Abstract
This paper deals with Rb-Sr whole-rock dates on supracrustal and plutonic rocks of the Oros Belt and their basement, in the SE corner of Ceara State, Northeastern Brazil. Geologically, the region belongs to the so-called Borborema Orogenic Province, of Brasiliano age. The dated rock units include metavolcanics (rhyolites and dacites) and augen gneisses that intrude them in structural conformity, as well as basement granodioritic orthogneisses. Isochronic dates ca. 1.7 Ga obtained from both the metavolcanics and augen gneisses are regarded as emplacement ages, during the initial phases of basin development. Initial Ro values are quite low, compatible with mantle (or deep-level, Rb-depleted crust) derivation. The age values correlate with those obtained in Chapada Diamantina/Espinhaco (Bahia State) as well as in Hoggar (Algerian Sahara), for middle proterozoic anorogenic igneous-sedimentary sequences. It is proposed that the basin started its development as a middle proterozoic intracratonic rift system, which was much later involved in the Brasiliano (Pan-African) orogenic event. A monocyclic evolution for the Oros Belt is thus favoured. The basement orthogneisses furnished an errorchron of ca. 2.4 Ga, regarded as a minimum age of a thermotectonic event in high amphibolite facies, that affected these rocks before deposition of the supracrustal pile.
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