NEOPROTEROZOIC OCEANIC REMNANTS IN EASTERN BRAZIL - FURTHER EVIDENCE AND REFUTATION OF AN EXCLUSIVELY ENSIALIC EVOLUTION FOR THE ARACUAI WEST CONGO OROGEN

The Aracuai (eastern Brazil) and West Congo (southwestern Africa) belt s are counterparts of the same Neoproterozoic orogen located between t he Sao Francisco and Congo cratons. The Macaubas Group represents a ma jor passive margin sequence and is a key unit for interpreting the evo lution of that orogen. The Salinas Formation is the distal rock assemb lage of the Macaubas Group and consists of a deep-sea sand-mud sequenc e, and a volcanic-sedimentary unit called the Ribeirao da Folha facies . The latter includes metamorphosed volcanic-exhalative sediments asso ciated with ocean-floor basalts (amphibolites). The magmatic protolith s of these amphibolites crystallized at about 816 +/- 72 Ma (Sm-Nd who le-rock isochron, epsilon(Nd(t)) = +3.8 +/- 0.2). Regional metamorphis m reached the amphibolite facies at about 630 Ma (Rb-Sr whole-rock iso chron), when slabs of ultramafic rocks were tectonically emplaced over the Ribeirao da Folha facies. We consider this volcanic-sedimentary f acies and the coeval slabs of ultramafic rocks to be remnants of a bra nch of the Adamastor-Brazilide ocean. The extensive occurrence of synt ectonic to rate tectonic calc-alkalic granitoids along the internal do main of the Aracuai belt implies that a reasonably large amount of oce an crust was consumed, via an east-dipping subduction zone, during for mation of the Aracuai-West Congo orogen.