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Título

PRELIMINARY CARACTERIZATION OF THE (META)PERIDOTITE FROM AN ULTRAMAFIC-MAFIC ASSOCIATION, VIGIA COMPLEX, BAGÉ, RS

Texto do resumo

The study area is located within the municipality of Bagé, RS, in the Vigia Complex, part of the Tijucas Terrain (TT). The TT is in the central portion of the Dom Feliciano Belt (DFB) between the São Gabriel and Pelotas Terrain. Delimited to the west by the Irapuã Fault System, covered by Camaquã Basin sediments and, to the east, along the Dorsal de Canguçu Shear Zone. The TT is composed by different types of accretionary rocks, including the Paleoproterozoic orthogneiss of Encantadas and Vigia Complex (VG), the Statherian metagranites, and Neoproterozoic metasedimentary and metavolcanic rocks of the Porongos Complex. The VC is an elongated structure with dimensions of about 10km wide by 25km long, oriented in ~N25°E direction. Recently, were superficially described meta ultramafic rocks inserted into Vigia Complex. This study aims to preliminary characterization of peridotite rocks, in the ultramafic principal body of the Vigia Complex. The methodology employed focused on fieldwork, petrographic, electron microprobe and scanning electron microscopy (SEM) analysis. In the studied area, the metamafic and mafic rocks outcrops as metric to hundreds of meters, elongated and discontinuous bodies. The main body has an approximate area of 850x350 meters, composed by serpentinites, peridotites, gabbro, dikes, tremolite schists and plagiogranite. Intruded, sometimes, by granitic veins and enclosed in granitic and gneiss rocks of the Vigia Complex. The studied samples outcrops are along the drainage with a dark grey color, low degree of alteration and massive structure. In thin section, the mineral assemblage is represented by olivine, orthopyroxene, amphibole, opaques, rare epidote, chlorite, and phyllosilicate. The textures are essentially granoblastic in mosaic with portions most preserved of olivine and orthopyroxene, can be potentially relicts of preserved igneous phaneritic texture. The opaque minerals occur both in mineral framework crystals and included in olivine. The average proportion of the rock, present contents 40% to 60% of olivine, 20-30% orthopyroxene, rare clinopyroxenes, 10% to 15% of amphiboles, about 2-3% of opaques minerals and <1% of chlorites, epidotes and phyllosilicates. In scanning electron microscopy and electron microprobe analysis, it was detect primary mineralogy composed by forsterite-chrysotile (olivine), enstatite and chromite. The secondary association consists essentially in tremolite, antigorite, phlogopite, peninnite and magnetite. The chromites exhibt skeletal texture without specific orientation, with varity size between 0,03 to 0,1 cm. The clinopyroxene, show chemical of augite and pigeonite chacaracteristic. Based on the mineralogic composition, texture, structure and minerals quantitative estimation, the rock was classified as a (meta)peridotite, specifically as a harzburgite. The peridotite is probably related to the basal portion of a preserved remnant of an ophiolitic sequence in the Dom Feliciano Belt.

Palavras Chave

meta-ultramafic; mineralogic caracterization; (meta)peridotite; Vigia Complex.