Fibrous orthopyroxene in the Etnean volcanics of S. Maria di Licodia (Sicily, Italy)

The south-western area of the Mt. Etna volcano has been studied because the presence of peculiar mineralogical association. In our previous study we investigated the Biancavilla area, a dome and dyke system with autobrecciated portion. In the Biancavilla volcanic system the metasomatic process led to the formation of new minerals, fluoro-edenite and fluoroflogopite [1] [2], and some rare minerals, such as pseudobrookite and As-apatite(Ca, F) (under study). In this work we studied the Santa Maria di Licodia area, a dome located on the East side of the Biancavilla volcanic area. Some significant differences are present respect to the Biancavilla, such as the presence of a fibrous orthopyroxene instead of fibrous fluoro-edenite. Fibrous orthopyroxene is rare, and was found in ultramafic xenoliths and interpreted as an autometasomatic product of hydrous magmas [3]. With the aim to investigate a possible metasomatic process in S. Maria di Licodia area, a detailed sampling was performed, and the sampling was executed on both the unaltered lava portion (massive rocks) and altered portion (massive and brecciated rocks) of the dome. Combining mineralogical and petrographic data the presence of fibrous orthopyroxene in both massive and brecciated portion was evidenced. Besides the fibrous opx, the secondary mineral phases also include alkali-feldspars (Na), augitic clinopyroxene, apatite, Fe-Ti oxides. Orthopyroxene show different morphology, such as prismatic, acicular and fibrous. In the massive rocks, fibrous orthopyroxene appears at the expense of probable olivine, in veins and in reaction rim. Here the orthopyroxene forms radiating aggregates totally replacing the previous crystal. The acicular morphology is only present in the pores of the rocks. Finally, in the brecciated portions the orthopyroxene is characterized by loose fibers. Different morphology correspond to differences in composition, in particular in the iron content. The EMPA analyses on the orthopyroxene form the massive rocks evidenced high content of FeO (until 18 wt%), while SEM-EDS micro-analyses on the fibers of the brecciated portion highlighted a more large variability, ranging from 4 to 14 wt % FeO. X-ray investigation on brecciated (fine) portion allowed to individuate and identify the orthopyroxene as an enstatite ferroan, also confirmed by the micro-analyses. [1]Gianfagna A., Oberti R. (2001) - Fluoro-edenite from Biancavilla (Catania, Sicily, Italy): crystal chemistry of a new amphibole end-member. American Mineralogist, 86, 1489-1493. [2]Gianfagna A., Scordari F, Mazziotti-Tagliani S, Ventruti G. (2007) - Fluorophlogopite from Biancavilla (Mt Etna, Sicily, Italy): crystal chemistry of a new F-dominant analogue of phlogopite. American Mineralogist, 92, 1601-1609. [3]Bryant J. A., Yogodzinski G. M., Churikova T. G. (2007) - Melt-mantle interactions beneath the Kamchatka arc: Evidence from ultramafic xenoliths from Shiveluch volcano. Geochemistry Geophysics Geosystems, 8, 4, 24 pp.