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

• Published in: Moscow University geology bulletin 2019-11, Vol.74 (6), p.582-591
• Main Authors: Spiridonov, E. M., Korotayeva, N. N., Krivitskaya, N. N., Ladygin, V. M., Ovsyannikov, G. N., Putintseva, E. V., Semikolennykh, E. S., Frolova, Yu. V.
• Format: Article
• Language: English
• Subjects: Apatite; Augite; Chlorine; Cores; Crystallization; Earth and Environmental Science; Earth Sciences; Fluorine; Geology; Haematite; Ilmenite; Island arcs; Magma; Manganese; Mountains; Plagioclase; Silica; Volcanic rocks; Zinc
• ISSN: 0145-8752; 1934-8436
• DOI: 10.3103/S0145875219060115

Island-arc calc-alkaline dacites (66.7% SiO 2 , 3.4% Na 2 O, 1.9% K 2 O) 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 75–72 Na 24–27 K 0.5–1 confined to the cores and labradorite Ca 67–52 Na 32–47 K 1 to the inner and outer rims) and low-Ti augite (augite Ca 44 Mg 45 Fe 11 with 4.5% Al 2 O 3 confined to the cores and augite Ca 43–41 Mg 41–38 Fe 16–21 with 1–2% Al 2 O 3 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 , 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% SiO 2 , 3.3% Na 2 O, and 2.5% K 2 O) consists of labradorite microlites Ca 52–50 Na 46–48 K 2–3 with quartz, minor andesine Ca 49–46 Na 49–52 K 2–3 , oligoclase Ca 27 Na 68 K 5 , 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 1 log unit higher than the QFM buffer. Their late crystallization occurs at ~880°С with 2 log units higher than the QFM buffer.