Mixing of crystal mushes and melts in the genesis of the Bogács Ignimbrite suite, northern Hungary: An integrated geochemical investigation of mineral phases and glasses

Abstract

The 16.4Ma old Bogács Ignimbrite, located south of the Bükk Mountains, northern Hungary, was formed during a silicic ignimbrite flare-up in the Pannonian Basin that occurred from 20Ma to 13Ma. It comprises two main units, a lower, variably welded pumiceous and an upper, scoriaceous pyroclastic flow unit. Bulk chemistry of the juvenile clasts indicates a gradual change of geochemical character with an upward decreasing SiO2 content through the stratigraphic section. A detailed in-situ major and trace element investigation of the main mineral phases and glasses combined with petrogenetic model calculations reveals complex magma reservoir processes. Based on the major and trace element variability, six juvenile clast types were distinguished and each contain fresh glass fractions with distinct compositions. The mineral assemblage consists of plagioclase, orthopyroxene, biotite with minor and variable amounts of quartz, amphibole, ilmenite, zircon and allanite. The anorthite content of the plagioclases varies from 20 to 90mol%, whilst the Enstatite content of orthopyroxenes covers also a wide range from 40 to 90mol%. This large compositional variation can be detected even in single crystals. This extreme geochemical variability can be explained by mixing of crystal mush bodies evolved from both basaltic and more silicic magmas. The calcic plagioclases (An=80–90mol%) and magnesian orthopyroxenes (En=70–90mol%) clearly indicate the role of primitive mafic magmas in the growth of the silicic magma reservoir, even though no basaltic volcanic activity was associated with the Miocene silicic volcanism in the Pannonian basin. The prolonged crystallization in the mushy sills resulted in compositionally different residual melt fractions that moved upwards and accumulated in separated melt pods at the roof of the magma reservoir. Intermittent intrusions of mafic and intermediate magmas into this silicic magma system could have resulted in thorough stirring of the crystal mush bodies and the melt pods, leading to eruptive products having compositionally heterogeneous glass and mineral assemblage.