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

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 Ca_75–72Na_24–27K_0.5–1 confined to the cores and labradorite Ca_67–52Na_32–47K₁ to the inner and outer rims) and low-Ti augite (augite Ca₄₄Mg₄₅Fe₁₁ with 4.5% Al₂O₃ confined to the cores and augite Ca_43–41Mg_41–38Fe_16–21 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 ulvöspinel 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 Ca_52–50Na_46–48K_2–3 with quartz, minor andesine Ca_49–46Na_49–52K_2–3, 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.