The Oka carbonatite complex, Quebec: geology and evidence for silicate-carbonate liquid immiscibility

Associated carbonate and silicate igneous rocks from the northern intrusive center of the Oka complex (Quebec, Canada) probably formed from immiscible carbonate and silicate magmas in situ, as their structure, petrography, isotope ratios and mineral chemistry are consistent with immiscibility. The intrusion consists of intimately intermixed rocks derived from carbonate magma (some jacupirangites, okaites, melilite-bearing carbonatites) and silicate magma (some jacupirangites, melilitites). Some jacupirangites and okaites are crystal cumulates. Inferred order of crystallization is: jacupirangites; nepheline okaite and nepheline melilitite; okaite and melilitite; and melilite-diopside carbonatite and diopside melilitite. Carbonate-rich ocelli are common in melilitites, and silicate-rich ocelli are present in rocks derived from carbonate magma. Rare earth elements are six times more abundant in carbonatite than in melilitite, consistent with REE partition experiments on immiscible carbonate and silicate liquids. Initial strontium isotope ratios (∼ 0.7033) are identical in silicate and carbonate rocks, as required by immiscibility. The strongest evidence for immiscibility comes from identity of compositions of magmatic minerals in rocks derived from contemporaneous immiscible fractions (required by liquid immiscibility, but inconsistent with other modes of fractionation). Fassaite pyroxenes in the jacupirangites (derived from both silicate-rich and carbonate-rich magmas) span identical compositional ranges (9.3–16% A1₂O₃; 3.2–4.7% TiO₂). Diopsidic pyroxenes in carbonatites and melilitites also have identical ranges (1.45–2.0% A1₂O₃; 0.31–0.45% TiO₂). Perovskites with magmatic compositions are preserved only in nepheline okaite and nepheline melilitite; here, the perovskites are of identical composition. Other minerals (calcite, melilite, nepheline) do not preserve magmatic compositions.