Pyrometamorphic processes at the magma-hydrothermal system interface of active volcanoes: evidence from buchite ejecta of Stromboli (Aeolian Islands, Italy)

Pyrometamorphic ejecta erupted during recent paroxysmal explosions and hydrothermally altered volcanic and subvolcanic rocks from Stromboli volcano (Aeolian Islands, Italy) were studied by combined polarizing microscopy, electron microprobe, scanning electron microscope, X-ray diffraction, transmission electron microscope and whole-rock inductively coupled plasma optical emission spectrometry-mass spectrometry analysis. The pyrometamorphic ejecta are compositionally heterogeneous at the macro- and micro-scale, and are classified as buchites; that is, glass coupled with a high-temperature, low-pressure modal mineralogy. Three distinct facies (i.e. cordierite-plagioclase, mullite-plagioclase palisade and tridymite-clinopyroxene) are recognized among the buchites, whose bulk-rock chemistry is compatible with aluminous- to silica-rich protoliths. Abundant mineral growth (e.g. plagioclase palisades and clinopyroxene coronae) occurs at the contact between the ejecta and the crystal-rich host shoshonitic basalt, which typically coats the pyrometamorphic blocks. Mineral constraints, pseudoporphyritic microstructures, plagioclase relict phenocrysts and major and trace element compositions strongly support the origin of the buchite ejecta as hydrothermally altered rocks from theVancori edifice (a period of activity spanning >10 kyr), which underwent high-temperature, low-pressure contact metamorphism. In the uppermost magmatic system of an active island arc volcano, such as Stromboli, volcanic and subvolcanic rocks usually undergo alteration processes induced by acid-sulphate hydrothermal systems. The circulation of acidic fluids results in leaching of the Stromboli rocks (in the upper part of the volcano edifice) and, depending on the pH, temperature and exposure time to the hydrothermal fluids, production of different alteration facies. The hydrothermally altered Stromboli rocks show facies consistent with argillic to silicic alteration, containing abundant hydrous sulphate and hydroxy-sulphate minerals such as the alunite group minerals, hexahydrite, millosevichite, pickeringite and kieserite. The hydrothermally altered wall-rocks, heated by contact with the basaltic magma of the feeder dyke system of Stromboli, can easily undergo pyrometamorphic processes, inducing partial melting and subsolidus crystallization of the volcanic protoliths. This results in the development of buchite rocks at the magma-hydrothermal system interface.