Portal de Periódicos da CAPES

Gneissic and igneous rocks of the Quadrilatero Ferrifero, Minas Gerais, Brazil

1970; United States Government Publishing Office; Linguagem: Inglês

10.3133/pp641b

2330-7102

Norman Herz,

Geological Modeling and Analysis

Bl B3gneissic and granodioritic rocks have a mixed origin.Differentiation indices in the eighties and nineties, high normative quartz and low normative anorthite, and high Y, La, and F in some rocks also suggest mixe,d origins.Ultramafic rocks and amphibolites are found in the Caete area.Mafic dikes and plugs are common in the Carac,a region and also as elongate intrusives, especially along the eastern Quadrilatero Ferrifero margin.Granitic and gneissic rocks underlie most of the northeastern part of the Quadrilatero Ferrifero.They are largely of post-Minas age and granitic in composition, and show both gradational and intrusive contacts with metasedimentary rocks.Relict staurolite, garnet, and kyanite in the metasedimentary rocks suggest high pressure and temperature assemblages that were later masked by a regional greenschist metamorphism that produced chlorite, albite, and epidote in the same rocks.Radioactive ages based on dates on the post-Minas Borrachudos Granite yield 1,230 and 486 m.y.Other dates in this area show 45(M545 m.y.Alkali feldspars show indicated temperatures of 380°-400°C in gneiss and 610°C in the Borrachudos.High Rb and Li in the feldspar and high Nb, Li, Rb, and F coupled with low Mg in biotite also suggest high temperatures of formation, under late magmatic or pneumatolytic conditions.Four varieties of gneiss show varying degrees of igneous and apparent relict sedimentary textures and structures.They consist commonly of granoblastic aggregates of quartz, plagioclase (Anio-zo), sheared microcline, micas, chlorite, and clinozoisite.Granodiorites range from tonalite to adamellite.The youngest igneous rock, the Borrachudos Granite, is unique in containing fiuorite and large porphyritic perthitic feldspar crystals that range in size between 2 and 5 mm.Biotite, almost pure annite, forms elongate unfoliated knots.Pegmatites are absent from the Borrachudos, although present in the granodiorites.Ultramafic and mafic rocks in the Itabira district are intrusive into the Rio de Velhas Series and include soapstone, serpentinite, and talc schist.Metagabbroic rocks throughout the area are largely amphibolites.The following igneous cycle was repeated two or more times:1. High-grade metamorphism, perhaps to granulite facies level; 2. Formation of anatectic liquids and crystallization of granodiorites and gneisses; and 3. Introduction of some liquids of granitic composition and the formation of granites.A low-grade regional metamorphism, structural deformation and alkali metasomatism associated with thermal gradients occurred one or more times.Temperatures associated with these later events were 380°-430°C; temperatures of earlier, largely igneous events were 570°-610°C.Tie lines for coexisting feldspars are parallel the Or-Ab join in an anorthite-albite-anorthite triangular diagram, suggesting low metamorphic temperatures.On a normative quartz-albiteorthoclase "residua" diagram, older granodiorites and younger granites fall within a low pressure-temperature trough, the older being more albitic than the younger.Differentiation indices for the granodiorites and older gneisses range from 67.1 to 92.4, whereas the granites and younger gneisses range from 85.7 to 96.1.Differentiation trends of oxides of the older rocks seem to be similar to igneous rocks elsewhere.The serpentine-steatite rocks are chemically and texturally metamorphosed ultramafics.The later mafic rocks are TiO-rrich and have quartz in their norms.They are largely metamorphosed or unmetamorphosed rocks of gabbroic to diabasic composition.