Transition from calc-alkaline to potassium-rich magmatism in subduction environments: geochemical and Sr, Nd, Pb isotopic constraints from the islandof Vulcano (Aeolian arc)

The problem of mantle metasomatism vs. crustal contamination in the genesis of are magmas with different potassium contents has been investigated usin g new trace element and Sr-Nd-Pb isotopic data on the island of Vulcano, Ae olian are. The analysed rocks range in age from 120 ka to the present day, and cover a compositional range from basalt to rhyolite of the high-K calc- alkaline (HKCA) to shoshonitic (SKO) and potassic (KS) series. Older Vulcan o products (> 30 ka) consist of HKCA-SHO rocks with SiO2 = 48-56%. They sho w lower contents of K2O, Rb and of several other incompatible trace element abundances and ratios than younger rocks with comparable degree of evoluti on. Sr-87/Sr-86 ranges from 0.70417 to 0.70504 and increases with decreasin g MgO and compatible element contents. Pb-206/Pb-204 ratios display signifi cant variations (19.31 to 19.76) and are positively correlated with MgO, Nd -143/Nd-144 (0.512532-0.512768), Pb-207/Pb-204 (15.66-15.71) and Pb-208/Pb- 204 (39.21-39.49). Overall, geochemical and isotopic data suggest that the evolution of the older series was dominated by assimilation-fractional crys tallisation (AFC) with an important role for continuous mixing with mafic l iquids. Magmas erupted within the last 30 ka consist mostly of SHO and KS i ntermediate and acid rocks, with minor mafic products. Except for a few aci d rocks, they display moderate isotopic variations (e.g. Sr-87/Sr-86, 0.704 57-0.70484; Pb-206/Pb-204 = 29.28-19.55, but Pb-207/Pb-204 = 15.66-15.82), which suggest an evolution by fractional crystallisation, or in some cases by mixing, with little interaction with crustal material. The higher Sr iso topic ratios (Sr-87/Sr-86 = 0.70494-0.70587) of a few, low-volume, intermed iate to acid rocks support differentiation by AFC at shallow depths for som e magma batches. New radiogenic isotope data on the Aeolian islands of Alic udi and Stromboli, as well as new data for lamproites from central Italy, a re also reported in order to discuss along-are compositional variations and to evaluate the role of mantle metasomsatism. Geochemical and petrological data demonstrate that the younger K-rich mafic magmas from Vulcano cannot be related to the older HKCA and SHO ones by intra-crustal evolutionary pro cesses and point to a derivation from different mantle sources. The data fr om Alicudi and Stromboli suggest that, even though interaction between magm a and wall rocks of the Calabrian basement during shallow level magma evolu tion was an important process locally, a similar interpretation can be exte nded to the entire Aeolian are.