Mantle source heterogeneity in the Campanian Region (South Italy) as inferred from geochemical and isotopic features of mafic volcanic rocks with shoshonitic affinity

摘要

Major, trace element and Sr-Nd-isotopic data onmafic volcanic rocks with shoshonitic affinity from the islands ofProcida and Ventotene (Campanian Region, South Italy) arereported. Within-suite chemical and isotopic variations areinterpreted in terms of open system differentiation processes heldresponsible for enrichment in a number of trace elements and forradiogenic Sr observed in both Procida and Ventotene volcanicsequences. Conversely, trace element contents and Sr-Nd-isotopiccompositions of the least-evolved among the investigated samplesare thought to reflect source region characteristics with onlynegligible crustal contamination of the primary magmas.The data support an origin from distinct source regions forthe primary magmas. In the case of Ventotene, the source wouldhave been asthenospheric and/or lithospheric mantle with featuressimilar to transitional mid-ocean ridge basalts (T-MORB),modified by enriching agents made up mainly of subducted slab-released components coming from marly sediments (about 7%)similar to cherts from Tuscan Apennines plus a limestonecontribution. In the case of Procida, the source would have beenasthenospheric and/or lithospheric mantle with a slightly moreenriched T-MORB-like feature with respect to Ventotene,modified by slab-derived enriching agents, likely represented by asmall amount of sediments (about 2.5%) of oceanic nature(mostly terrigenous/pelagic). These differences are constrained bylower Zr/Nb, Y/Nb, Sc/Nb, V/Nb, LILE/HFSE, LREE/HFSE and87Sr/86Sr, higher Ba/La and 143Nd/144Nd ratios, as well as by thelack of a detectable negative Eu anomaly, in the least-evolvedvolcanic rocks of Procida, with respect to those of Ventotene,which show conversely a significant negative Eu anomaly. Thus, asharp heterogeneity exists in the mantle beneath the CampanianRegion. It is thought to be the consequence of two different slabswhich are in contact along the Volturno River Plain tectonic line:a continental slab, seismically inactive, descending beneath theLatium Region, and an oceanic slab, seismically active, descendingbeneath the Calabria, up to the Campanian Region.