Hanna Jordt Evangelista

Quartzito azul com dumortierita e fosfatos de alumínio do Espinhaço setentrional, Bahia: mineralogia e petrogênese

In the Northern physiographic domain of the Espinhaco Ridge, in Bahia State, Brazil, blue quartzite is mined as a valuable dimension stone. The meter- to decameter-sized lenses are irregularly distributed within the whitish quartzites belonging to the Vereda Formation. Optical studies and microprobe analyses have identified dumortierite, kyanite, chromium-bearing muscovite, and lazulite as the main minerals responsible for the bluish and greenish colors. Phosphates, such as trolleite, augelite, svanbergite-goyazite and monazite, occur sporadically. Dumortierite and kyanite, the most abundant color-causing minerals, are mostly concentrated along millimeter-wide, plane-parallel to crossed sedimentary beds. The dumortierite is also found filling discordant fractures, evidencing the high mobility of boron during metamorphism. The estimated metamorphic conditions of P ≥ 3.5 kbar and T = 475 to 560oC are of higher greenschist to lower amphibolite facies. Rounded quartz grains inherited from the sedimentary protolith are conspicuous in the Northern part and absent in the Southern region of the studied area, which indicates increasing deformation southwards. In terms of the depositional milieu, the lithofacies of the Vereda Formation is interpreted as the initial filling product of a Mesoproterozoic rift associated with alluvial planes and lagoons or tidal planes in continental and coastal systems, where the boron that generated dumortierite could have been concentrated in sediments with contribution of evaporitic material.

Petrografia e geoquímica dos granulitos do Complexo Acaiaca, região Centro-Sudeste de Minas Gerais.

The Acaiaca Complex, located near the town of Acaiaca, Minas Gerais, is composed predominantly of granulite facies rocks. The Complex extends for at least 36 km in the N-S direction and is about 6 km wide in the central area. Felsic (biotite granulites and charnockites), mafic (pyroxene ± hornblende granulites), ultramafic (olivine-pyroxene granofels) and aluminous granulites are the main rock types. Felsic granulites are derived from rocks of rhyolitic composition. Mafic granulites are derived from rocks geochemically similar to island arc basalts. Aluminous granulites are derived from pelitic rocks and greywackes. The olivine-pyroxene granofels present a chemical composition similar to harzburgites. The mineral assemblages of granulites indicate an intermediate-pressure granulite facies metamorphism for their formation. Metamorphic rocks of lower grade found in the area of the Acaiaca Complex commonly present mylonitic microstructures and evidences of generation by retrometamorphism of the granulites in shear zones.

The Unusual Mössbauer Spectrum of Beryl

The Mössbauer spectra of several aquamarine samples 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 (ΔEQ∼2.7 mm/s, δ∼1.1 mm/s), with a very broad low-velocity peak. This asymmetry is not caused by preferred orientation since the spectrum collected under the magic angle did not show any difference in the line intensities, nor is it caused by the superposition of a Fe3+ doublet. At 4.2 K the spectrum of a deep-blue beryl could be well fitted with three symmetrical doublets, with the major Fe2+ doublet accounting for 87% of the total spectral area. At 14 K the symmetry remains, but at 30 K the low-velocity peak is again broad. 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 before the heating. A meaningful fit for the room-temperature spectrum, as well as an explanation for the temperature dependence of the Mössbauer spectra, are discussed.

Rochas ultramáficas plutônicas do greenstone belt Rio das Velhas na porção central do Quadrilátero Ferrífero, Minas Gerais, Brasil

In Amarantina, district of Ouro Preto (State of Minas Gerais, Brazil), ultramafic rock exposures are found along two areas of about 500 m2 each. The rocks crop out in the Bacao complex, which is the gneissic basement of the Rio das Velhas greenstone belt in the central portion of the Quadrilatero Ferrifero (QF). The interest in a petrogenetic study of the ultramafic rocks is the partial preservation of igneous minerals, which are not observed in most of the completely metamorphosed ultramafic rocks in the QF. Among them, the steatites and the serpentinites are the best studied because of their economic importance. The ultramafic rocks from Amarantina are classified as metaperidotites due to the equigranular texture characteristic of plutonic origin. The rocks are made up of large grains of olivine, pyroxene, and spinel preserved from de original magmatic rock, which are distributed in a fine grained mass with talc, serpentine, chlorite, amphibole, and opaque minerals. Scarce arite (NiSbAs) and breithauptite (NiSb) were generated after pentlandite during hydrothermal metamorphism. Comparison of the chemical composition with a metakomatiite with spinifex texture from de QF as well as with known komatiitic rocks from other parts of the world reveals that the metaperidotites are chemically similar to non Al-depleted komatiites. Therefore it is probable that the studied ultramafic rocks correspond to the plutonic portion of the komatiitic magmatism of the Nova Lima group, at the base of the Rio das Velhas greenstone belt.

Chemical zoning of muscovite megacrystal from the Brazilian Pegmatite Province

Macroscopically homogenous muscovite plate from the Cruzeiro pegmatite, located in the Eastern Pegmatite Province in Minas Gerais, may show complex distribution patterns of some trace elements. In geochronological and petrological studies, as for example in the distinction of magmatic and post-magmatic mica, the cause of zoning could be taken into consideration. The complex chemical zoning in the studied mica plate can be best explained by growth in an evolving magma followed by alteration due to percolation of hydrothermal fluids. Enrichment of Rb towards the border is interpreted as resulting from the chemical evolution of the residual magma during crystal growth. The depletion in ( IV Al+ VI Al) as well as the increase in (Fe+Mg) and Si along a fracture could be due to the hydrothermal celadonitic substitution of muscovite. This alteration also caused depletion in the contents of Rb, Ga, Y, Nb, Sn, and Zn and residual concentration of Ti. Elements such as Ga, Y, Nb, Sn, and Zn, rarely considered in the discussion of differentiation or alteration processes in micas, have been shown to be as significant as the alkali-elements.

Ocorrência de rochas da fácies granulito no Cinturão Mineiro, Minas Gerais, Brasil

Paleoproterozoic granitoids intruding the Archean greenstone belt of Rio das Velhas are found in the region of Lagoa Dourada, which belongs to the Mineiro Belt. The granitoids are cut by amphibolitized mafi c rocks which contain xenoliths of granulite facies rocks. The rocks are mafi c granulites, characterized by their granoblastic texture and by parageneses orthopyroxene-clinopyroxene-amphibole-plagioclase. As secondary minerals, actinolite, carbonate, epidote, cummingtonite, garnet and quartz can be found. Geothermometric determinations based on the mineral pairs orthopyroxene-clinopyroxene and amphibole-plagioclase yielded temperature values between 700oC and 853oC. The secondary mineral phases are interpreted as a result of a younger metamorphic event.