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

Abstract

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. % ZrO 2 ), 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 , poor in CaO (0.26-0.37 wt. %), MgO (0.01-0.11 wt. %), and P 2 O 5 (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. Received: 29 October, 2015; accepted: 12 January, 2016; handling editor: K. Schulmann This paper is intended to provide new geochemical and geochronological data on the astrophyllite-and alkali amphibole-bearing rhyolite dyke cutting the Lower Paleozoic volcanosedimentary and plutonic sequence in the Hovd Zone in Western Mongolia. Keywords: rhyolite, A-type, astrophyllite, K-Ar geochronology, CAOB

Main mafic and accessory mineralogy in granites and greisens from selected occurrences in the A-type Graciosa Province, S-SE Brazil, and petrological implications

“…Compositions similar to the main Ast-1 crystals were reported in primary astrophyllite crystals from the Oslo Rift Valley, the Oktyabrsky Massif, and the Mongolian Altai (Andersen et al, 2013;Kryvdik and Sharygin, 2014;Žáček et al, 2016); while rim compositions approached those of secondary astrophyllite in the Strange Lake (Piilonen et al, 2003a). As compared with Mandira, the Ast-1 crystals from the Papanduva Pluton present higher Ti contents (up to 1.68 apfu), accompanied by somewhat lower Zr (up to 0.45 apfu) and Nb (up to 0.28 apfu) abundances.…”

Anorogenic Early Permian dykes in the western Mongolian Altai - petrography, geochemistry and K-Ar geochronology

“…Brown slightly pleochroic Fe-dominant astrophyllite occurs as radiating aggregates of up to 0.3 mm long and forms two generations: Zr-Nbrich astrophyllite I and Zr-poor astrophyllite II. Both generations share an expressive dominance of Fe over Mn, from the literature up till now unknown (seeŽáček et al 2016 and compare Piilonen et al 2003). Accessories are zircon, ilmenite, hematite, monazite, and REE-rich fluorite; probable chevkinite and bastnäsite.…”