Authors:Réka Lukács, György Czuppon, Szabolcs Harangi, Csaba Szabó, Theodor Ntaflos, and Friedrich Koller
Silicate melt inclusions are frequent in the phenocryst phases (quartz, plagioclase, orthopyroxene, ilmenite and accessory minerals) of the Miocene silicic pyroclastic rocks of the Bükkalja Volcanic Field, Northern Hungary. These melt inclusions were trapped at different stages of magma evolution; therefore, they provide important information on the petrogenetic processes. The melt inclusions in the Bükkalja pyroclastic rocks show various textures such as (1) wholly enclosed type; (2) hourglass inclusions and (3) reentrant or embayment glass. Among the wholly enclosed type melt inclusions further textural subgroups can be distinguished based on their shape: negative crystal, rounded, elongated and irregular shaped. These various textures reflect differences in the time of entrapment prior to eruption and in the post-entrapment condition in the magma chamber. The largest textural variation was found in the quartz-hosted melt inclusions. However, the major element compositions of these melt inclusions do not differ from one another in the same unit. In general, compositions of the melt inclusions are similar to the chemistry of the glass shards. Comparing the composition of the quartz-hosted melt inclusions from three main ignimbrite units (Lower, Middle and Harsány Ignimbrite Units), slight differences have been recognized, suggesting distinct erupted host magmas. Melt inclusions from the andesitic lithic clast of the Lower Ignimbrite Unit could represent heterogeneous interstitial melt in the crystal mush zone at the magma chamber wall. The largest geochemical variation was found in the melt inclusion of the Middle Ignimbrite Unit, even in single samples. This compositional variation overlaps that of the rhyolitic juvenile clasts, but does not match that of the glasses of scoria clasts. We suggest that syn-eruptive magma mixing (mingling) occurred in a compositionally heterogeneous magma chamber of the Middle Ignimbrite Unit.