Sekaninaite (Fe-dominant cordierite) from Devonian S-type rhyolites in Victoria, Australia: genesis and comparisons with other occurrences in volcanic rocks

Abstract Fe-dominant cordierite, the mineral sekaninaite, is relatively rare in calc-alkaline volcanic rocks, with only a handful of recorded localities. The compositionally zoned phenocrysts in the Late Devonian Rubicon Rhyolite in central Victoria, Australia are the most Fe-rich known, with core compositions having Mg# of 10–30. Similar cordierite–sekaninaite (Crd–Skn) phenocrysts occur in: rhyolites and rhyodacites from San Vincenzo and Roccastrada in Tuscany; on Lipari Island in Italy; in the Nefza province in northern Tunisia; and in a newly discovered Devonian rhyolite at Arthurs Seat, Victoria. All show well-developed sector twinning in response to structural ordering during cooling. The Victorian sekaninaite crystals all show strong zonation from Fe-rich cores to rims richer in Mg, i.e. reverse zoning. In contrast, the Italian and Tunisian examples have relatively weak normal zoning. In the Roccastrada and Lipari rocks, a second type of Crd–Skn occurs as turbid fragments and as groundmass crystals, with the latter showing reverse zoning. Reverse zoning in the Rubicon and Arthurs Seat rhyolite phenocrysts and the Lipari groundmass crystals is attributed to the reheating of their host magmas due to interaction, such as mixing, with a higher-temperature magmas prior to eruption. The ranges of Crd–Skn compositions in all volcanic rocks bear no systematic relationship to the bulk compositions of their host rocks. Assuming that the whole-rock compositions approximate the original magmatic liquids, and an initial H 2 O content of 5 wt.% throughout, enabled a comparison to be made between the relative P–T conditions of most occurrences, apart from Lipari. Results suggest that the Rubicon Rhyolite had the lowest P–T conditions with Roccastrada next, and San Vincenzo the highest. It appears that sekaninaite might be stable in silicic volcanic magmas over a wide range of melt compositions, pressures and temperatures but is favoured for low-Mg# bulk compositions at low P and low T .