the samples RC-02 and RC-34, with an 80% age peak of detrital
zircon grains from 2158 Ma to 1830 Ma, εHf(t) from −2.2 to −22.7, and Hf TDM model ages from 3530 Ma to
2440 Ma, suggest sediment sources located in the São Francisco craton basement. The samples RC-03, also
fromthewestern sector, and RC-46 fromthe southern sector, have a more complex assemblage of detrital zircon
grains with an 87% age peak from 987 Ma to 592 Ma, εHf(t) from +14.9 to −2.9, and Hf TDM model ages from
2220Ma to 720 Ma, 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 systemof 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 650 Ma and 552 Ma 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 621 Ma to 480 Ma, with the main collisional
activity between 580Ma and 540 Ma. 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 = 6.43 kbar. 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 614 Ma; iii) the final amalgamation of Western
Gondwana took place around 540 Ma in the focused region; iv) an important re-heating period (520–480 Ma)
can be related to the AROS gravitational collapse, afterWestern Gondwana assembly.
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Keywords
Sediment provenance, Timing of metamorphism
Citation
DEGLER, R. et al. Contrasting provenance and timing of metamorphism fromparagneisses of the Araçuaí-Ribeira orogenic system, Brazil: hints for Western Gondwana assembly. Gondwana Rerearch, v. 57, p. 30-50, 2017. Disponível em: <http://www.sciencedirect.com/science/article/pii/S1342937X16303446>. Acesso em: 25 ago. 2017.