Subduction-related enrichment of the Phlegrean Volcanic District (Southern Italy) mantle source: new constraints on the characteristics of the sedimentary components

The Neapolitan volcanic area (Southern Italy), which includes the Phlegrean Volcanic District and the Somma– Vesuvius complex, has been the site of intense Plio-Quaternary magmatic activity and has produced volcanic rocks with a subduction-related geochemical and isotopic signature. High-Mg, K-basaltic lithic lava fragments dispersed within hydromagmatic tuff of the Solchiaro eruption (Procida Island) provide constraints on the nature and role of both the mantle source prior to enrichment and the subduction-related components. The geochemical data (Nb/Yb, Nb/Y, Zr/Hf) indicate a pre-enrichment source similar to that of enriched MORB mantle. In order to constrain the characteristics of subducted slab-derived components added to this mantle sector, new geochemical and Sr–Nd-isotopic data have been acquired on meta-sediments and pillow lavas from Timpa delle Murge ophiolites. These represent fragments of Tethyan oceanic crust (basalts and sediments) obducted during the Apennine orogeny, and may be similar to sediments subducted during the closure of the Tethys Ocean. Based on trace element compositions (e.g., Th/Nd, Nb/Th, Yb/Th and Ba/Th) and Nd-isotopic ratio, we hypothesize the addition of several distinct subducted slab-derived components to the mantle wedge: partial melts from shales and limestones, and aqueous fluids from shales, but the most important contribution is provided by melts from pelitic sediments. Also, trace elements and Sr–Nd-isotopic ratios seem to rule out a significant role for altered oceanic crust. Modeling based on variations of trace elements and isotopic ratios indicates that the pre-subduction mantle source of the Phlegrean Volcanic District and Somma–Vesuvius was enriched by 2–4% of subducted slab-derived components. This enrichment event might have stabilized amphibole and/or phlogopite in the mantle source. 6% degree of partial melting of a phlogopite-bearing enriched source, occurring initially in the garnet stability field and then in the spinel stability field can generate a melt with trace elements and Sr– Nd-isotopic features matching those of high-Mg, K-basalts of Procida Island. Furthermore, 2% partial melting of the same enriched source can reproduce the trace elements and isotopic features of the most primitive magmas of Somma–Vesuvius, subsequently modified by assimilation of continental crust during fractional crystallization processes at mid-lower depth. Combined trace element and Sr–Nd isotope modeling constrains the age of the enrichment event to 45 Ma ago, suggesting that the Plio-Quaternary magmatism of the Neapolitan area is postorogenic, and related to the subduction of oceanic crust belonging to the Tethys Ocean.