Eventos Magmáticos Arqlieanos De Natureza Cálcio-Alcalina E Tholeiítica No Quadrilátero Ferrífero E Suas Implicações Tectônicas

Abstract

TECTONIC IMPLICATIONS OF ARCHEAN CALC-ALKALIME AND THOLEIITIC MAG-MATIC EVENTS IN THE QUADRILÁTERO FERRÍFERO, BRAZIL Mafic amphibolitic dikes cutting tonalitic rocks with little evidence of metamorphism are one of the singularities in the crusta, evolution of the Quadrilátero Ferrífero, in Minas Gerais. Petrographic, geochemical and geochronogical studies show that in the Bonfim Metamorphic Complex these rock types constitute two distinct magmatic series, one tholeiitic and the other calcalkaline. They were partially metamorphosed under the conditions of higher greenschist to lower amphibolite facies. In the calcalkaline tonalites the metamorphic reactions, which required the introduction of water, were difficulted by their massive structure. Therefore primary textures and mineralogy were mostly preserved. In the tholeiitic rocks of the dikes the metamorphism was more homogeneous because the primary igneous mineralogy, rich in pyroxenes, is more sensible to hydration reactions than the mineralogy of the tonalites. The access of the needed aqueous fluids certainly was more effective to the mafic rocks because fluids tend to be concentrated in zones of crustal weakness such as dikes. We conclude that the non-penetrative metamorphic recrystallization of the two rocks series under the conditions of the higher greenschist to lower amphibolite facies, and also the preservation of early Archean U/Pb ages in titanites of the tonalites, indicate that the studied region was not submitted to any penetrative tectonometamorphic event of higher metamorphic conditions than the lower amphibolite facies during the Proterozoic. Therefore we also conclude that the tectonometamorphic overprinting of the Transamazonian Event (2.2 My) in the Bonfim Metamorphic Complex and probably also in the rest of the Quadrilátero Ferrífero was heterogeneous, not affecting all áreas with the same intensity.

ENSIALIC TECTONIC SETTING OF THE ARCHAEAN RIO DAS VELHAS GREENSTONE BELT: Nd AND Pb ISOTOPIC EVIDENCE FROM THE BONFIM METAMORPHIC COMPLEX, QUADRILÁTERO FERRÍFERO, BRAZIL

“…The felsic units are the Alberto Flores and Souza Noschese Gneisses (Herz 1970), the Samambaia Tonalite, and the Brumadinho Granite (Carneiro 1992, Carneiro et al 1997. The two mafic units are the Paraopeba and Candeias Amphibolites (Carneiro 1992, Carneiro et al 1997. In the next paragraphs we summarize the main geological and petrographic features of NeoArchaean NBMC units, from oldest to youngest, according to the field relations and, basically, U-Pb geochronological evidence.…”

“…Four units of the NBMC are felsic and two are mafic in composition. The felsic units are the Alberto Flores and Souza Noschese Gneisses (Herz 1970), the Samambaia Tonalite, and the Brumadinho Granite (Carneiro 1992, Carneiro et al 1997. The two mafic units are the Paraopeba and Candeias Amphibolites (Carneiro 1992, Carneiro et al 1997.…”

“…The AFG has a granoblastic texture, and consists of plagioclase, microcline, and quartz with minor biotite, apatite, (after Carneiro 1992, Carneiro et al 1997 Carneiro 1992, Carneiro et al 1997. Area emoldurada corresponde ao Complexo Metamorfico Bonfim.…”

Litogeoquímica do Complexo Campos Gerais e granitoides intrusivos, Arqueano/Paleoproterozoico, Brasil

“…Idades para migmatização mais ou menos expressiva ocorrem entre 2,2 e 2,1 Ga (Carneiro et al 1997;Noce, Machado, Teixeira 1998).…”

Gold Deposit Formation during Collapse Tectonics:Structural, Mineralogical, Geochronological, and Fluid InclusionConstraints in the Ouro Preto Gold Mines, Quadril ateroFerrifero,Brazil

“…600 Ma) tectonic events (e.g., Marshak and Alkmim, 1989;Chauvet et al, 1994;Carneiro et al, 1997). 2 Ga) and Brasiliano (ca.…”