Tectonic evolution of the Juvenile Tonian Serra da Prata magmatic arc in the Ribeira belt, SE Brazil: Implications for early west Gondwana amalgamation

The evolution of the Ribeira belt resulted from the progressive amalgamation of several terranes against the eastern margin of the São Francisco Craton between ca. 620 and 580 Ma. This work brings new field, U-Pb geochronology, geochemistry and isotopic (Sm-Nd and Sr) data on the evolution primitive rocks from the Serra da Prata magmatic arc and their relationships with the previously described Rio Negro arc. The new U-Pb data allow the distinction of two episodes of arc generation: the Serra da Prata Arc (856–838 Ma) and the Rio Negro Arc (790–620 Ma). Rocks from the oldest stage are composed of metaluminous calc-alkaline diorites, tonalites and granodiorites, and geochemical signatures compatible with magmatic arc scenarios. Their rocks are associated to a metamorphosed volcano-sedimentary of intra or back-arc basin setting platform carbonates, amphibolites (basaltic lavas) and psammitic rocks of the Italva group. Whole-rock Nd and Sr isotope data indicate more primitive contribution than earliest stage: initial εNd = −3.7 to +5.2, TDM = 1.68–0.92 Ga and ⁸⁷Sr/⁸⁶Sr initial ratios between 0.7061 and 0.7113. The second stage – Rio Negro arc – yielded more mature arc signatures: initial εNd = −8.4 to −2.5, TDM = 1.93–1.33 Ga and ⁸⁷Sr/⁸⁶Sr initial ratios between 0.7098 and 0.7211. The new data have been interpreted as an evolution of a Tonian primitive intra-oceanic stage of the magmatic arc generation, followed by more continental or transitional arcs during the Rio Negro stage. The data from both arc stages contrast with the younger Serra da Bolívia and Rio Doce continental arcs (570–590 Ma) developed in a proximal location. The data are similar to other Tonian-Ediacaran magmatic arcs: the Goiás arc in the Brasília Belt (ca. 862–630 Ma) and the São Gabriel arc (ca. 840–690 Ma), located respectively along the western margin of the São Francisco and Rio de La Plata cratons. In a Western Gondwana scenario, the juvenile signature indicates intra-oceanic tectonic settings. The combination of the older Tonian arcs with the more evolved Cryogenian to Ediacaran arcs within the Neoproterozoic belts, suggests more than 200 m.y. of subduction around the older cratonic blocks that made up Western Gondwana.