PETROGÊNESE DO COMPLEXO ACAIACA, MG

AUTOR(ES)
DATA DE PUBLICAÇÃO

2009

RESUMO

The Acaiaca Complex (AC) is composed of mainly granulite facies rocks distributed along the Acaiaca county and surroundings, east of the Quadrilátero Ferrífero. The present work aimed to investigate the lithological diversity of the AC with emphasis on the mineralogical, microstructural, geochemical and petrogenetic characterization. The lithostructural survey showed that the rocks of the AC are distributed along a narrow, up to 6 km wide stripe extending for about 36 km in the NS-direction, which is inserted in a terrain of amphibolite facies gneisses. The granulites commonly have subvertical mylonitic foliation which parallels the mineralogical banding trending NNE-SSW. Felsic, mafic and ultramafic ortho-derived granulites as well as para-derived granulites are found in the complex. Other rock types in the area of the AC are amphibolite facies gneisses, meta-gabbros, meta-diabases, quartzites, meta-granites and pegmatites. Felsic granulites show geochemical similarity to orogenic to post-orogenic peraluminous granitoids. They are composed of biotite + garnet + plagioclase + quartz k-feldspar orthopyroxene. Results of garnet-biotite geothermometry indicate average temperatures between 622 and 700C which are relatively low due to partial retrometamorphic re-equilibration of mineral compositions. Mafic granulites present chemical affinity to island-arc basalts. They are composed of the paragenesis orthopyroxene + clinopyroxene + plagioclase hornblende, typical of low-pressure granulite facies. Results of geothermobarometric calculations indicate conditions of formation between 720 and 820C and 6.5 and 7 kbar. Granulites derived from pelitic rocks are composed of two distinct mineral assemblages: garnet + biotite + sillimanite + plagioclase + quartz k-feldspar cordierite and biotite + kyanite + plagioclase + quartz + cordierite garnet staurolite. The first assemblage is typical for intermediate-pressure conditions of the granulite facies. The second assemblage presents minerals in textural disequilibrium, which can be assigned to two distinct paragenesis: staurolite + plagioclase + kyanite + biotite + quartz, typical of intermediate and low-pressure conditions of the amphibolite facies, and cordierite + garnet + sillimanite + plagioclase + biotite + quartz, typical of low and intermediate-pressure conditions of the granulite facies. Geothermobarometric calculations indicate PT-conditions between 662 and 732C and around 6.5 kbar for the granulites derived of pelitic rocks. The results of this study allow to conclude that the main metamorphic event that affected the rocks of the Acaiaca Complex was of granulite facies, under temperature conditions around 800C and intermediate-pressures around 6.5 kbar, being followed by a near-isothermal retrometamorphic decompression path.

ASSUNTO(S)

engenharias 1. geologia - acaiaca (mg) - teses. 2. petrogênese - acaiaca (mg) - teses. 3. granulito - teses. i. universidade federal de ouro preto. ii. título.

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