Mediterranean island arcs and origin of high potash volcanoes

Abstract

Active volcanoes of the Mediterranean Sea are distributed along two arc structures: the Hellenic arc in the Aegean Sea and the Calabrian arc in the Tyrrhenian Sea. The active volcanoes in both arcs lie above earthquakes with focal depth greater than 100 km. The depth of these earthquakes increases generally northward reaching a maximum depth of about 200 km in the Aegean Sea and more than 300 km in the Tyrrhenian Sea. The K 2O versus SiO 2 value in volcanic rocks of the active volcanoes of the Hellenic and Calabrin arcs increases with increasing depth of underlying earthquakes which is similar to the pattern found in Pacific and Indonesian volcanic arcs. The high potash rocks of the Mediterranean suite are the culmination of this trend toward increasing potash as earthquake depth increases. The ratio of the trace element rubidium to silica also increases with increasing depth of earthquakes. Both the intermediate earthquakes and volcanic activity in the Mediterranean, as in Indo-Pacific volcanic arcs, are considered to have originated in dehydration of oceanic crust underthrusting the Aegean and Tyrrhenian Seas as a consequence of a counterclockwise rotation of Africa relative to Eurasia. The released water, we believe, works its way toward the surface in a hydrous melt scavenging potash and other alkali on the way. The ultimate ratio of K 2O versus SiO 2 and Rb versus SiO 2 will be determined by the temperature of the hydrous melt and the distance traveled through the asthenosphere.