Lava – substrate interaction: Constraints on flow emplacement and basal sintering, Lebuj rhyolitic flow, Tokaj Mountains, Carpathian-Pannonian region

Szepesi, János and Vonák, Alexandra and Kovács, István János and Fintor, Krisztián and Molnár, Katalin and Scarani, Alex and Giordano, Guido and Lukács, Réka (2023) Lava – substrate interaction: Constraints on flow emplacement and basal sintering, Lebuj rhyolitic flow, Tokaj Mountains, Carpathian-Pannonian region. JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH (441). No-107878. ISSN 0377-0273

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Abstract

The surface extrusion of rhyolite lava is linked with a complex edifice morphology and texturally diverse internal structure. The rarely exposed basal flow/dome zones provide a textural record of the mechanical and thermal stress between the overlying lava and existing topography. The Lebuj flow (Tokaj Mountains, Hungary) devel- oped in a Miocene caldera setting, where the erosion exposed its basal zone, including various textures due to the lava substrate interaction. The flow margin is contacted with underlying volcaniclastics along a steeply inclined (50–75◦) plane. The lithology describes a complex ductile-brittle transition in the flow and lithification process in the substrate. The relict obsidian grains (marekanite, 0.1% H2O) in the perlite (2–3% H2O) are proof of an incomplete hydration process below the glass transition temperature. The dynamic loading of the lava caused irregular fragmentation developing a brittle basal shear zone with lens-like glass deformation. The substrate has suffered syn-emplacement lithification and the primary glass structure is completely re-crystallized. FTIR and Raman measurements identified low-temperature phyllosilicate minerals and SiO2 polymorphs, which caused additional porosity loss and densification. Using cooling time-temperature-porosity information given by this reconstruction, we suggest parallel-acting processes at elevated (groundmass crystallization-devitrification) and lower temperature (hydration-secondary mineralization) range. These define a relative timescale for the textural development in the lithologically heterogeneous contact zone.

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