EM - Escola de Minas

Agora exibindo 1 - 2 de 2

Time and isotopic constraints for Early Tonian basaltic magmatism in a large igneous province of the São Francisco – Congo paleocontinent (Macaúbas basin, Southeast Brazil).
(2022) Souza, Maria Eugênia Silva de; Martins, Maximiliano de Souza; Queiroga, Gláucia Nascimento; Soares, Antônio Carlos Pedrosa; Dussin, Ivo Antonio; Castro, Marco Paulo de; Serrano, Paula
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.
Paleoenvironment, sediment provenance and tectonic setting of Tonian basal deposits of the Macaúbas basin system, Araçuaí orogen, southeast Brazil.
(2019) Souza, Maria Eugênia Silva de; Martins, Maximiliano de Souza; Queiroga, Gláucia Nascimento; Leite, Mariana; Oliveira, Rosana Gonçalves; Dussin, Ivo Antonio; Soares, Antônio Carlos Pedrosa
The Neoproterozoic Macaúbas Group represents the main precursor basin system of the Araçuaí orogen, including a Tonian volcano-sedimentary succession, at the base, overlain by a Cryogenian sequence. Belonging to the Tonian succession, the Matão – Duas Barras Formation, exposed at the southern border of the Porteirinha basement block in the proximal (western) Araçuaí orogen, represents the very first stage of rifting ever recorded in the Macaúbas basin system. Sedimentological analysis on that formation has revealed two distinct and consecutive facies associations, both indicating half-graben sedimentary filling: i) fault-sourced alluvial fans; and ii) fluvial deposits associated with braided rivers. U-Pb ages of detrital zircon grains from the Matão – Duas Barras Formation and its underlain unit (ascribed to the Espinhaço Supergroup) uniformly show a dominant Rhyacian to Orosirian peak at ca. 2182 Ma – 1863 Ma, as well as several minor Archean age peaks from ca. 3186 Ma to ca. 2655 Ma. The Matão – Duas Barras fluvial deposits also show two Mesoproterozoic age clusters at ca. 1520 Ma and ca. 1156 Ma. Together with sedimentological features, the Archean and Paleoproterozoic zircon ages indicate sediment provenance from the granite-gneiss sources located in the neighboring Porteirinha block, which would have acted as a basement high (a large horst) at least during the initial filling of the Early Tonian Macaúbas rift. The ca. 1520 Ma and ca. 1156 Ma peaks are in good agreement with the Calymmian and Stenian development stages (II and III) of the Espinhaço Supergroup, which acted as an important secondary sediment sources for the basal Macaúbas Group. This also suggests that the Mesoproterozoic sources were exposed by the Porteirinha block uplift coeval with the Matão – Duas Barras basin filling. The maximum sedimentary ages given by the youngest zircon grains, together with a thorough literature compilation, bracket a depositional interval for the basal Macaúbas deposits from 1000 Ma to 933 Ma. Changes in sedimentary style and geochronological spectra suggest variations in the tectonic activity and subsidence rates of the Early Tonian Macaúbas basin, marking its first stage as an entirely siliciclastic sedimentation.

EM - Escola de Minas

Notícias

Navegar

Filtros

Configurações

Ordenar por

Resultados por página

Resultados da Pesquisa