GEOLOGIA E PETROLOGIA DAS ROCHAS METAMÓRFICAS E GRANÍTICAS ASSOCIADAS DO VALE DO RIO RIBEIRA DE IGUAPE, SP E PR

Abstract

An area of 16900 square kilometers, located between 48o to 49° 30' west longitude and 21° to 25° south latitude, was geologically mapped in 1:100.000 scale. Flat lying Devonian and Permo-carboniferous sediments occur in the northern part of the region, but most of the area is underlain by late Precambrian and Eopalaeozic igneous and metamorphic rocks. Argillaceous low grade metasediments are widespread, although typical phyllites are subordinate. These rocks usually exhibit clear bedding planes and contain dominant sericite and quartz. Graphite, magnetite, apatite, tourmaline, rutile, and epidote are acessories. A variety of micaschists occurs in different parts of the area. Quartz and muscovite are the main minerals, but also biotite, chlorite, garnet, staurolite, kyanite, and hornblende may occur in various proportions. Apatite, sphene, zircon, rutile, and opaque minerals are accessories. Metamorphic rocks derived from siltstones and sandstones contain essentially the same minerals, but occasionally feldspars are abundant in these rocks. True quartzites form some of the more conspicuous ridges in the area. Subordinate conglomerate lenses are also found. Limestones, dolomites and carbonate schists occur in three main belts in the area. Usually individual layers of carbonate minerals are from a few centimeters to one meter thick and alternate with thin sandy or micaceous beds. Impure limestones have produced a large variety of carbonate schists which may contain carbonate, quartz, tremolite, garnet, actinolite, hornblende, phlogopit, plagioclase. microcline, dinpside. epidote, sphene, tourmaline, scapolite, and opaque minerals. Many occurrences of amphibolites and amphibole schists are intercalated in the metasediments, but they are mostly too small to be mapped. The amphibolites contain, almost exclusively, common green hornblende and andesine. Gneisses and migmatites are the dominant rocks in the southern part of the area. Some migmatites are quite homogeneous and contain mostly quartz and feldspar with subordinate irregular bands of biotite. Typical banded gneisses are also found. The metamorphic rocks are cut by many granitic bodies of different sizes, ranging from small stocks only a few hundreds of meters in diameter to large batholiths. With one single exception, the Itapeuna massif, they represent late or post tectonic intrusions. The most common variety is a porphyroidal granite which contains large microcline crysta ls in a much finer matrix of feldspars, quartz, biotite, and hornblende. In the southern part of the area several granite bodies, occur with alkaline affinities. These rocks are made up essentially by orthoclase, oligoclase, quartz, biotite, and some hornblende or a more sodic amphibole. Granite porphyries occur as dykes or small stocks. A large part of the rocks that belong to the Acungui Group were submitted to a low grade metamorphism, corresponding to the green schist facies. Many original sedimentary features are still preserved and schistosity is only incipient. In the southern part of the area, a stronger degree of regional metamorphism affected the rocks and produced mineral assemblages which include biotite, garnet, staurolite, and kyanite. A progressive increase in the intensity of the regional metamorphism, from northwest to southeast, can clearly be observed. Near the borders of some granitic intrusions contact or thermal metamorphism produced mineral assemblages which correspond to the albite-epidote hornfels, hornblende hornfels or pyroxene hornfels facies. Retrograde metamorphism apparently took place in limited zones that were deformed after the main period of metamorphism.