Interaction of mantle magmas and fluids with crustal fluids at the 1894Â Ma Montviel alkaline-carbonatite complex, Canada: Insights from metasomatic and hydrothermal carbonates

Alkaline and carbonatite rocks are relatively rare but offer the opportunity to study the contribution of fluids in the genesis of mantle and crustal rocks because they are commonly affected by metasomatism. Carbonate minerals represent versatile archives of mantle and crustal magmatic-hydrothermal processes because they can have magmatic, metasomatic or hydrothermal origins and because they host the trace elements, stable and radiogenic isotopes required to unravel their petrogenesis. Previous studies have shown that the 1894 Ma Montviel alkaline‑carbonatite complex was emplaced through four injections of volatile-saturated, mantle magmas which evolved through fractional crystallization, mixing of mantle and crustal fluids and metasomatism. Trace element analyses and δ18O, δ13C, 87Sr/86Sr and 143Nd/144Nd isotope compositions of metasomatic and hydrothermal carbonates further support that each magma injection was accompanied by a volatile phase. Variations in trace element concentrations suggest that the carbonatite might have exsolved from a metasomatized mantle or hybrid silicate‑carbonatite magma, and that the fluid composition evolved towards higher REE and lower HFSE with increasing degree of segregation of the carbonatite magma and the silicate source. A strong correlation between the C-O-Sr isotopic systems show that mantle fluids mixed with crustal fluids, increasing the 87Sr/86Sr from mantle to crustal values, and driving the C and O isotopic ratios towards respectively lighter and heavier values. The Sm/Nd isotopic system was weakly coupled with the other isotopic systems as depleted mantle fluids mixed with crustal fluids and metasomatized the crystallizing magmas, thereby redistributing the REE and affecting their Sm/Nd ratios. The Nd isotopes suggest that the mixed mantle/crustal fluids redistributed the rare earth elements, producing ultra-depleted (εNd = + 10), normally depleted (εNd = + 4) and slightly enriched (εNd = − 2) isotopic compositions.