PETROGENESIS OF THE LATE PROTEROZOIC CURACA MAFIC DYKE SWARM, BRAZIL - ASTHENOSPHERIC MAGMATISM ASSOCIATED WITH CONTINENTAL COLLISION

The late Proterozoic Curaca mafic dyke swarm is one of the more alkali ne swarms of the Brazilian Precambrian. Intra-dyke chemical variations are characterised by higher abundances of incompatible elements in dy ke margins than in the dyke centres, a feature that may have resulted from flowage differentiation, from progressive evacuation of liquids f rom zoned magma chambers, or possibly through dynamic melting of an up rising diapir. Inter-dyke chemical differences are best interpreted by different extents of melting of a mantle source with a small modal pr oportion of garnet. This source was heterogeneous and had the trace el ement compositions of ocean island basalts with Dupal and non-Dupal ch aracteristics. The emplacement of the Curaca dykes from 650-700 Ma may have been facilitated by the extensional tectonics perpendicular to t he collision zone between the Sao Francisco craton and the Pernambuco- Alagoas massif during the evolution of the Sergipano orogen. The energ y in the system was insufficient to promote much melting of, or intera ction with, the continental lithosphere or overlying crust.