Detrital zircon U–Pb and Lu–Hf constraints on the age, provenance and tectonic setting of arc-related high-grade units of the transition zone of the Araçuaí and Ribeira orogens (SE Brazil).
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2022
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Sedimentary basins with epiclastic and volcanoclastic components are important geotectonic components for
understanding the complex evolution of orogenic systems. The age, provenance and tectonic setting of ancient
basins are, however, often hard to constrain, especially in the high-grade core of ancient mountain belts where
original sedimentary and stratigraphic relations have been largely obliterated by deformation and metamorphism. In this case, analytical tools such as whole-rock geochemistry and U–Pb and Lu–Hf analyses on
detrital zircon grains can be deployed to unravel the provenance and tectonic setting of high-grade metasedimentary and metavolcaniclastic units. Here, we apply these analytical tools on samples from gneissic paleosomes
of migmatites found in the eastern transition zone between the Neoproterozoic–Eopaleozoic Araçuaí and Ribeira
orogens (southeastern Brazil). The studied paleosomes comprise peraluminous paragneiss rich in biotite, garnet
and sillimanite, metaluminous hornblende-bearing gneiss free of peraluminous silicates, and intermediate terms
between these compositions, all of them with geochemical characteristics pointing to sources located in
magmatic arc systems. Overall, U–Pb data from detrital zircon grains bracket a maximum sedimentation age for
all samples between 600 and 627 Ma, while εHf(t): +5.7 to − 18.7 and Hf TDM model ages of 1.0–2.2 Ga suggest
moderately juvenile to evolved sources for the gneiss protoliths. However, despite the similarity in age range, the
analyzed samples show contrasting isotope data indicating significant variability in the source areas. Sample 7
only has grains with negative εHf (− 5.9 to − 18.7), most of them ranging in age from 708 Ma to 758 Ma, with
very few older (1021–1143 Ma) and younger (642–651 Ma) zircons grains, indicating evolved crustal sources of
distinct ages (e.g., the Late Tonian – Early Cryogenian South Bahia Alkaline Province, the Early Cryogenian
magmatism found in the West Congo Belt, the Stenian to Early Tonian Espinhaço and Matadi-Noqui igneous
rocks, and magmatic rocks of the early Rio Doce arc). Conversely, sample 13 has twenty grains with positive εHf
values and ages between 602 and 758 Ma, suggesting sourcing from (meta)igneous rocks of the Rio Negro arc
and their remelting products, and forty grains with negative εHf values with ages between 591 and 646 Ma
suggesting magmatic sources in the Rio Doce arc. Additionally, the two youngest spot ages (553 Ma and 571 Ma)
have highly negative εHf values (− 11.1 and − 16.4) suggesting their origin from collisional anatexis. Similarly,
sample 12 shows detrital grains within the consistent time interval (ca. 596 Ma – 798 Ma) of the Rio Negro arc
and Rio Doce arcs, reinforcing that the fragments of these arcs, or zircons that were inherited from these terranes,
are potential sources to the sedimentary protoliths of the paragneisses. The regional scenario together with our
field and analytical data, compared with data available in the literature, suggest the sedimentary protoliths of the
studied samples filled orogenic basins related to the Rio Doce magmatic arc. Most detrital zircon ages are close to
the maximum depositional age, a classical signature of orogenic basins related to magmatic arcs. The studied
paragneiss unit correlates with the Nova Ven´ecia Complex, located on the intra-arc to back-arc zones of the
Araçuaí orogen, and with the orogenic succession of the distal Andrelândia Group, shared by both the Araçuaí and Ribeira orogens. Furthermore, our data also suggest the juvenile Rio Negro - Serra da Prata arc system was either amalgamated with the Rio Doce arc or that fragments of these arcs were incorporated as unmelted xenoliths and xenocrystals within the Rio Doce arc rocks before ca. 614 Ma, providing key hints to unravel the evolution of the Araçuaí - Ribeira orogenic system (AROS) and its role in the Gondwana assembly.
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Gondwana, Sedimentary provenance, Magmatic arc, Brasiliano Orogeny, Detrital zircon
Citação
SANTIAGO, R. et al. Detrital zircon U–Pb and Lu–Hf constraints on the age, provenance and tectonic setting of arc-related high-grade units of the transition zone of the Araçuaí and Ribeira orogens (SE Brazil). Journal of South American Earth Sciences, v. 116, artigo 103861, 2022. Disponível em: <https://www.sciencedirect.com/science/article/pii/S0895981122001511>. Acesso em: 15 mar. 2023.