Origin of high silica liquids at Stromboli volcano (Aeolian Islands, Italy) inferred from crustal xenoliths

High-silica igneous xenoliths (granophyre and obsidian fragments with SiO2 ca. 75 wt.%) and hi,oh-silica glass (SiO2 between ca. 64 and 75 wt.%) withi n glass-bearing tonalitic and dioritic xenoliths have been discovered at St romboli. They are well beyond the silica range of the volcanic rocks erupte d during the subaerial activity of the volcano. The granophyre and the obsi dian fragments occur within the Petrazza pyroclastics (high-K andesite pumi ceous scoriae) of the Paleostromboli I period (<100 to >61 ka), whereas the glass-bearing tonalites and diorites are hasted in the Omo basaltic-andesi te lava flows of the Paleostromboli II period (64-55 ka). The obsidian repr esents an extremely evolved liquid derived from low-pressure fractional cry stallization of high-K calc-alkaline magmas of Stromboli, coupled with mino r assimilation of upper crust or terrigenous sediments. The other studied h igh-silica products have an anatectic origin. The granophyre composition is compatible with a genesis by low-degree melting of leucotonalites similar to some Calabrian Are lithotypes. Partial melting of crustal rocks from the Stromboli basement is also demonstrated by interstitial glass in the tonal ite and diorite xenoliths. Textural evidence and the presence of variable g lass compositions suggest that in these plutonic xenoliths the melt chemist ry was controlled by dehydration melting of biotite and non-modal melting o f local mineralogy. High-degree undercooling of granophyre melt and of the partially melted xenoliths is consistent with rapid rise of the host magmas leading to eruption. Whether partial melting was initiated during stoping and transport of xenoliths within the rising magma or in situ during magma storage within the crust has not been unequivocally established.