Magma feeding an early hydromagmatic activity in the potassic period of Stromboli: the cases of the COA pyroclastic sequence and San Vincenzo scoria cone

A hydromagmatic volcanoclastic sequence (Semaforo Nuovo succession), cropping out close to the Advanced Operations Center (COA) of the Department of Civil Defense [the so called “COA succession” of Porreca et al., 2006] has been until now attributed to the final activity of the Neostromboli period, nearly coeval with the eruption of the Secche di Lazzaro pyroclastic sequences [Bertagnini and Landi, 1996; SDL-S and SDL-N of Petrone et al., 2009]. The Semaforo Nuovo pyroclastic succession is an approximately 3-4 mt thick deposit made up of mostly thinly bedded to massive lapilli-tuffs with abundant accretionary lapilli deposited from mainly PDCs with variable clast concentration. This sequence has been sampled in detail, collecting pumices from each stratigraphic layer for a total of about 10 samples. The juvenile fragments (scoriae and abundant quite small golden pumices) of the Semaforo Nuovo pyroclastic sequence have evolved high-K shoshonite compositions (K2O around 4.5 wt%, with silica of 53.5-55.2 wt%), with low MgO contents (2.3-3.0 wt%), which is the typical composition of the KS evolved rocks. In detail, pumices have composition slightly different than the grey scoria, suggesting slight zoning of magma reservoir. All the samples have a paragenesis characterized by plagioclase, clinopyroxene, olivine and opaques. Mineral chemistry analyses show quite variable composition for clinopyroxene (Mg# 0.68-0.87) and plagioclase An77-86, whereas olivine has slightly constant composition with Fo67-68. The Semaforo Nuovo pyroclastic sequence directly overlies the deposits of the San Vincenzo scoria cone, formed in the area of the Stromboli village and representing a relatively recent eccentric vent, whose activity is dated back to 12.5 ka [Wijbrans et al., 2010]. It consist of a succession of welded to loose, strombolian fallout scoriae, lava spatter and flows and minor pumices emitted from a slightly NE-SWelongated eruptive fissure [Calvari et al., 2010; Francalanci et al., in press). A total of 16 samples were taken on the scoria cone flank, from several new outcrops near the football pitch. All the samples show hypocrystalline groundmass and highly vesiculate, glomeroporphyritic textures with a paragenesis dominated by clinopyroxene, plagioclase, olivine and opaques [Klaver, 2008]. Compositional data show that both the San Vincenzo scoriae and lava flows are high-K shoshonites belonging to the evolved group of the KS rocks. They also reveal that the San Vincenzo products have a strict geochemical and isotopic similarity with the Semaforo Nuovo succession, thus the two deposits have been related to a unique period of mild explosive activity that took place during the early stages of the Neostromboli potassic activity. This study seems therefore to validate the existence of an evolved hydromagmatic explosive phase during the effusive potassic period and not only at the end of the period. References Bertagnini, A. and Landi, P. (1996). The Secche di Lazzaro pyroclastics of Stromboli volcano: a phreatomagmatic eruption related to the Sciara del Fuoco sector collapse. Bulletin of Volcanology 58, pp 239-245. Calvari, S., Branca, S., Corsaro, R.A., De Beni, E. Miraglia, L. Norini, G. Wijbrans, J.R. and Boschi, E. (2010). Reconstruction of the eruptive activity on the NE sector of Stromboli volcano: timing of flank eruptions since 15 ka. Bulletin of Volcanology, doi:10.1007/s00445-010-0412-5. Klaver, M. (2008). A Neostromboli eccentric vent in Stromboli village, Stromboli, Aeolian islands, Italy. Geochemistry and petrography of new outcrops. Bachelor thesis, Vrije Universiteit, Amsterdam. Porreca, M., Giordano, G., Mattei, M. and Musacchio, P., (2006). Evidence of two Holocene phreatomagmatic eruptions at Stromboli (Aeolian Islands) from paleomagnetic data. Geophysical Research Letters 33, L21316, doi:10.1029/2006GL027575. 35CONFERENZA A. RITTMANN Nicolosi (Catania) 12 | 14 dicembre 2012 Wijbrans, J.R., Schneider, B., Calvari, S., Branca, S., De Beni, E., Norini, G., Corsaro, R.A. and Miraglia, L. (2010). 40Ar/39Ar geochronology of Holocene basalts; examples from Stromboli, Italy. Journal of Quaternary Geochronology. 10.1016/j.quageo.2010.10.003.