Abstract

A swarm of pegmatitic bodies occurs in the central portion of the Mineiro Belt at the meridional border of the São Francisco craton (Brazil), forming the São João del Rei Pegmatitic Province. These bodies are genetically related to one common source, the granitic magmatism precursor to the Ritápolis granitoid, whose age of 2121 ± 7 Ma marks the last pulse of paleoproterozoic plutonism in the Mineiro Belt. A selected group of gahnite crystals from several pegmatite bodies hosted in the Brumado metadiorite, Ritápolis metagranitoid, and Rio das Mortes amphibolites and phyllites has been analyzed by optical stereomicroscopy, Raman spectroscopy, SEM/EDS, and EMPA/WDS. The gahnites have compositions defined by the ranges Ghn65.52–85.38Spl0–2.16Hc13.71–31.92Jac0–0.92Mfr0–1.22Glx0–1.11 (in mol.%), plot almost exclusively on the gahnite–hercynite axis, and fall into the granitic pegmatite field in the gahnite–spinel–hercynite ternary diagram. The composition of gahnite varies depending on the country rock the pegmatite cuts, and the interaction between the pegmatite and host rocks is more evident in grains found in pegmatite bodies associated with amphibolites due to higher Fe and Mg in these rocks. These compositions also indicate a higher degree of fractionation of the pegmatites hosted by the Brumado metadiorite in comparison to those hosted by the Rio das Mortes metavolcano-sedimentary sequence (amphibolites and phyllites) and the Ritápolis metagranitoid, reflecting different crustal levels of emplacement of these pegmatitic bodies.

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