Parental Melts and Magma Storage of a Large-volume Dacite Eruption at Vetrovoy Isthmus (Iturup Island, Southern Kuril Islands): Insights into the Genesis of Subduction-zone Dacites

• Published in: Journal of petrology 2019-07, Vol.60 (7), p.1349-1370
• Main Authors: Smirnov, S Z, Rybin, A V, Kruk, N N, Timina, T Yu, Sokolova, E N, Kuzmin, D V, Maksimovich, I A, Kotov, A A, Shevko, A Ya, Nizametdinov, I R, Abersteiner, A
• Format: Article
• Language: English
• ISSN: 0022-3530; 1460-2415
• DOI: 10.1093/petrology/egz032

Abstract Detailed mineralogical and melt and fluid inclusion constraints on magma storage, and the evolution of melts, are presented for the large-volume caldera eruption on the Vetrovoy Isthmus on Itutrup Island (Kuril Islands, Russia). The shallow magma reservoir beneath the Vetrovoy Isthmus is composed of a mush of plagio-rhyolitic melt, phenocrysts and the products of peritectic reaction(s). The melt appears to have formed as a result of partial melting of previously erupted rocks, which probably had andesitic to basaltic compositions and were metamorphosed into amphibole-bearing assemblages. The breakdown of amphibole in the partially melted precursor rocks led to the formation of early Mg-rich clino- and orthopyroxene, along with plagioclase and Fe–Ti oxides, and the release of aqueous fluids. Variations in fluid pressure are recorded by a strong increase of An contents in plagioclase. Crystallization took place at around 850°C with pressure ranging from 0·9 to 3 kbar. This study demonstrates that dacitic magmas erupted during the course of a 20 kyr voluminous eruption were the result of mixing between plagio-rhyolitic partial melts and the breakdown reaction minerals (i.e. pyroxenes, plagioclase and Fe–Ti oxides). Plagioclase and quartz were the last minerals to crystallize from these melts prior to eruption.