Caracterização dos minerais opacos dos enclaves máficos hospedados em gnaisses aflorantes na vila Cruzeiro do Sul, porção Sul de domínio Bacajá

Abstract

This research aims to obtain magnetic susceptibility data from mafic enclaves and study their opaque minerals. These mafic enclaves are hosted by gneissic rocks from the Vila Cruzeiro do Sul area, southern Bacajá domain, Transamazonic Tectonic Province, eastern portion of the Amazonian Craton. The mafic enclaves are greenish gray, medium grain size with granoblastic texture, are predominantly rounded, and display structural trend similar to host rocks. These mafic enclaves showed heterogeneous magnetic susceptibility values, ranging from 0.177 to 31.2 x 10-3 (IS) and mean value of 5.524x10-3 (IS). Based on frequency histograms and probability diagrams representing the magnetic susceptibility values, it was possible to divide the rocks into three groups: A, B, and C, where they represent the low, intermediate, and high values, respectively. The mafic enclaves contain magnetite and ilmenite, as main opaque phases with subordinate, pyrite, chalcopyrite, and goethite. Altogether, the rocks belong to A population exhibit Altogether, the rocks belong to A population exhibit high modal content of magnetite in comparison to rocks from other populations. Another characteristic of the A population, refers to regular shapes and an average size of 0.4 m of magnetite crystals and the high magnetite/ilmenite ratio. The low and medium SM populations exhibit less modal content of magnetite and magnetite/ilmenite ratio. In both populations, textual features in magnetite crystals were identified, which are indicate of oxy- exsolution processes.