Lava – substrate interaction: constraints on perlitic hydration and low temperature mineralization, Lebuj rhyolitic flow, Tokaj Mountains, Carpathian-Pannonian region

The silicic flows and domes can impose mechanical and thermal stress on the underlying substrate causing mineralization and lithification of granular bodies. In addition, the released water from the permeable substrate as dominant volatile species can contribute to the glass hydration of the flow. This fluid-lava interaction can be directly studied in ancient successions with exposed contacts. The Lebuj flow (Tokaj Mountains, Hungary) developed in a Miocene caldera setting, where the erosion revealed its basal zone including lava-substrate interaction textures. The main textural units comprise (1) a rhyolitic lava flow (F₁: perlitic glass with obsidian marekanite, F₂: microcrystalline-glass transition and F₃: a basal breccia layer) and (2) the underlying mixed substrate unit (S₁: massive rhyolite and breccia S₂: enclosed partially sintered rhyolite tuff). The thin section textural analyses were completed by BSE imaging, Raman mapping (SiO₂ polymorphs) and FTIR spot measurement (perlite H₂O, clays). Glass transition temperature (T_g) was estimated using the chemical based GRD model.

The flow margin contacted with underlying volcanoclastic deposits along a steeply inclined (50-75°) plane with subordinate fragmentation. The substrate suffered re-heating by the flow where porosity loss and welding (solid-state sintering) occurred. The silica polymorphs are observed growing into open pore spaces and fractures and interpreted as precipitates from vapor phase fluids passing through the permeable lithologies. The smectite group minerals typically record acidic type alteration, where the water-rock interaction commonly produces glass replacement minerals. The FTIR-identified clays (mixed layer kaolinite/montmorillonite or beidellite) indicate low-to medium alteration degree (estimated temperature between 50-100 °C).

The lithophysae, spherulites and microcrystalline bands in the flow unit are textural evidence for prolonged groundmass crystallization above T_g. The relict obsidian grains in the glass are proofs of an incomplete hydration process. The FTIR and BSE investigations demonstrate the presence of sharp transitions from the hydrated ~3 wt.% perlitic rims to non-hydrated obsidian cores.

Textural and mineralogical evidence suggest that the following series of events occurred as the consequences of the lava-substrate interaction: a) a viscous rhyolite flow advanced on an irregular topography; b) shear and brittle fracturing occurred at the contact; c) groundmass crystallization (above T_g, ~ 690-715 °C) and hydration (below T_g) acted in the flow; d) low temperature mineralization and variable scale sintering occurred in the substrate (below T_g). According to the fluid exchange model beneath silicic lava domes (Ball et al. 2015), the water – rock interaction resulted in weak hydrothermal alteration of the substrate and water flux to the quenched glass (flow). As an interaction of the two processes, the increased sintering and mineralization reduced the porosity of the substrate which probably restricted further water uptake for hydration. Thus the obsidian results from a ‘quenched’ hydration front (Bindeman and Lowenstern 2016).

 

Ball J.L., Stauffer P.H., Calder E.S., Valentine G.A. (2015). Bull Volcanol 77:1–16.

Bindeman I.N., Lowenstern J.B.  (2016).  Contrib to Mineral Petrol 171:89

 

Aknowledgements

This research has been funded by the Hungarian–Italian MTA-CNR bilateral research project 2019–2022. The research was also supported by Development and Innovation Office–NKFIH No. 131869 OTKA project. RL was supported by the Bolyai János Research Fellowship.