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

Abstract

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.