PETROLOGY OF FINAL-STAGE LATERA LAVAS (VULSINI MTS) - MINERALOGICAL, GEOCHEMICAL AND SR-ISOTOPIC DATA AND THEIR BEARING ON THE GENESIS OF SOME POTASSIC MAGMAS IN CENTRAL ITALY

A detailed investigation on the final-stage Latera lavas has shown the presence of three groups of magma having different petrochemical affinities: (1) typical high-potassium series (HKS) magmas, represented only by evolved rocks (group A); (2) HKS-like volcanics with slightly lower K2O/Na2O ratios and lower K2O and incompatible trace-element contents than the typical HKS of the Roman Magmatic Province (group B); (3) potassic series (KS) with higher K2O/Na2O ratios and higher SiO2, MgO, Cr, Ni, Rb, Th and LREE contents and lower Ba and Sr contents than the typical Roman KS (group C). Compared to the HKS volcanics, the latter group of volcanics have higher Th contents and Rb/Ba and Ce/Sr ratios, and lower LREE, Sr, Rb and Ba contents, and lower LILE/HFSE and LREE/HFSE ratios. It has been also shown that the group-C lavas display a general composition similar to that of Radicofani volcanics, suggesting a transitional character between the typical Roman-type KS rocks and the Tuscan lamproites. Crystal fractionation was mainly responsible for the petrological variations observed within the two HKS groups, which are believed to derive from two distinct parental magmas, probably a leucite-tephrite liquid (group A) and a leucite-basanite liquid (group B). Limited and variable crustal assimilation processes were associated with crystal fractionation in the evolution of the group-B magmas. On the other hand, the group-C magmas underwent virtually no fractionation during their ascent to surface, thus their composition can be considered almost primary. Different degrees of partial melting, at high-P and high-X(CO2) conditions, of a metasomatized phlogopite-bearing peridotitic mantle source are suggested to explain the genesis of the group-A and -B primary magmas. On the other hand, low-P and low-X(CO2) conditions during the partial melting of a phlogopite-bearing mantle source, slightly depleted in clinopyroxene, are suggested for the genesis of the group-C magmas. Moreover, the different incompatible-element contents and ratios between the HKS (group-A and -B) and KS (group-C) rocks studied should reflect primary characteristics of the mantle sources.