OROSIRIAN CALC-ALKALINE VOLCANISM AND THE OROCAIMA EVENT IN THE NORTHERN AMAZONIAN CRATON, EASTERN RORAIMA STATE, BRAZIL

Abstract

In the eastern portion of the Brazilian state of Roraima there are two main volcanic terranes of Paleoproterozoic age: the Surumu Group to the north and the Iricoumé Group to the south. The Central Guyana Belt is located between these groups. Geochemical and geochronological data point to a dissociation with the anorogenic granitoids related to Saracura and Mapuera suites, presently grouped into the Uatuma Supergroup (Uatumã Event, 1.90-1.70Ga). Chemical characteristics of volcanic rocks are very similar to those observed in the calcalkaline granitoids such as the Pedra Pintada and Água Branca suites (1.96-1.91Ga) pointing to a post-collisional setting, related to the end of Trans-Amazônian Orogeny. The Surumu Group is usually metamorphosed in the lower greenschist facies, with steep foliation and open folding. The Surumu - Iricoumé groups represent a series of acidic to intermediate volcanics, metaluminous and calc-alkaline affinities, similar to those observed in modern magmatic arcs. The REE pattern shows two characteristic groups of volcanics, suggesting distinctive magmatic evolution. Both volcanic-plutonic associated Surumu - Pedra Pintada and Iricoumé - Água Branca and previous molassic sedimentary covers show a new geological framework and are postulated to the Orocaima Event (1.98-1.88 Ga), Orosirian Period