THE TECTONIC SIGNIFICANCE OF ITALIAN MAGMATISM - AN ALTERNATIVE VIEW TO THE POPULAR INTERPRETATION

The Tyrrhenian rift zone has been the site of widespread magmatism sin ce late Tortonian times. A pronounced asymmetrical distribution, refle cting the tectonic structure, characterizes Italian magmatism. Sodic b asalts occur on the western Tyrrhenian flank and transitional-MORB bas alts occur in the Tyrhenian Sea. The eastern flank, however, is charac terized by K-alkaline and HK- to ultra-alkaline (e.g. carbonatites and melilitites) rocks. Major trace elements and isotopic compositions al low two major magmatic lineages to be identified: one relating to a no n-radiogenic basaltic end-member and the other to a mantle end-member enriched in Ca, with high LILE/HFSE ratio and high Sr isotopic ratios. Their mantle sources are located within the lithosphere thermal bound ary layer (TBL) and the metasomatized phlogopite-carbonate asthenosphe re at the base of the TBL, respectively. The composition and spatial d istribution of volcanism and relative mantle sources tend to map the g eometry of the lithospheric mantle and to define a pronounced increase in depth from less than 60 km to about 100 km across the boundary bet ween the thinned Tyrrhenian lithosphere and the Adriatic lithosphere. A mechanism of intracontinental passive rifting, which drives mantle u pwelling, is sufficient to satisfy the petrological and geochemical co nstraints and the observed tectonic environment without requiring a su bduction plane.