Silica-rich Melts in Quartz Xenoliths from Vulcano Island and their Bearing on Processes of Crustal Anatexis and Crust-Magma Interaction beneath the Aeolian Arc, Southern Italy

Abstract

Quartz-rich xenoliths in lavas and pyroclastic rocks from Vulcano Island, part of the Aeolian arc, Italy, contain silicic melt inclusions with high SiO2 (73–80 wt %) and K2O (3–6 wt %) contents. Two types of inclusions can be distinguished based on their time of entrapment and incompatible trace element (ITE) concentrations. One type (late, ITE-enriched inclusions) has trace element characteristics that resemble those of the metamorphic rocks of the Calabro-Peloritano basement of the adjacent mainland. Other inclusions (early, ITE-depleted) have variable Ba, Rb, Sr and Cs, and low Nb, Zr and rare earth element (REE) contents. Their REE patterns are unfractionated, with a marked positive Eu anomaly. Geochemical modelling suggests that the ITE-depleted inclusions cannot be derived from equilibrium melting of Calabro-Peloritano metamorphic rocks. ITE-enriched inclusions can be modelled by large degrees (>80%) of melting of basement gneisses and schists, leaving a quartz-rich residue represented by the quartz-rich xenoliths. Glass inclusions in quartz-rich xenoliths represent potential contaminants of Aeolian arc magmas. Interaction between calc-alkaline magmas and crustal anatectic melts with a composition similar to the analysed inclusions may generate significant enrichment in potassium in the magmas. However, ITE contents of the melt inclusions are comparable with or lower than those of Vulcano calc-alkaline and potassic rocks. This precludes the possibility that potassic magmas in the Aeolian arc may originate from calc-alkaline parents through different degrees of incorporation of crustal melts.