Island-Arc Augite–Bytownite–Labradorite Dacites of the Kara-Dag Massif, Crimea

Abstract

Island-arc calc-alkaline dacites (66.7% SiO2, 3.4% Na2O, 1.9% K2O) form a subvolcanic body hosted within andesitic and trachyandesitic tuffs on the east of the Kara-Dag Volcanic Massif of the Crimean Mountains. These dacites characteristically contain many phenocrysts of plagioclase (bytownite Ca75–72Na24–27K0.5–1 confined to the cores and labradorite Ca67–52Na32–47K1 to the inner and outer rims) and low-Ti augite (augite Ca44Mg45Fe11 with 4.5% Al2O3 confined to the cores and augite Ca43–41Mg41–38Fe16–21 with 1–2% Al2O3 to the inner and outer rims). Titanomagnetite, ilmenite, and apatite are intergrown with augite. Low-Mg titanomagnetite is enriched in manganese (up to 4.5 wt % MnO) and zinc (up to 1.6% ZnO) and contains the ulvospinel endmember ranging between 39 and 28%. Low-Mn ilmenite contains 10 to 25 mol % of the hematite endmember, suggesting crystallization at elevated $${{f}_{{{{{\text{O}}}_{2}}}}}$$, that is, under water-saturated melt conditions. The Sr, Ce, and S contents in the apatite are low. The fluorine trend shows an increase in the F content from chlorine-hydroxyl-fluorapatite to fluorapatite. The groundmass of rhyolitic dacites (77.3% SiO2, 3.3% Na2O, and 2.5% K2O) consists of labradorite microlites Ca52–50Na46–48K2–3 with quartz, minor andesine Ca49–46Na49–52K2–3, oligoclase Ca27Na68K5, and anorthoclase in the interstitial spaces. An extremely high anorthite content of plagioclase that is typical of island-arc volcanic rocks is characteristic of these dacites. The crystallization temperature for augite is ~1050–950°C. Early crystallization of the coexisting titanomagnetite and ilmenite occurs at ~900°С with $${{f}_{{{{{\text{O}}}_{2}}}}}$$ 1 log unit higher than the QFM buffer. Their late crystallization occurs at ~880°С with $${{f}_{{{{{\text{O}}}_{2}}}}}$$ 2 log units higher than the QFM buffer.