PETROGRAFIA E LITOGEOQUÍMICA DOS METATONALITOS E SEUS ENCLAVES DE GRANULITOS BÁSICOS DA PARTE SUL DO CINTURÃO ITABUNA-SALVADOR-CURAÇÁ, BAHIA

Abstract

The research has led to the definition of an important lithologic unit composed by archean metatonalites containing enclaves of biotite-bearing basic granulites, hornblende-bearing basic granulites and garnet-bearing basic granulites. The essential minerals in metatonalites include plagioclase, quartz, orthopyroxene and clinopyroxene whereas garnet is rare and hornblende and biotite are secondary ones. The basic granulites are, in general, composed by the same mineralogy, except quartz but they differ from the metatonalites by an enrichment of biotite, hornblende and garnet. Petrochemically, the metatonalites are low potassium-calc-alkaline rocks with REE concentration showing a strong fractionated spectrum, with LREE enrichment and HREE impoverishment. In comparison, the basic granulites show tholeiitic filiation and they exhibit little fractionated-flat-REE patterns that suggest basalt and/or gabbro protholiths. Both metatonalites and basic granulites do not show Eu anomaly. The metatonalites are similar to the Lewisian Complex in Scotland, such haracteristic leds to an interpretation that they were formed by fractional crystallization from a basic magma, which had been produced by partial melting of hydrated basalts/gabbros. The tectonic model for the region involves the collision of archean crustal segments during the paleoproterozoic and that the existing oceanic crust between them may have produced tonalites similar to those studied. The basic granulites which occur as enclaves, especially those containing garnet, can represent this oceanic crust and possible source of the metatonalite’s protholiths.