Estudo petrográfico e suscetibilidade magnética das rochas máficas e metamáficas da vila ouro verde, Canaã dos Carajás-Pa

Abstract

A semi-detailed geological mapping (1:100.0000) was carried out in the Vila Ouro Verde area, western portion of the Canaã dos Carajás, and allowed the identification of mafic and metamafics bodies as well as to distinguish three major groups of rocks: gabbros (gabbro, gabronorito and ortopiroxênio-gabro), amphibolites and metabasalts. Such rocks occur as enclaves in the enderbites, Planalto granites in TTGs. Gabbronorites are composed by plagioclase + clinopyroxene + orthopyroxene + amphibole + biotite and their differences are restricted to the modal proportions of ortho- and clinopyroxene, olivine and quartz. As amphibolitic rocks and metabasalts are essentially constituted by plagioclase, hornblende, biotite and quartz, with their restricted difficulties as proportions of pyroxenes and their textures. In amphibolites the pyroxene crystals occur as reliquary crystals, and their textures are granoblastic to granonatoblastic whereas in metabasaltic rocks the pyroxene crystals occur in greater proportions. These rocks with fiber texture, with plagioclase vias associated to mafic aggregates. The magnetic behavior of these lithotypes is mainly defined by the nature of the Fe-Ti oxides minerals and their respective modal contents. Based on magnetic petrology studies it was possible to distinguish two groups of rocks: population A is predominantly composed of amphibolites and exhibits low values of magnetic susceptibility (0,298 × 10-3 to 1,687 × 10-3 SIv), whereas in the population B the gabbronoritic variety is more abundant, magnetite is the main Fe-Ti oxide and the magnetic susceptibility values are higher (4,870 × 10-3 to 95,425 × 10-3 SIv). Textural evidences show that the formation of opaque minerals in the gabbronorites and amphibolites are marked by the following evolution: (i) early crystallization of individual ilmenite and ilmenite composite with crystals of magnetite and titanite; (ii) oxy-exsolution process on magnetite crystals producing magnetite-ilmenite intergrowth (ilmenite patch and ilmenite sandwich); and (iii) partial destabilization of the individual ilmenite giving rise to titanite.