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Island-arc calc-alcaline dacites (66,7% of SiO2, 3,4% of Na2O, 1,9% of K2O) compose a subvolcanic body among tuffs, andesites and trachyandesites in the east of the Kara-Dag volcanic massif of the Rocky Crimea. The unique features of dacites is abundance of plagioclase phenocrysts (the central zone is bytownite Ca75–72Na24–27K0,5–1; the intermediate and external zones is labradorite Ca67–52Na32–47K1) and low-Ti augite (augite Ca43–41Mg41–38Fe16–21 with 1–2% of Al2O3 composes the core; the intermediate and external zones is augite Ca43–41Mg41–38Fe16–21 with 1–2% Al2O3). Titanomagnetite, ilmenite and apatite form intergrowths with augite. Lowmagnesian titanomagnetite is enriched with manganese (up to 4,5 wt.% MnO) and zinc (up to 1,6% of ZnO); it contains from 39 to 28% of ulvospinel minal. Ilmenite, poor in Mn, contains from 10 to 25 mol.% of hematite minal that demonstrates the crystallization with the raised fO2, in other words, the water saturation of fusion. Apatite is poor in Sr, Ce and S. The trend with standard accumulation of fluorine from chlorine-hydroxyl-fluorapatite up to fluorapatite is shown. Plagioclase microlites — labradorite Ca52–50Na46–48K2–3 composes the cementing mass of rhyolitic composition (77,3% SiO2, 3,3% Na2O, 2,5% K2O) with quartz, small amounts of andesine Ca49–46Na49–52K2–3, oligoclase Ca27Na68K5 and anorthoclase in interstitions. The speciality of the described dacites is plagioclase wealth in anorthite component, what is typical for island-arc volcanites. The crystallization temperature of augite is ~1050–950 ᵒC. The crystallization temperature of associated titanomagnetite and ilmenite of early origin is ~900 ᵒC, fO2 exceed by 1 logarithmic unit the QFM buffer, their late origin crystallization temperature is ~880 ᵒC, fO2 exceed by 2 logarithmic units the QFM buffer. |
