Extensive and Continuous Solid Solutions in Spinel-Subgroup Minerals in the Rogomu Complex, Lapland–Belomorian Belt, Kola Peninsula, Russia

Abstract The Paleoproterozoic Rogomu complex is one of a large number of basic–ultrabasic intrusions that populate the Lapland–Belomorian Belt in the Kola Peninsula, Russia. The small, zoned complex contains a peridotitic core containing fresh olivine (Fo82–87) and an anthophyllitic amphibole, both with inclusions of texturally unusual spinel-subgroup minerals. Members of two solid solutions, labeled the magnesian aluminous chromite–magnetite and the spinel–hercynite series, crystallized from a relatively high-temperature melt. The analyzed products (n = 549) involve Cr → Fe3+ and Cr → Al substitutions and merge into a single arc-like band parallel to the miscibility gap on an Al–Cr–Fe3+ plot. Skeletal crystals are consistent with the hypabyssal setting. Some grains are smoothly zoned from a chromite-based chromian spinel-subgroup mineral in the core to a spinel-enriched chromian spinel-subgroup mineral phase in the rim. The solid solutions are unusually extensive and continuous. There is no hiatus between Cr-enriched and Fe3+-enriched spinels in the peridotite core. The chromian spinel-subgroup mineral compositions are enriched in chromite–magnesiochromite components at Pados-Tundra–Chapesvara–Lotmvara, located near the hypothetical center of a komatiitic plume. At Rogomu, a distal complex, the chromian spinel-subgroup minerals attained more aluminous compositions.