Juvenile contribution of the Neoproterozoic Rio Negro Magmatic Arc (Ribeira Belt, Brazil): Implications for Western Gondwana amalgamation

• Published in: Gondwana research Vol. 21; no. 2-3; pp. 422 - 438
• Main Authors: Tupinambá, Miguel, Heilbron, Monica, Valeriano, Claudio, Júnior, Rubem Porto, de Dios, Fátima Blanco, Machado, Nuno, Silva, Luiz Guilherme do Eirado, de Almeida, Júlio Cesar Horta
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-03-2012
• Subjects: Magmatic arc; Neoproterozoic; Ribeira belt; Tectonics; Western Gondwana; Magmatic arc; Western Gondwana; Tectonics; Ribeira belt; Neoproterozoic
• ISSN: 1342-937X; 1878-0571
• DOI: 10.1016/j.gr.2011.05.012

The ca. 790–600Ma Rio Negro Complex (RNC) of the Ribeira belt (Brazil) consists of a plutonic portion of a magmatic arc built by the E-vergent subduction of the ESE border of the São Francisco paleoplate during the amalgamation of Western Gondwana.
The plutonic series comprises low- to medium-K granitoids (ca. 790–620Ma) and high-K granitoids and shoshonite rocks (ca. 610–605). The age span of 185m.y. is suggestive of a long history of arc-related magmatism, continuously or not in time. The Nd isotopic signatures of the RNC consist of εNd(t) ratios from −3 to +5 for the medium-K series shoshonite series and from −14 to −3 for the younger high-K group. This time-dependent trend of Nd isotopes is indicative of progressive maturity of the arc over time. The same evolution is indicated by Sr data, as the medium-K rocks have 87Sr/86Sr initial ratios<0.705 while the high-K rocks yield values between 0.705 and 0.710. The predominance of intermediate rocks over mafic ones suggests an initial intra-oceanic to transitional stage, possibly developed in a distal portion of a passive margin, such as the Japanese arc, evolving to a more developed, differentiated felsic rock associations.
The role of transform fault zones, such as the Luanda shear zone, is emphasized in order to explain the consumption of a wide oceanic plate in the inner portion of Western Gondwana.
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► Rio Negro arc: plutonic section of a magmatic arc in the interior of West Gondwana. ► Geochemical and isotopic signature similar to modern oceanic island arcs. ► Low- to medium-K group, 790Ma; medium-K group, 630Ma; high-K group, 600Ma.