Evolution of Montviel alkaline-carbonatite complex by coupled fractional crystallization, fluid mixing and metasomatism - part I: Petrography and geochemistry of metasomatic aegirine-augite and biotite: Implications for REE-Nb mineralization

Magmatic volatiles are critically important in the petrogenesis of igneous rocks but their inherent transience hampers the identification of their role in magmatic and mineralization processes. We present evidence that magmatic volatiles played a critical role in the formation of the 1894 Ma Paleoproterozoic Montviel alkaline-carbonatite complex, Canada, and the related carbonatite-hosted REE-Nb deposit. Field and drill core relationships indicate that lithological units of the complex were emplaced in the following order: clinopyroxenites, melteigites, ijolites, melanosyenites, leucosyenites, granites, lamprophyric silicocarbonatites, rare magnesiocarbonatites, calciocarbonatites, ferrocarbonatites, late mixed carbonatites, kimberlitic silicocarbonatites and polygenic breccias. Magmatic minerals within these units were systematically metasomatized. In undersaturated silicate rocks, augite recrystallized to aegirine-augite and aegirine, plagioclase recrystallized to albite, and nepheline recrystallized with analcime, cancrinite and albite. Primary biotite was replaced by secondary, REE-rich metasomatic biotite, particularly along fractures and alteration pockets. In carbonatites, liquidus phases consisted of calcite and dolomite and were recrystallized to ferroan dolomite, ankerite, siderite, barytocalcite, witherite and strontianite, which are intimately related to the REE-bearing carbonates and fluorocarbonates. Biotite is common to all lithologies, ranges in REE concentrations from 1.5 to 230 ppm and yielded subsolidus crystallization temperatures ranging from 770 °C to 370 °C. Sm-Nd isotope analyses from biotite and aegirine-augite yield a range of εNd values (+ 3.4 to − 3.0) that suggests mixing of fluids from three sources during the crystallization of the Montviel magmas. The clinopyroxenites to melteigite, ijolites and melanosyenites crystallized augite and biotite with initial εNd value ≥ 3.4 and these minerals were metasomatized by a 1st fluid, lowering their εNd to values comprised between 0.8 and 3.4. Silicocarbonatites and carbonatites subsequently crystallized aegirine-augite and biotite with initial εNd value ≥ 2.6 and a 2nd fluid metasomatized the minerals to lower ɛ values. Both the 1st and the 2nd fluids eventually mixed with a 3rd recrystallizing aegirine-augite and biotite and lower their εNd values down to − 3.0. The results presented herein suggest that the mantle magmas evolved through 4 distinct mantle pulses by fractional crystallization, mixing of depleted mantle fluids with crustal fluids, and metasomatism. Some of the silicate rocks also show evidence of assimilation of wall rock as part of their petrogenetic evolution. During the last stages of its evolution in carbonatites, the fluid source transited from the depleted mantle to the crust and we speculate that this resulted in a violent explosive eruption creating the diatreme-shaped, HREE-rich polygenic breccia.

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