Mineral chemistry of the calcalkaline lavas from Marsili seamount (southeast Tyrrhenian sea): some magmatological and geodynamic considerations

Mineral chemistry data from lavas recovered over a large depth interval of the deep-water volcano of Marsili (Aeolian back-arc, SE Tyrrhenian Sea) are presented. The volcano is built up by products of calcalkaline affinity (20 samples) with the only exception of one outcrop of sub- alkaline, within-plate basalts in the crestal area. Petrography and mineral chemistry data indicate that the low parts of edifice consist of basalts which present a porphyritic vesicular texture. Basalt phenocryst assemblages consist either of plagioclase and little olivine and clinopyroxene (porphyritic index, P.I. about 13%) or of olivine and scarce pyroxene and plagioclase (PA. < 5%). The summit area is constructed by andesites which have a porphyritic texture (PA. about 15%) with plagioclase and little clino- and ortho-pyroxene and magnetite. On the whole, the calcalkaline lavas of Marsili have bimodal composition, and consist of basalts and andesites with, respectively, calcalkaline and high-K serial affinity. The andesites mark a change in the geodynamic conditions that is reflected by the clockwise rotation of the seamount rifting from the early NNE-SSW orientation, associated with the extrusion of the basalts, to a dominant tensional regime trending almost NE-SW. This evolution is discussed in the context of some magmatic- structural features in the south-east Tyrrhenian volcanic area. The region is characterized by continental arc volcanism migration from the island of Sardinia, to the central Tyrrhenian Sea, to the Aeolian area.