Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

Vladimír Žáček, David Buriánek, Zoltán Pécskay, Radek Škoda

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

Journal of Geosciences, volume 61 (2016), issue 1, 93 - 103

DOI: http://doi.org/10.3190/jgeosci.205

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:
(K_1.71 Na_0.01Rb_0.08Cs_0.01) (Na_0.93Ca_0.07) (Fe²⁺_6.52Mn_0.31Zn_0.06) (Ti_0.84Zr_0.50Nb_0.55) Si_7.68Al_0.32 O₂₆ (OH)_3.78 F_0.66 (astrophyllite I, Zr-Nb-rich);
(K_1.52Rb_0.07) (Na_0.81Ca_0.19) (Fe²⁺_6.31 Mn_0.28Zn_0.06) (Ti_1.28Nb_0.30Zr_0.28) Si_7.68Al_0.32 O₂₆ (OH)_2.85 F_0.67 (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.