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A dyke of alkali rhyolite intrudes the Tsetseg and Zuun Nuruu volcanosedimentary sequence of Ordovician–Silurian age (Hovd Zone, Central Asian Orogenic Belt) at the Botgon bag, Mankhan Soum, Hovd District in Western Mongolia. The rock consists of quartz and K-feldspar phenocrysts set in fine-grained groundmass composed of quartz, K-feldspar, albite, blue alkali amphibole (riebeckite–arfvedsonite containing up to 1.94 wt. % ZrO2), tiny brown radial astrophyllite, annite and accessory zircon, ilmenite, fluorite, monazite, hematite, chevkinite and bastnäsite. Astrophyllite has unusual, highly ferroan composition and occurs as two sharply bound zones of astrophyllite I and II with the average empirical formulae: (K1.71 Na0.01Rb0.08Cs0.01) (Na0.93Ca0.07) (Fe 2+ 6.52Mn0.31Zn0.06) (Ti0.84Zr0.50Nb0.55) Si7.68Al0.32 O26 (OH)3.78 F0.66 (astrophyllite I, Zr–Nb-rich); (K1.52Rb0.07) (Na0.81Ca0.19) (Fe 2+ 6.31 Mn0.28Zn0.06) (Ti1.28Nb0.30Zr0.28) Si7.68Al0.32 O26 (OH)2.85 F0.67 (astrophyllite II). Geochemically, the rhyolite corresponds to strongly fractionated silicic alkaline A-type (ferroan) magmatic rock with 75.5–75.9 wt. % SiO2, 4.4 wt. % K2O, 3.9–4.3 wt. % Na2O and 1.98–2.23 wt. % Fe2O3 t, poor in CaO (0.26–0.37 wt. %), MgO (0.01–0.11 wt. %), and P2O5 (0.01 wt. %). The rock is enriched in Zr, Nb, Ta, Ga, Sn, Y, Rb, Cs, U and Th, depleted in V, Sr, Ba, Sc, and exhibits a pronounced negative Eu anomaly (Eu/Eu* = 0.03–0.05). The conventional whole-rock K–Ar geochronology yielded an age of 299.9 ± 9.1 Ma (1σ), which indicates latest Carboniferous or early Permian extension associated with the A-type alkaline volcanic activity.

Astrophyllite-alkali amphibole rhyolite, an evidence of early Permian A-type alkaline volcanism in the western Mongolian Altai