Formation of esseneite and kushiroite in calc-silicate skarnoid xenoliths from Southern Slovakia

Skarnoid calc-silicate xenoliths composed of anorthite, clinopyroxene and Mg-Al spinel were discovered in an alkali basalt quarry located in the Belinsky vrch lava flow, near Fiľakovo (Southern Slovakia). Randomly oriented tschermakite pseudomorphs are replaced by olivine, spinel, and plagioclase. The relict amphibole within the pseudomorphs is characterized by high ^VIAl (1.95 to 2.1 apfu), and very low occupancy of the A-site (<0.1 apfu), which are a diagnostic feature of high-pressure metamorphic rocks. Pyroxene compositions plot along continuous mixing line extending from nearly pure diopside-augite towards a Ca(Fe³⁺Al)AlSiO₆ endmember with an equal proportion of ^VIAl³⁺ and Fe³⁺. Forsterite (Fo_72–83) and Fe³⁺-rich ilmenite crystallized from the melt, leaving behind the residual calcic carbonate with minor MgO (1–3 wt%). Euhedral aragonite and apatite embedded in the fine-grained calcite or aragonite groundmass indicate slow crystallization of residual carbonatite around the calcite-aragonite stability boundary. Olivine-ilmenite thermometry (Andersen & Lindsley, 1981) yielded temperatures between 770 and 860 °C. Pressures of 1.8–2.1 GPa were estimated by intersection of the olivine-ilmenite thermometer with the calcite-aragonite stability boundary calculated for a CO₂ saturated environment using Perple_X (Connolly, 1990). Tschermakite touching interstitial plagioclase was suitable for the application of the barometer of (Molina et al., 2021), which yielded 781±13 °C and 2.05±0.03 GPa consistent with the olivine-ilmenite-calcite-aragonite thermobarometry. The estimated PT conditions fall well inside the garnet stability field, although no garnet has been observed in the mineral assemblage. However, the presence of esseneite and kushiroite with melilite inclusions suggest high CO₂ partial pressure, low SiO₂ activity and strongly oxidizing conditions, in which the high Al, Fe pyroxenes are formed at the expense of the garnet (Ohashi & Hariya, 1975). The protolith is still ambiguous, and two options have been considered. The relict tschermakite in spinel-plagioclase-forsterite pseudomorphs suggests a metamorphosed calc-silicate marble originating from a sedimentary protolith. High Cr contents in spinel and pyroxene, abundant Cu-sulfides, and high CaO contents, 0.3–1.0 wt% CaO, in forsterite, suggest a magmatic protolith, similar to layered gabbro-anorthosite complexes modified by interaction with calcic carbonatite melt.

References:

Andersen, D., & Lindsley, D. (1981). A valid Margules formulation for an asymmetric ternary solution: revision of the olivine-ilmenite thermometer, with applications. Geochimica et Cosmochimica Acta, 45(6), 847–853.

Connolly, J. (1990). Multivariable phase diagrams; an algorithm based on generalized thermodynamics. American Journal of Science, 290(6), 666–718.

Molina, J., Cambeses, A., Moreno, J., Morales, I., Montero, P., & Bea, F. (2021). A reassessment of the amphibole-plagioclase NaSi-CaAl exchange thermometer with applications to igneous and high-grade metamorphic rocks. American Mineralogist, 106(5), 782–800.

Ohashi, H., & Hariya, Y. (1975). Phase relation of CaFeAlSiO6 pyroxene at high pressures and temperatures. The Journal of the Japanese Association of Mineralogists, Petrologists and Economic Geologists, 70(3), 93–95.