Silicate melt inclusions in the phenocrysts of the Szomolya Ignimbrite, Bükkalja Volcanic Field (Northern Hungary): Implications for magma chamber processes

Abstract

Silicate melt inclusions provide important information on the magma evolution processes and therefore they are the subjects of rapidly growing geochemical studies. However, the original geochemical fingerprint of the trapped melts could be often masked by post-entrapment processes. In this paper, we investigate silicate melt inclusions hosted by various minerals (plagioclase, orthopyroxene, clinopyroxene, quartz) of a silicic ignimbrite and the enclosed igneous lithic clasts in order to reconstruct the magma chamber processes. The Szomolya Ignimbrite was formed at the initiation of the widespread Neogene volcanism of the Pannonian Basin, eastern–central Europe. It is composed of fresh rhyolitic pyroclastics and contains abundant andesitic lithic clasts. The melt inclusions in the phenocrysts are composed mostly of glass with or without shrinkage bubble suggesting rapid cooling. They all have rhyolitic composition, but show significant chemical variability. The melt inclusions in quartz and plagioclase of the ignimbrite have similar compositions as the glass shards. They represent a strongly silicic and potassic (SiO 2 = 76.5 to 79 wt.%; K 2O / Na 2O = > 2) melt resided at the top of the magma chamber at about 2–3 km depth. The melt inclusions in the phenocrysts of the lithic clast show larger chemical variation. It is remarkable that the plagioclase-hosted melt inclusions differ significantly from all the other glass data. This can be explained by the influence of significant post-entrapment crystallization (PEC), i.e. chemical modifications between the entrapment and quenching. Based on major and trace element mass balance modeling, we show that about 10% PEC took place in the plagioclase-hosted melt inclusions, whereas PEC was less in the pyroxene-hosted melt inclusions (about 3%). After correcting this modification, the melt inclusions from the pyroxenes still show wide chemical variation forming linear trends in the Harker diagrams overlapping partly the compositional trend of the groundmass glass. We interpret this as incorporation of heterogeneous melt batches during the mineral growth at the solidification front of the magma chamber at about 5–6 km depth. We assume that there is a genetic relationship between the andesitic lithic clasts and the host ignimbrite. The former ones could reflect crystallization at the solidification front at greater depth, whereas the latter one could represent the silicic residual melt accumulating at the top of the magma chamber. Difference in trace element compositions can be explained by late stage removal of accessory minerals such as zircon and allanite, which strongly controlled the trace element budget of the most differentiated melt. Withdrawal of the rhyolitic magma could enhance the disruption of the deeper solidification front. Its mineral assemblage (clinopyroxene, orthopyroxene and plagioclase) came in contact with a slightly more evolved melt, which quenched suddenly due to the eruption of the silicic magma.