THE POST-PALEOZOIC VOLCANISM OF NORTHEAST BRAZIL AND ITS TECTONIC SIGNIFICANCE

Four main Post-Paleozoic groups of vol­ canic rocks OCC\lrin Northeast Brazil: (1) A Mesozoic diabase dike swarm in Rio Grande do Norte which extends into the States of Ceara and Paraiba; (2) a Tertiary group of predomi­ nantly alkali basaltic rocks, in central Rio Grande do Norte and northern Paraiba; (3) a Mesozoic tholeiitic flow and some instrusive forms, in the State of Maranhao, cOl).stitutingthe most voluminous basaltic activity of Northeast; and (4) a Cretaceous magmatic province of Cabo Santo Agostinho in the State of Pernambuco, wherein andesites, rhyolites and trachytes are also present. The diabasic dike swarm which is composed of six main east-west dike sub-swarms in the Pre-Cambrian basement has ages between 125 and 130 m. y. Coarse grained, olivine-bearing diabases predominate in the southern sub-swarms, while porphyritic olivine-free types are more frequent in the northern ones. Coarse-grained, .hypersthene-rich diabases of uncertain relation to the sub-swarms are also observed.

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