THE RIO GRANDE DO NORTE ALKALI - OLIVINE BASALT ASSOCIATION, NORTHEAST BRAZIL

Abstract

The Tertiary volcanic suite of Rio Grande do Norte, Brazil, composed of ankaratrites, basanites, and olivine-basalts with basanitic or tholeiitic affinities formed plugs, necks, flows and dikes which cut the Precambrian basement and Cretaceous sediments of the Apodi Basin in a north-south trend. Crystallization began with olivine followed in sequence by pyroxene, iron oxides, apatite and nepheline whereas the olivine-basalts started with olivine, followed in sequence by plagioclase, augite and iron oxides. The magma crystallized under fO 2 equivalent to the FMQ buffer. Xenocrysts of olivine, orthopyroxene, clinopyroxene, spinels and granular and sheared spinel-lherzolite and harzburgite nodules are found as inclusions in several necks and plugs. The entire xenolith suite was derived from depth of approximately 64 km. The alkali basaltic magma formed by partial melting under pressure of at least 20 kb. Fractionation gave rise to the ankaratrites and basanites. The increase of partial melting generated an olivine basaltic magma which gave rise to the flows, dikes and some necks. The emplacement of this suite is related either to internal readjustment with in the South American plate during its westward displacement or to the Tertiary pressure release of arched zones formed in the Upper Mesozoic during the opening of the South Atlantic Ocean.