U-PB AND SM-ND GEOCHRONOLOGICAL CONSTRAINTS ON THE CRUSTAL EVOLUTION AND BASEMENT ARCHITECTURE OF CEARÁ STATE, NW BORBOREMA PROVINCE, NE BRAZIL: IMPLICATIONS FOR THE EXISTENCE OF THE PALEOPROTEROZOIC SUPERCONTINENT "ATLANTICA"

Geological and geochronological data for the northwestern part of the Brasiliano Borborema Province are described and compared with their counterparts in the Pan-African Dahomey (Pharusian) belt that flanks the southeastern margin of the West African Craton, where outcrops are sufficiently continuous to discern the nature of the collision during West Gondwana assembly. In the Médio Coreaú domain, NW Borborema Province, U–Pb and Sm–Nd data have revealed unusual basement rocks representing 2.35–2.30 Ga juvenile crust, along with large tracts of 2.15–2.10 Ga juvenile gneisses in the Ceará Central domain. These basement blocks were affected by two pulses of intracratonic extension at 1785 and 775 Ma. Prior to West Gondwana collision, a continental arc (the Santa Quitéria batholith) developed between 665 Ma and 620 Ma. The presence of this arc strengthens the hypothesis that convergence between the Borborema Province and the São Luis craton involved closure of an oceanic basin. New geochronological data are presented showing that Palaeoproterozoic orthogneisses (U–Pb upper intercept 2288±2 Ma) were affected by a major late Neoproterozoic event (554±4 Ma U–Pb lower intercept, 558±3 Ma Sm–Nd whole-rock and mineral isochron). Exhumation and cooling of granulite rocks between 568 and 550 Ma in the Médio Coreaú domain and between c . 587 and 576 in the West African Dahomey Belt indicate that the final tectonic phase was not simultaneous along this front of the orogen.