Neoproterozoic (740-680 Ma) arc-back-arc magmatism in the Sergipano Belt, southern Borborema Province, Brazil

摘要

The Novo Gosto amphibolites represent the oldest unit of the Caninde ' domain, a Neoproterozoic sequence located in the NW Sergipano Belt, southern Borborema Province, Brazil. The domain comprises an association of metavolcano-sedimentary rocks, gabbroic intrusions, bimodal magmatic rocks, and syn-to post-tectonic granitoids. Due to the lithostratigraphic and geochemical complexity of this domain, several controversial interpretations of the tectonic setting have been proposed over the years. The protholits of the Novo Gosto amphibolites are here classified as high Fe tholeiitic basalt type rocks. Three geochemical groups were identified: Group 1 is more depleted in incompatible elements and has a pattern similar to island arc basalts with HFSE depletion (Nb-Ta, Zr-Hf, weak Ti); Group 2 displays a pattern similar to Group 1, but with more evident anomalies (Nb-Ta, Ti) and varied enrichment in incompatible elements; Group 3 is more enriched in incompatible elements than Group 1 rocks, with the pattern being flatter with milder HFSE depletion (Nb-Ta, Zr-Hf, weak Ti, Y) and resembling a geochemical pattern slightly more enriched than E-MORB. Our results suggest an environment similar to the Ryukyu-Okinawa (Japan) and Kamchatka arc-back-arc (Russia) setting. LA-ICP-MS U-Pb zircon dating of one amphibolite sample resulted in a date of 743 +/- 3 Ma and confirms the geological field relationships that indicate this unit to be the oldest in the domain. The new petrographic, geochemical, and isotopic data for the Novo Gosto amphibolites lead to the recognition of an ancient Neoproterozoic arc-back-arc system, a rare setting in the earlier phases of the Brasiliano Orogeny within the Borborema Province, which ended with the closure of the restricted Caninde ' sea and the associated Sergipano ocean at around 630 Ma.