The Szigliget maar/diatreme, Bakony- Balaton Highland Volcanic Field (Hungary)

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Massey University.
A preliminary volcanological mapping has been carried out in the western part of the Bakony- Balaton Highland Volcanic Field (BBHVF) around Szigliget (Hungary) village. Pyroclastic rocks have been found building up the three distinct hillsides in the area. The pyroclastic rock beds in each hillside show similar north-westward dip direction and similar textural and compositional characteristics, suggestive of a complex but closely related volcanic system in the area. The pyroclastic deposits have been grouped into three units according to their textural, compositional and stratigraphic characteristics. Unit 1 which represents the lowermost stratigraphic position crops out in the southern side of the study area. It consists of coarse-grained, matrix-supported massive to weakly bedded, accidental lithic clast-rich, block-bearing lapilli tuffs / tuff breccias, extremely rich in deep-seated accidental lithic and peridotite lherzolite clasts. Unit 2 which represents an intermediate stratigraphic position crops out in the southern and north-east hilltops. It consists of coarse-grained accidental lithic clast-rich, normal graded, bedded, vitric lapilli tuffs / tuff beds. Deep-seated accidental lithic clasts are common, but large peridotite lherzolite fragments are relatively rare. Unit 3 which represents the highest stratigraphic position in the area crops out in the northwestern side. It consists of fine-to-coarse grained, bedded, accidental lithic clast-rich vitric lapilli tuff / tuff beds. Deep-seated lithic clasts as well as peridotite lherzolite fragments are rare. Accidental lithic clasts, derived from shallow prevolcanic strata (Neogene sediments), have a dominant proportion of pyroclastic rocks in this unit. In each unit the volcanic glasses are angular, non- to highly vesiculated tephrite to phono-tephrite shards. The presence of sideromelane glass shards and the large amount of accidental lithic clasts in beds from each units indicate subsurface phreatomagmatic explosive processes during formation of pyroclastic rocks at Szigliget. The pyroclastic rocks are interpreted as part of a former crater rim deposits around maar basin which subsequently subsided inward into a vent. Unit 1 is interpreted to be a lower diatreme deposit and Unit 2 and Unit 3 a series of near-vent pyroclastic density currents and fallout tephra.