Petrologia magnética do granito anorogênico Bannach, terreno granitogreenstone de rio Maria, Pará

Abstract

The Paleoproterozoic Bannach Granite is intrusive in Archean units of the Rio Maria Granite-Greenstone Terrane. The batholith is composed of three group of monzogranites rocks: a coarsegrained amphibole, biotite and sometimes clinopyroxene-bearing facies; a porphyritic facies with biotite; and leucogranitic fácies with varied textures. Magnetite, ilmenite, hematite, pyrite, chalcopyrite, sphalerite and goethite, are the opaque oxide minerals observed. Six textural types of ilmenite are distinguished: individual Ilmenite; internal and external composite Ilmenite; sandwich Ilmenite; patch Ilmenite; trellis Ilm. Texture features suggest that titanomagnetite and individual and composite ilmenite crystallized in early magmatic stage. The Ilmenite was destabilized and partially replaced by titanite still in the magmatic stage. During the subsolidus stage,titanomagnetite was transformed by oxidation-exsolution in to intergrowths of almost pure magnetite and ilmenite (sandwich, patch, and trellis ilmenite). Hematization of magnetite and oxidation of the pirite the latter with goethite formation occurred later at lower temperatures. Compared to the average of magnetic susceptibility (MS) for granites, the different facies of the Bannach pluton are characterized by high values of the MS. There is good correlation between MS and magmatic differentiation of the facies forming the batholith. MS decrease from the amphibole+biotite± clinopyroxene-bearing facies to the biotite porphyritic facies, attaining the lowest values in the leucogranites. The modal magnetite contents are high for granites. Opaque minerals, amphibole, total of mafic minerals and biotite modal contents show a positive correlations with MS. Positive correlations among MS and Fe 2 O 3 , FeO, MgO and TiO 2 contents and negative correlations between MS and K 2 O e SiO 2 are consistent with the mentioned relationships between MS and modal mineral content. High MS, significant modal contents of magnetite, the high Fe 2 O 3 /FeO ratios and magmatic assemblage titanite + Mt + quartz demonstrate that the Bannach Granite evolved in relatively oxidizing conditions estimated as situated near those of the NNO buffer.