Origin and evolution of the potassic magmatism in the Campanian region (South Italy) as inferred from new geochemical and isotopic data for Mt. Vesuvius and Phlegrean Volcanic District.

In the Campanian region (South Italy) potassic magmas are erupted through a thinned continental lithosphere. Magma generation is related to NW subduction of the Ionian plate, and rising is a consequence of extension of the Tyrrhenian Basin. Magmas are variably enriched in K and related trace elements. New geochemical and isotopic investigations have been carried out on potassic volcanic rocks representative of the magmatic activity of the Phlegraean Volcanic District, which includes the Campi Flegrei caldera and the islands of Procida and Ischia, and Mt. Vesuvius. The results of these investigations allowed us to define at least three distinct components in the petrogenesis of the magmas erupted in the Campanian region. Two components, located in the mantle, include a T-MORB type mantle source and a slab-derived component, likely representing fluids and partial melts of sediments of the subducting Ionian plate. The third component is represented by continental material (crust-derived fluids or partial melts) which are incorporated into mantle-derived magmas. This last component is acquired in the crust, probably at 8-10 km of depth where seismic tomography and other geophysical studies, as well as pressure estimates resulting from melt inclusion analyses, identify an horizontally extended low-velocity layer. This layer is interpreted as the top of the present magma reservoirs in the areas beneath Mt. Vesuvius and the Phlegraean Volcanic District. We have detected an eastward spatial variation in the isotopic characteristics of the volcanic rocks, from Procida and Ischia islands to the Campi Flegrei caldera and Mt. Vesuvius. This variation can be interpreted as the result of variable contribution of the slab-derived component to the mantle source, due to the variable position of the volcanoes with respect to the Ionian plate, presently sinking beneath the Southern Tyrrhenian sea. We speculate that within the crust there are multiple sill-like bodies where magmas stop, permeate and interact with the crust. This structure would consist of rocks permeated by various portions of melt, in such a way to individuate different reservoirs at different depths. The existence of different depths of cristallization (10-22 km) and the occurrence of discrete isotope values in the volcanic rocks strongly suggest that different magma batches, starting from the mantle source, stop at various depths during their ascent towards the surface. Such a condition would determine a large surface of contact between magma and whole rock which could favour contamination.