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

Abstract

High-silica igneous xenoliths (granophyre and obsidian fragments with SiO2 ca. 75 wt.%) and high-silica glass (SiO2 between ca. 64 and 75 wt.%) within glass-bearing tonalitic and dioritic xenoliths have been discovered at Stromboli. They are well beyond the silica range of the volcanic rocks erupted during the subaerial activity of the volcano. The granophyre and the obsidian fragments occur within the Petrazza pyroclastics (high-K andesite pumiceous scoriae) of the Paleostromboli I period ( 61 ka), whereas the glass-bearing tonalites and diorites are hosted in the Omo basaltic-andesite lava flows of the Paleostromboli II period (64–55 ka). The obsidian represents an extremely evolved liquid derived from low-pressure fractional crystallization of high-K calc-alkaline magmas of Stromboli, coupled with minor assimilation of upper crust or terrigenous sediments. The other studied high-silica products have an anatectic origin. The granophyre composition is compatible with a genesis by low-degree melting of leucotonalites similar to some Calabrian Arc lithotypes. Partial melting of crustal rocks from the Stromboli basement is also demonstrated by interstitial glass in the tonalite and diorite xenoliths. Textural evidence and the presence of variable glass compositions suggest that in these plutonic xenoliths the melt chemistry was controlled by dehydration melting of biotite and non-modal melting of 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.