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 05Myr. 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 mag- mas (87Sr/86 Srinitial ˆ 071037---071115) 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 differen- tiated 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/86Srinitial ˆ 070812---070948) were derived from a potassic trachybasalt by assimilation---fractional crystallization with 20---40% of solid removed and r ˆ 04---05 (where r is assimilation rate/crystallization rate) at different pressures. Silica-saturated magmas of Period II (87Sr/86Srinitial ˆ 071044---071052) 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.