Decoding a post-collisional multistage magma system: The Quaternary ignimbrites of Aragats stratovolcano, western Armenia

• Published in: Lithos 2018-10, Vol.318-319, p.267-282
• Main Authors: Gevorgyan, Hripsime, Repstock, Alexander, Schulz, Bernhard, Meliksetian, Khachatur, Breitkreuz, Christoph, Israyelyan, Arsen
• Format: Article
• Language: English
• Subjects: Aragats; Geothermobarometry; Glass chemistry; Ignimbrite; Magma mingling; Mineral texture; Mixing
• ISSN: 0024-4937; 1872-6143
• DOI: 10.1016/j.lithos.2018.07.024

Quaternary eruptions from an intraplate multistage magma reservoir of the Aragats stratovolcano in western Armenia formed pyroclastic density currents and fallouts deposited in an area around 2,000 km2, comprising six ignimbrite units and three fallout deposits. Published K/Ar and Ar/Ar age data allow for a distinction between older (Calabrian, 1.8–0.90 Ma) and younger (Middle Pleistocene, 0.75–0.65 Ma) ignimbrites. The detailed characterization of the crystal-poor to crystal-rich, trachyandesitic to rhyolitic ignimbrite sequence deciphers the pre-eruptive magmatic conditions. It also contributes to the understanding of Plinian eruptions of the largest volcanic complex in the Lesser Caucasus. The chemical variation of pumice fragments reveals the conditions of both, homogeneous and mingled heterogeneous magma systems. The phenocryst assemblage of the older crystal-poor units (Ab-rich plagioclase + amphibole ± pyroxene) indicates growth in a cooler hydrous magma chamber, whereas the partially resorbed phenocrysts of the younger crystal-rich units (An- to Ab-rich plagioclase + pyroxene) suggest growth in a superheated magma chamber fueled by mafic, mantle-derived melts. In the younger units, external (e.g. sieve fabrics) and internal phenocryst textures have been generated by decompression and convection during ascent. With the older units, the chemical variation in amphibole from core to the narrow break-down rim reveals magma ascent from a deep (13 km depth) to a shallow (up to 8 km) level. Tracking the p-T paths of cumulate fragments (pyroxene, plagioclase) in the younger units, we estimate the depth of the anhydrous crystal-rich magmatic chamber at around 25 km. Based on the composition of clinopyroxene cumulates, we propose a multi-stage evolution of melts from subalkaline and tholeiitic to calc-alkaline composition. The pronounced high Zr content and Zr/Hf ratios of the entire Upper Pliocene to Quaternary post-collisional magmatism could be attributed to partial melting of an enriched mantle source in the Lesser Caucasus. •Revised Aragats ignimbrite lithostratigraphy•Combined textural and chemical analyses unravel the Aragats magma system•Contribution to the knowledge on dynamics of Lesser Caucasus intraplate magmatism