Chemical Trends In the Meech Lake, Québec, Carbonatites and Fenites

Abstract

Abstract Near Meech Lake, Quebec, the edges of Mesoproterozoic carbonatite dikes are composed of calcite, dolomite, fluorapatite, phlogopite, amphibole, and pyrochlore. The carbonatite is separated from amphibole-fenite by a narrow, fine-grained reaction selvage of phlogopite pierced with long prisms of amphibole. The amphibole is mainly richterite, but it extends to magnesio-arfvedsonite (overgrowth, crystal rim). Uranium-rich pyrochlore is metamict and ranges from calciopyrochlore to kenopyrochlore with Ta-U enrichment in crystal rims. Chemical characteristics of the suite are: (1) F and Nb highest in the selvage, and (2) decline of Sr and Ce outwards from the carbonatite. A similar pattern (this research) is found at Fen, Norway. Rare earths are enriched in LREE with smooth downward-sloping patterns, in chondrite-normalized curves, to HREE. Two major surges of mineralization are suggested: (1) early, metasomatic-alkalic, creating fenites with enrichment in Mg, Na, and K; and (2) later igneous depositing carbonatites and introducing first F, P, and Nb, then Ca, Sr, and Ce. Thermochemical and geochronological data place carbonate equilibration at 700 °C and the emplacement at 1026 Ma b.p. Calciocarbonatites, in monzonitic orthogneiss, are enriched in Ba and Ce. They are composed of baryte, calcite, phlogopite, fluorapatite, magnesio-riebeckite, and non-metamict allanite-(Ce). A mica selvage is present, but amphibole fenite is almost completely lacking. Magnesiocarbonatite has a well-developed selvage against granite but lacks significant amphibole fenite. In breccia cement at nearby Fortune Lake, pyrochlore is associated with abundant fluorapatite but lacks carbonates. The Cambro-Proterozoic calciocarbonatite near Fen, Norway is particularly Nb-rich in breccia zones, and pyroxene fenite takes the place of amphibole fenite at Meech Lake. In contrast to a relatively anorogenic regime during carbonatite petrogenesis at Fen, metamorphism has obscured pyrochlore zonation and enhanced amphibole growth at Meech Lake.