Time and isotopic constraints for Early Tonian basaltic magmatism in a large igneous province of the São Francisco – Congo paleocontinent (Macaúbas basin, Southeast Brazil).
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2022
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Abstract
The Neoproterozoic Macaúbas basin (southeast Brazil) provides key hints to unravel the history of the São
Francisco – Congo paleocontinent before its amalgamation as part of Gondwana. Among the several Proterozoic
taphrogenic events and anorogenic igneous episodes documented for the São Francisco – Congo paleocontinent,
Early Tonian event has been particularly focused on in the literature owing to the large production of bimodal
(felsic-mafic) magmas now found in the Araçuaí (SE Brazil) – West Congo (SW Africa) orogenic system (AWCO)
and neighboring cratonic regions. Aiming to examine the stratigraphic relations, timing and isotopic signatures
of extensive basaltic magmatism related to the Macaúbas basin, we carried out detailed field, lithochemical,
isotope (whole-rock Sm-Nd and Lu-Hf in zircon) geochemistry and zircon U-Pb geochronological studies on the
Pedro Lessa suite and Planalto de Minas Formation, located in the central-western Araçuaí orogen. The studied
Pedro Lessa suite consists of a series of mafic dikes that crosscut the lowermost units of the Macaúbas Group but
not the Planalto de Minas Formation. The Pedro Lessa suite yielded zircon U-Pb ages between 951 ± 54 Ma and
939 ± 7 Ma. The Planalto de Minas Formation comprises volcaniclastic rocks crystallized at 889 ± 10 Ma, and
sedimentary rocks with maximum depositional age of 867 ± 10 Ma. The lithochemical and isotopic datasets
(Pedro Lessa dikes: εNd(t) = +0.60; εHf(t) = –22.21 to +4.66; Planalto de Minas volcanism: εNd(t) = +0.76 to
+0.78; εHf(t) = − 19.27 to +10.03) indicate predominantly continental tholeiite and minor enriched MORB-type
mantle magmas with variable amounts of crustal contamination. The (La/Sm)N ratios indicate a theoretical
partial melting (1–7%) of the spinel-lherzoite lithospheric zone for the Pedro Lessa suite, and of the spinel-garnet
transition zone (3–10%) for the Planalto de Minas metamafic rocks, in accordance with La/Nb and La/Ba ratios.
The new and complied isotopic and lithochemical data suggest a time-dependent evolution of magma sources,
with lithospheric mantle sources for the Early Tonian dike swarms (Pedro Lessa and correlatives) evolving to
predominantly asthenospheric mantle sources for the late basaltic volcanism recorded in the Planalto de Minas
Formation. Such an extensive and long-lasting anorogenic magmatism of Early Tonian age (c. 965 – 870 Ma)
found in the AWCO and São Francisco – Congo craton likely records more than one extensional event and related
aborted continental rifts, altogether driven by the same mantle plume. Although long-lived, this mantle plume
acted beneath a strong continental lithosphere so that it was unable to break up the São Francisco – Congo
paleocontinent into two completely separated landmasses. Highlighting the main stages of anorogenic magmatism, we also present an updated barcode for the São Francisco – Congo paleocontinent.
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Keywords
U-Pb geochronology, Sm-Nd isotopes, Lu-Hf isotopes, Rifting, Araçuaí orogen
Citation
SOUZA, M. E. S. de et al. Time and isotopic constraints for Early Tonian basaltic magmatism in a large igneous province of the São Francisco – Congo paleocontinent (Macaúbas basin, Southeast Brazil). Precambrian Research, v. 373, 2022. Disponível em: <https://www.sciencedirect.com/science/article/pii/S0301926822000651>. Acesso em: 29 abr. 2022.