Abstract

The Lake Brisson peralkaline granite in Labrador, which by way of its age of 1189 Ma is the most recent phase of the Gardar magmatic stage, was intruded in the Middle Proterozoic at the margin of a granulitic complex, retrograded to an amphibolite facies during Aphebian, and of an Elsonian adamellite pluton. It shows a petrographie zonation ("feldspathic" facies at the center, "quartzose" facies including early quartz at the edge) suggestive of a permissive multiphase intrusion, and is characterized by deuteritic alteration via metalliferous fluids (Zr, Y, Nb, Be; rare earths). All facies are Na-peralkaline, well evolved, and represent end-members of a differentiated magmatic suite of the designated A type. The relative behavior of the major elements indicates that the facies differentiation was controlled by fractional crystallization and was also greatly influenced by alkaline feldspath and by increase of f(O2) in the final stage of evolution. The trace elements contents, significantly higher than those reported for other peralkaline complexes, are a confirmation of the influence of fractional crystallization. The unsual trace element enrichment in an altered quartzose facies is the result of the effects of a final oxidizing fluid phase, rich in F; the relative depletion of Na and the enrichment in Sr and Ca of the fluid are explained by its having been contaminated by the wall rocks. [Journal Translation]

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