Hydrothermally altered volcanic rocks metamorphosed at granulite-facies conditions: an example from the Grenville Province

A 1.2 Ga association of aluminous gneisses, garnetites, and white felsic gneisses of andesitic composition in the southern Manicouagan area (central Grenville Province) provides evidence consistent with protolith formation and hydrothermal alteration in a submarine volcanic environment. In addition to field relations, potential relics of quartz phenocrysts in the aluminous gneisses, revealed by SEM-MLA (scanning electron microscope with a mineral liberation analysis software) imaging, are consistent with a volcanic precursor. Moreover, in these rocks, aluminous nodules and seams of sillimanite are considered to represent metamorphosed hydrothermal mineral assemblages and to reflect former pathways of hydrothermal fluid. These features are preserved despite the Grenvillian granulite-facies metamorphic overprint and evidence of partial melting. In addition, the garnetites are inferred to represent hydrothermally altered products of the white gneisses, based on the gradational contacts between the two rock types. The compositional ranges of minerals are generally similar to those of granulite-facies metapelites, but moderately elevated contents of Mn in garnet from the garnetites, and Zn in spinel from the aluminous gneisses, are consistent with hydrothermal addition of these elements to the protolith. The most prominent alteration trends are an increase in Fe-Mg-Mn from the white gneisses to the aluminous gneisses and the garnetites, and a trend of increasing alumina index in some white gneisses, suggesting mild argillic alteration. The new findings highlight the preservation of early hydrothermal alteration in high-grade metamorphic belts in the Grenville Province, and these altered rocks are potential targets for exploration.