Long-term magmatism in the Ribeira Orogen, Western Gondwana: SHRIMP U-Pb and O isotope data for two Ediacaran-Cambrian magmatic events in the Rio de Janeiro area

Abstract

In the southeastern Brazilian margin, a Neoproterozoic to the Early Paleozoic long-term magmatic interval is recorded within the Oriental Terrane, part of the Ribeira Orogen, characterizing it as a hot orogenic belt. Metaigneous and igneous rock units of ca. 570–540 Ma and 510–480 Ma are coeval with two main collisional orogenies related to the Western Gondwana assembly: the Ribeira Orogeny (590–565 Ma) and the Búzios Orogeny (540–490 Ma). The Oriental Terrane, interpreted as a continental microplate amalgamated during the convergence between the São Francisco and Congo cratons, comprises Cryogenian to Ediacaran metaigneous rocks and Cambro-Ordovician plutons intrusive in metasedimentary sequences. Zircons from an orthogneiss and a porphyritic leucogneiss sample gave SHRIMP U-Pb crystallization ages of 558 ± 2 Ma and 560 ± 2 Ma, respectively, with high δ18O values from 7 ‰ to 10 ‰. Zircons from a tonalite pluton and tonalitic dikes, gave SHRIMP U-Pb crystallization ages of 483 ± 2 Ma and 492 ± 2 Ma, with lower δ18O values, between 6 ‰ to 8 ‰. These two magmatic pulses, integrated with a compiled database from other plutons in this terrane, corroborate with the hypothesis that they represent crustal scale magmatic provinces here interpreted as pre- and post-collisional in relation with the Búzios Orogeny. Their distribution within the tectonic map of the Oriental terrane reflect a paleogeography linked to their tectonic origin. The 570–540 Ma plutons are aligned ENE -WSW, parallel and close to the suture zone with the Cabo Frio Tectonic Domain, representing a crustal melt related to a NW-subduction. A subsequent magmatic hiatus of 30 ma is interpreted as the collision of the Cabo Frio Tectonic Domain. The post collisional period is marked the intrusion of non– linear 510–480 Ma pluton, widespread in the Oriental terrane.