The Monticellite-Bearing Rocks of the Krestovskaya Intrusion: the Genesis According to Melt Inclusion Study

Abstract

The investigation of monticellitolites and olivine-monticellite rocks from the Krestovskaya Intrusion shows that the principal minerals (olivine and monticellite) contain higher amount of MgO than same minerals in olivinites and kugdites of the Intrusion. In the studied rocks olivine contains 90–93 mol. % Fo and monticellite has 41.6–42.3 mol. % Fo whereas in olivinites and kugdites olivine and monticellite contains 86–87 mol. % Fo, and 37.2–41.2 mol. % Fo, respectively. Melt inclusion study in minerals of monticellite rocks evidenced that the monticellite rocks of the Krestovskaya Intrusion were formed due to mixing melts different in composition and volatiles: K-rich high-iron low-alumina kamafugitic melt and Na-rich high-magnesia high-alumina picritic one. Minerals crystallized at high temperatures in the following sequence: perovskite I (1250–1230°C) → perovskite II (≥1200°C) ↔ olivine (1200°C) → monticellite (1150°C). Perovskite I in monticellite rocks, as well as olivine in olivinites, crystallized from K-rich high-iron (Mg # = MgO/(MgO + FeO) – 0.37), low-alumina kamafugitic melt. During crystallization of late perovskite II in monticellite rocks, the melt became more magnesian (Mg# = 0.41) and richer in Na2O and Al2O3, which is intermediate in composition between kamafugite and alkali picritoid. Olivine in monticellite rocks crystallized from melts similar in composition to melilitite, having a K-rich composition with Mg # = 0.39, whereas monticellite formed from a heterogeneous high-magnesian Si-undersaturated melt, which is highly enriched with volatile components (including H2O) and salts. The crystallization of minerals was accompanied by accumulation of volatile components in the mixing melts and as a result by the further processes of silicate-carbonate liquid immiscibility under 1250–1190°C and by multiphase carbonate-salt immiscibility under below 1190°C. In the latter event, the separated carbonate melt began to decompose into simpler immiscible fractions: alkali-sulfate-carbonate, alkali-phosphate-carbonate and calcio-carbonate.