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Subduction-related enrichment of the Neapolitan volcanoes (Southern Italy) mantle source: New constraints on the characteristics of the slab-derived components

 

Mazzeo Fabio Carmine1, D’Antonio Massimo1,2, Arienzo Ilenia2, Aulinas Meritxell3, Di Renzo Valeria4, Gimeno Domingo3

 
1 Dipartimento di Scienze della Terra, dell’Ambiente e delle Risorse, Università degli Studi di Napoli Federico II, Largo San Marcellino 10, 80138 Napoli, Italy
2 Istituto Nazionale di Geofisica e Vulcanologia, sezione di Napoli Osservatorio Vesuviano, Via Diocleziano 328, 80124 Napoli, Italy
3 Departamento de Geoquímica, Petrologia i Prospecció Geològica, Facultat de Geologia, Universitat de Barcelona (UB), Martí i Franquès s/n, 08028 Barcelona, Spain
4 Dipartimento di Ingegneria Civile, Design, Edilizia e Ambiente, Seconda Università degli Studi di Napoli, Via Roma 29, 81031 Aversa, CE, Italy
Chemical Geology, 386, 165-183, 2014
 
Abstract
dantonioThe 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 post-orogenic, and related to the subduction of oceanic crust belonging to the Tethys Ocean.

 

Key-words: Neapolitan volcanic area; Phlegrean Volcanic District; Somma–Vesuvius complex; Basilicata ophiolites; Pre-enrichment mantle wedge; Subducted slab-derived melts and fluids

 

Legenda: 

Idealized sketch showing the stages of enrichment of the PVD/Somma–Vesuvius mantle source, based on paleogeographic reconstructions of Carminati et al., 2012 and Vitale and Ciarcia, 2013.

a: During the Eocene, the African oceanic lithosphere (Tethys) began to subduct under the European continental margin represented by the Corsica–Sardinia block and the recently formed Alpine Chain; fluids and melts derived from progressive dehydration and melting of the sedimentary cover of the oceanic crust migrated into the mantle wedge, locally causing its chemical, isotopic and mineralogical modification.

b: In the Present, in the mantle below the Campania region enriched veins consisting mainly of phlogopite, formed during the Eocene, occur. Melting of this enriched source, during Pliocene–Holocene times, has generated the primary magmas of Procida, progenitor of the entire PVD rock suite, and the Somma–Vesuvius.