PETROGENESIS AND TECTONIC SETTING OF THE ROMAN VOLCANIC PROVINCE, ITALY

The volcanism in the Roman Province of Italy can be modelled by the partial melting of heterogeneously enriched mantle sources. The heterogeneity was created by materials derived from a subducted slab which can still be traced geophysically beneath the central Apennines. New petrographical and chemical data are presented for the high-K calc-alkaline and the shoshonitic volcanics of the Campania region. Primary magmas are present only locally. The existence of spatial zonation in the volcanism of Campania is documented for the first time. The shoshonitic, leucite-basanitic and leucititic volcanics of the Phlegraean Fields-Procida-Ischia and the Somma-Vesuvius areas are, at similar degrees of evolution, about two times richer in Nb and Ba than those of northwestern Campania and the Latium part of the Roman Province. Accordingly, distinct north-western and south-eastern sub-provinces can be defined. The evaluation of enrichment factors, that is the abundance ratio between the average contents of each element in the relatively primitive lavas of the low K- and the high K suites, shows that the mantle sources prior to K-metasomatism were different in the two sub-provinces of the Roman Province. In the north-western one, they resembled the sources of ocean-island tholeiites and moderately enriched MOR-basalts. In the south-east they were similar to those of ocean island alkaline lavas and enriched MORB's. Modelling based on K, P, Ce, Sr, Rb, Ba, Th, Sm, Eu, Gd, Y, Nb and 87Sr/86Sr was carried out. It indicates that the range of mantle sources of the volcanics in northwestern Campania and Latium can be modelled by the addition of 3 to 20% of materials derived by partial melting of carbonaceous pelites to a Sr-enriched mantle wedge comparable to the Honolulu mantle source least enriched in Nb. The production of Sr-enriched mantle wedge requires either the action of fluids produced by dehydration of subducted oceanic crust, or a small amount of metasomatism caused by the presence of carbonatite melts. The near absence of Ti, Ta, Nb, Yb and the highly fractionated REE in the metasomatizing component requires the presence of residual garnet and accessory Ti-rich minerals during the partial melting of the subducted sediments. The writers propose that the mantle wedge overlying the subducted slab was hybridized by melts produced by partial fusion of subducted material derived from the continental crust, probably sediments. This process played a dominant role in the generation of the mantle sources from which the high-K calc-alkaline, shoshonitic, leucite-basanitic and leucititic magmas of the Roman Province were derived.