GEOLOGIA E IMPLICAÇÕES GEOTECTÔNICAS DO GREENSTONE BELT MORRO DO FERRO (FORTALEZA DE MINAS, MG)

NOEVALDO A. TEIXEIRA, JOSÉ C. GASPAR, THOMAS L. BRENNER, JOHN T. CHENEY, CELINA M. L. MARCHETTO

Resumo


The volcano-sedimentary rocks of the Morro do Ferro Greenstone Belt occur in linear or irregularly isolated strips within a sialíc substrate. Thís is characterized by gneissic terrains cut by large shear zones, marking the boundary between lhe cratonic block (Paramirim) and the marginal mobile belt (Alfenas). The lower unit of lhe Morro do Ferro Greenstone Belt is composed of peridotite, pyroxenite and basaltic flows of komatiitic affinities, intercalated with siliceous chemical precipitates, tuffaceous material, and tholeiitic basalts. The metamorphic grade between greenschist and amphibolite facies, with 400-500ºC and 3-5 kb. The peridotitic komatiite flows show a basal cumulate zone (MgO == 30) and an interrmediate zone with spinifex textures (CaO/A1203) ~ 1, A1203/Ti02 ratios 20). Such characteristics are comparable with those of the undepleted aluminium suítes. To lhe SE of Morro do Ferro itself, in lhe Bom Jesus da Penha-Jacuí region, the metamorphic grade is markedly hígher. Ultramafic flow are scarcer and sedimentary and tuffaceous rocks predominate. containing restricted basic volcanic components. The mineral associatíon in magnesium rich and aluminium-rich rocks of this belt include: gedrite-antophyllite-tscherrnakitic hornblendeplagioclase (An33)-kyanite; hornblend e-gamet-kyaníte ; muscovíte-biotite-staurolite. Such mineral assemblages indicate that metamorphic conditions reached the upper limit of the amphibolite facies (600-700ºC, 6-7 Kbar) which is an uncommon temperature and pressure condition for the Archaen. Two hypotheses can explain the dístinct characteristics of lhe Bom Jesus da Penha-Jacuí belt and the Morro do Ferro Greenstone Belt: 1. The Morro do Ferro Greenstone Belt may have been more extensive than presently defined. It may have extended to the SE, where the rock units were dynamically metamorphosed during the same event that affected the larger Alfenas Mobile Belt; and 2. each belt originally evolved as a part of different crustal zones. The Morro do Ferro Greenstone Belt could represent low grade terrains from the upper crustal levels, whereas the Bom Jesus-Jacuí belt could represent supracrustal belts of high grade metamorphic terrains. Both of these hypotheses are consistent with a mechanism of crustal evolution capable of provokíng strong tangential movements (overthrusting). In the suggested overthrusting and depth displacement scenery, high pressure zones would be developed as indicated by rocks containing kyanite and hornblende.

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