Evolution and Genesis of Magmas from Vico Volcano, Central Italy: Multiple Differentiation Pathways and Variable Parental Magmas.

Vico volcano has erupted potassic and ultrapotassic magmas, ranging from silica-saturated to silica-undersaturated types, in three distinct volcanic periods over the past 0·5 Myr. During Period I magma compositions changed from latite to trachyte and rhyolite, with minor phono-tephrite; during Periods II and III the erupted magmas were primarly phono-tephrite to tephri-phonolite and phonolite; however, magmatic episodes involving leucite-free eruptives with latitic, trachytic and olivine latitic compositions also occurred. In Period II, leucite-bearing magmas (⁸⁷Sr/⁸⁶Sr<inf>initial</inf> = 0·71037-0·71115) were derived from a primitive tephrite parental magma. Modelling of phonolites with different modal plagioclase and Sr contents indicates that low-Sr phonolitic lavas differentiated from tephri-phonolite by fractional crystallization of 7% olivine + 27% clinopyroxene + 54% plagioclase + 10% Fe-Ti oxides + 4% apatite at low pressure, whereas high-Sr phonolitic lavas were generated by fractional crystallization at higher pressure. More differentiated phonolites were generated from the parental magma of the high-Sr phonolitic tephra by fractional crystallization of 10-29% clinopyroxene + 12-15% plagioclase + 44-67% sanidine + 2-4% phlogopite + 1-3% apatite + 7-10% Fe-Ti oxides. In contrast, leucite-bearing rocks of Period III (⁸⁷Sr/⁸⁶Sr<inf>initial</inf> = 0·70812-0·70948) were derived from a potassic trachybasalt by assimilation-fractional crystallization with 20-40% of solid removed and r = 0·4-0·5 (where r is assimilation rate/crystallization rate) at different pressures. Silica-saturated magmas of Period II (⁸⁷Sr/⁸⁶Sr<inf>initial</inf> = 0·71044-0·71052) appear to have been generated from an olivine latite similar to some of the youngest erupted products. A primitive tephrite, a potassic trachybasalt and an olivine latite are inferred to be the parental magmas at Vico. These magmas were generated by partial melting of a veined lithospheric mantle sources with different vein-peridotite/wall-rock proportions, amount of residual apatite and distinct isolation times for the veins. [ABSTRACT FROM AUTHOR]