Um magmatismo bimodal arqueano no âmbito dos complexos metamórficos do quadrilátero ferrífero e suas implicações tectônicas

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Characterization of beryl (aquamarine variety) by Mössbauer spectroscopy

The Mossbauer spectra of several blue beryls have been obtained in the temperature range of 4.2-500 K. A common feature observed in all room-temperature spectra is the presence of an asymmetric Fe2+ doublet (DeltaE(Q) similar to 2.7 mm s(-1), delta similar to 1.1 mm s(-1)), with a very broad low-velocity peak. This asymmetry seems to be related to a relaxation process involving ferrous ions and water molecules in the structural channels, as suggested by Price et al. (1976). Surprisingly, the spectrum at 500 K also shows a broad, but symmetrical, doublet, with a clear splitting of the lines indicating the presence of at least two Fe2+ components. The room-temperature spectrum obtained after the 500 K run shows the same features as prior to the heating. At 4.2 K the spectrum of a deep blue beryl was well fitted with four symmetrical doublets, one of which could be related to Fe2+ in the structural channels. Ferrous ion was also found to occupy the octahedral and tetrahedral sites, whereas ferric ion is most probably located in the octahedral site. A meaningful fit of the room-temperature spectrum, as well as an explanation for the temperature dependence of the Mossbauer spectra, are discussed. Finally, it is believed that the color in beryl will be dictated by the relative proportions of Fe3+ in the octahedral sites and of Fe2+ in the channels

The southern Araçuaí belt and the Dom Silvério Group: geologic architecture and tectonic significance

The Araçuaí belt corresponds to the external portion of the western half of the Araçuaí-West Congo Neopro-terozoic orogen. TheAraçuaí belt fringes the São Francisco craton to east and is separated from the crystalline core of the orogen by the Abre Campo geophysical discontinuity. The southern Araçuaí belt involves four major lithologic units: the Archean and Paleoproterozoic gneisses of the Mantiqueira Complex, the Pedra Dourada Charnockite, the Paleoproterozoic Borrachudos Granitoid, and the metavolcanosedimentary rocks of the Dom Silvério Group. The Dom Silvério Group occurs in a NNE-SSW striking belt and consists of a thick package of metapelitic rocks with intercalations of quartzites, amphibolites, meta-ultramafics, banded iron formations, gondites and marbles. All units of the southern Araçuaí belt underwent four syn-metamorphic phases of deformation in the course of the Brasiliano event. The first phase, synchronous to a regional amphibolite facies metamorphism, was associated to a general tectonic transport towards north along the left-lateral Dom Silvério shear zone and its low angle segment. The second and third phases represent progressive stages of a west directed shortening, which led to the development of local thrusts and pervasive folds in several scales. The fourth phase is extensional and reflects the collapse of the orogen.<br>A Faixa Araçuaí corresponde à porção ocidental externa do orógeno neoproterozóico Araçuaí-Congo Ocidental. Margeia a borda leste do Cráton do São Francisco e é separada do núcleo cristalino do orógeno pela descontinuidade geofísica de Abre Campo. A porção meridional da Faixa Araçuaí envolve quatro unidades litológicas principais: os ortognaisses arqueanos e paleoproterozóicos do Complexo Mantiqueira, os charnoquitos Pedra Dourada, os granitóides paleoproterozóicos da Suíte Borrachudos e as rochas metavulcanossedimentares do Grupo Dom Silvério. O Grupo Dom Silvério ocorre em uma faixa NNE-SSW e engloba um pacote de rochas metapelíticas com intercalações de quartzitos, anfibolitos, meta-ultramáficas, formações ferríferas, gonditos e mármores. Todas as unidades da porção meridional da Faixa Araçuaí foram envolvidas em quatro fases deformacionais sin-metamórficas no curso do Evento Brasiliano. A primeira fase, sincrônica a um metamorfismo regional de fácies anfibolito, associou-se a um transporte tectônico geral para norte ao longo da zona de cisalhamento sinistral de Dom Silvério e no segmento de baixo ângulo a ela conectado. A segunda e terceira fases representam estágios progressivos de um encurtamento com movimentação geral para oeste, com desenvolvimento de empurrões localizados e intenso dobramento em todas as escalas. A quarta fase é extensional e reflete o colapso do orógeno