CALC-ALKALINE MAGMATISM AND RIFTING OF THE DEEP-WATER VOLCANO OF MARSILI (AEOLIAN BACK-ARC, TYRRHENIAN SEA)

Geochemistry and mineral chemistry data indicate that new lava samples recovered from a large depth interval of Marsili volcano (southern Tyrrhenian Sea) are calc-alkaline. The deep to intermediate portions of the volcano are made up by calc-alkaline basalts, whereas the summit consists of high-potassium calc-alkaline andesites. The contents of MgO, CaO, total iron, Cr, Ni and V decrease with increasing silica, while K2O, Rb, Ba, Th, Nb and LREE exhibit the opposite behavior. The andesite lavas of the top, at waterdepth shallower than about 800 m, are younger than 0.2 Ma. Composition of the basalts varies with the waterdepth; overall, the deepest products of Marsili are geochemically more mafic than those erupted in the higher physiographic positions. Geochemical features suggest the existence of a mantle source that was modified by metasomatic acquirement of subduction-derived incompatible elements. The seamount lies on oceanic crust which formed above subducting lithosphere not earlier than about 2 Ma ago. It is currently undergoing extension along linear faults. The major distensional fractures trend N-S to N15-degrees-E paralleling the physiographic elongation of the volcano. At the summit, later faults have an oblique NE-SW orientation which is associated with the eruption of high-potassium andesites. The transition from basalts to high-potassium andesites is discussed as a possible response to the extension and rapid subsidence that dominate the volcano and surrounding region.