MINERALOGIA QUÍMICA E PETROLOGIA DOS ANFIBOLITOS DA REGIÃO DE MORRETES-ANTONINA (PR)

E. RUBERTI, C. B. GOMES

Resumo


The Morretes-Antonina, Paraná, area has been affected by two métamorphic episodes. The first one is related to the Brazilian Cycle (-600 m.y.) aird shows mineral asgemblages consistent with the amphibolite facies. Oligoclase-andesine gneisses, andesine-labradorite amphibolites, oligoclase-andesine amphibolites and gondites make up the main rock types. The second event has local distribution, being apparently confined to the contact region with post-tectonic granitic massifs. It shows clear evidences of retrogressive metamorphism and mineral assemblages compatible with the greeenschist facies. In this area, albite gneisses occur interbedded with albite amphibolites, magnesian schists, chlorite schists and magnetite quartzites. Andesine-labradorite amphibolites arc cssentially made up of plagioclase (An40-75) and brown hornblende, and present no evidence of retrograde metamorphism at all. Oligoclase-andesine amphibolites have plagioclase (An20-50) and brown and green hornblende as major constituents. A few mineralogical features indicating some retrogressive metamorphism are also present. On the other hand, albite amphibolites consist essentially of albite (An0-8), actinolite, epidote, and chlorite. Observations on a geographic scale as well as in thin sections indicate that the transition from one facies to the lower one is not abrupt; lack ofequilibrium is observed in rocks showing textural and mineralogical transitions: amphibole zoning going from brown hornblende to the green one to actinolite; partial breakdown ofcalcic plagioclases to albite and epidote and ofhornblende into biotite plus chlorite. Chemical data as well indicate disequilibrium inhomogeneities in the transitional rocks such as extreme variations in plagioclase compositions (An0-75) and higher TiO2 , Al2O3 and alkali contents in the earlier brown horblendes as compared to the later green ones and to actinolites.


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