Ondrejka M., Broska I. & Uher P. , 2015: The late magmatic to subsolidus T-fO₂ evolution of the Lower Triassic A-type rhyolites (Silicic Superunit, Western Carpathians, Slovakia): Fe-Ti oxythermometry and petrological implications. Acta Geologica Slovaca, 7, 1, 51–61.

The late magmatic to subsolidus T-fO₂ evolution of the Lower Triassic A-type rhyolites (Silicic Superunit, Western Carpathians, Slovakia): Fe-Ti oxythermometry and petrological implications

Neskoro magmatický až subsolidový T-fO₂ vývoj spodnotriasových ryolitov A-typu (silicikum, Západné Karpaty, Slovensko): oxytermometria Fe-Ti oxidov a petrologické implikácie

Martin Ondrejka¹, Igor Broska² & Pavel Uher¹

¹Department of Mineralogy and Petrology, Faculty of Natural Sciences, Comenius University in Bratislava, Mlynská dolina, Ilkovičova 6, 842 15 Bratislava, Slovakia; mondrejka@fns.uniba.sk
²Geological Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 840 05 Bratislava, Slovakia

Abstract

Iron-titanium oxides represent significant mineral phase (≤1.5 vol. %) in the A-type rhyolites from the Silicic Superunit, Western Carpathians, central Slovakia. They are composed of magnetite, ilmenite, and rutile showing typical lamellae “trellis - sandwich” type texture as a result of oxy-exsolution of former Ti-rich magnetite. A composite type and complete decomposition of primary Ti-rich magnetite to a very fine lamellae aggregate of hematite/rutile or to the progressive C4 stage of Fe-rich rutile/hematite occurs in lesser extent. The compositional data show significant miscibility along the ilmenite–hematite join. The ulvöspinel component in magnetite reaches up to 18 mol. %, while the rutile is more uniform and close to end-member composition. However, in some cases rutile is enriched in Fe getting up to 5 wt. % FeO_total mainly with coexisting ilmenite–hematite_ss. Fe–Ti oxide geothermometry yields a relatively smooth evolutional T-trend ranging from late-magmatic to subsolidus/solidus stages with equilibrium temperatures from ~750 to ~400 °C and fO₂ values approaching the NiNiO buffer from –0.76 Δlog fO₂ (~626 °C) to 1.53 Δlog fO₂ (~655 °C). The presence of Ti-rich magnetite, Ti-poor magnetite, ilmenite to hematite, and discrete rutile in rhyolitic magma point to a long termed process from orthomagmatic to a low-temperature subsolidus/solidus stage showing a gradual decrease of oxygen fugacity. The Fe–Ti oxide investigation indicates a relatively short-term storage of the Permian A-type silicic melt at shallow depths prior to eruption.

Key words: Western Carpathians, A-type rhyolite, oxygen fugacity, oxide thermometry, Fe-Ti oxides, magnetite, ilmenite

Manuscript received: 2014-10-14

Revised version accepted: 2015-03-27