Abstract
Dikes which cut Archean and Proterozoic sequences and the Brasiliano structures in the Southern Espinhaço Region were dated using the conventional KAr method. The particular petrographic characteristics of the swarm in relation to those of the Precambrian, abundant in the region, provide no evidence of deformation or metamorphism. The geochronological results indicate that emplacement of the dikes was related to the Mesozoic extensional tectonics and point to a period of magmatic activity during Upper Triassic/Lower Jurassic, between 170 and 220 Ma. Data indicate that the swarm is earlier than the main Phanerozoic swarms of southeast Brazil which are related to the opening of the South Atlantic, and that the dikes were emplaced during the initial breakup of Gondwana.
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