Petrologia do maciço alcalino de Peixe-Goiás

Abstract

The Alkaline Fish Massif consists of a nepheline syenite gneiss body, leucocratic, foliated and banded. Foliation is accentuated at the edges and incipient or absent at the center, while banding is restricted to the edges. The Massif is surrounded by a narrow band of quartz syenite and granite gneisses, in contact with pre-Serra da Mesa Group metasediments, predominantly pelitic (graphite-sillimanite micaschist and sillimanite-quartz micaschist). The nepheline syenite gneiss consists essentially of nepheline, mesopertite, peristerite and microcline, with biotite being the mafic mineral and, more rarely, magnetite. Its agpaitic coefficient is typical of miaskitic rocks, and its mineralogy is also of this type. The different textural types of nepheline show variation in the Si/Al ratio, while the Na/K ratio is approximately constant. Peristerite has composition An4.0-5.1 -An0.5-2.8 and, in a narrow band of the edge, it is An7.0-10.7 -An0.4-4.0. The , microcline and potassium phase of mesopertite (microcline) have composition Or8 3.0-9 4.4, while the sodium phase of mesopertite is predominantly peristerite (An4.0-5.1 -An0.7-2.8) and sometimes it is albite (An0.4-2.6). Biotite is of the lepidomelane-siderophyllite type. The composition of nepheline syenite gneiss, similar to the minimum point in the Ne-Ks-Qz diagram, the variation in the ratio, Si/Al. of nepheline, typical of nepheline crystallized from a nepheline-like syenite magma under plutonic conditions, as well as the trends of the elements Si, Al, Na and K and the K/K + Na ratio in differentiation diagrams, which are coincident with the variation observed in series of alkaline igneous rocks, indicate that the nepheline syenite gneiss resulted from a nepheline syenitic magma, generated in the lower crust or upper mantle, in a tectonically stable environment. During the ascent of this magma, the increase in the geothermal gradient caused fusions in rocks of the crust, generating magmas of quartz syenitic and granitic composition. The nepheline syenite body, the quartz syenite and granite belt and the pre-Serra da Mesa Group metasediments were later affected, together with the sediments of the Uruaçú Development Belt (Serra da Mesa Group), by a regional medium pressure type metamorphism. in the amphibolite facies, with a temperature of 650 ± 30°C and a pressure of 6 ± 0.5 kb. The metamorphism took place in the presence of a fluid phase rich in H2O and CO2, in addition to S03, F and P originated by the decomposition of late-magmatic minerals from nepheline syenite, rich in volatile components, which had great importance during the metamorphism in the formation and recrystallization of minerals and, in local processes, of autometasomatism. This Massif presents striking analogies with the alkaline complexes of Blue Mountain, Bigwood and Darkainle.