The Petrology of the Golygin Ignimbrite (South Kamchatka)

Abstract

Abstract—

This paper presents the results of a detailed petrological study of a continuous section of the Golygin ignimbrite, related to the Pauzhetka caldera-forming eruption (443 ± 8 ka BP). The continuous zoning of the studied sequence suggests that a large magmatic chamber, partially emptied during the eruption, was also zoned and had significant vertical dimensions. Our research included determining the bulk composition of rocks, studying samples of the Golygin ignimbrites in thin sections, and detailed studying of quartz and melt inclusions in it. The roof of the magmatic chamber was at a depth of about 5 km; amphibole and plagioclase crystalloclasts in rocks at the base of the sequence show evidence of formation in a more basic melt at a depth of 24 to 25 km. The composition of the magmatic melt reconstructed from melt inclusions in quartz from this sequence corresponds to medium-K rhyolites (wt %): SiO₂ 78.1–77.9, Al₂O₃ 12.7–12, FeOt 0.8–0.1, MgO 0.1–0.0, CaO 1.0–0.2, K₂O 4.3–3.8, Na₂O 4.9–3.3, Cl₂ 0.1–0.2. Crystallization temperatures calculated for various minerals and their assemblages vary in the range 1009–784°С, which, together with estimates of formation depths of 25 km to 5 km, may reflect different stages of a crystallization of the magmatic melt, magma ascent and evolution in a relatively shallow chamber where it was slowly cooled and crystallized. The maximum water content of the magmatic melt during quartz crystallization was estimated in the range from 4.1 to 7.5 wt % H₂O.