Miocene shoshonite volcanism in Sardinia: Implications for magma sources and geodynamic evolution of the central-western Mediterranean

In this paper we document the existence of a Miocene shoshonite (SHO) volcanism in Northern Sardinia (Anglona). This occurrence completes the spectrum of orogenic magmas related to the subduction process which developed from the Eocene along the Palaeo-European continental margin, in concert with the opening of the Ligurian-Balearic back-arc basin and southeastward drift/rotation of the Sardinia-Corsica continental block. K-Ar ages show that the oldest volcanics of the area are calcalkaline (CA) basalts and andesites (~. 21. Ma), overlain by 19.7-18.4. Ma-old more potassic products such as high-potassium calcalkaline (HK-CA) and SHO lavas. CA, HK-CA and SHO suites include basalts and differentiated lavas of andesite and latite composition, respectively, that (according to the PELE software modelling) represent ~. 40-45% residual liquid fraction after shallow fractional crystallization. Application of the "Arc Magma Simulator" software suggests that the generation of primary melts of the distinct suites may occur at similar degrees of partial melting (5-8%) and melting pressures (2-2.2. GPa, ~. 60-70. km depth) in the mantle wedge. By contrast, the potassic character of parental melts of CA, HK-CA and SHO suites is controlled by 1) the amount of subducted continental components (possibly terrigenous sediments) and 2) the pressure (depth) at which these metasomatic agents are released from the slab. Results suggest that the slab depth beneath the volcanic district increased from ~. 80-100 to 100-120. km for CA and SHO magmas, respectively. Accordingly, the evolution from CA to SHO magmatism in the same plumbing system could be related to slab deepening and increase of the subduction angle of ~. 5-10° in the time span of 2-3. Ma. This tectono-magmatic scenario conforms to the major anticlockwise rotation (~. 30°) event of the Sardinia block (between 20.5 and 18. Ma). This geodynamic evolution preludes the development of the volcanism in the Apennine-Tyrrhenian domains, where the final collisional/post-collisional stages of subduction were characterized by accentuated slab retreat and roll back, inter-arc extension and eruption of highly potassic magmas in the frontal arc (Roman and Aeolian Provinces)