Shoshonite and sub-alkaline magmas from an ultrapotassic volcano: Sr–Nd–Pb isotope data on the Roccamonfina volcanic rocks, Roman Magmatic Province, Southern Italy

The Roccamonfina volcano is characterised by two stages of volcanic activity that are separated by volcano-tectonic caldera collapses. Ultrapotassic leucite-bearing rocks are confined to the pre-caldera stage and display geochemical characteristics similar to those of other volcanoes in the Roman Province. After the major sector collapse of the volcano, occurred at ca. 400 ka, shoshonitic rocks erupted from cinder cones and domes both within the caldera and on the external flanks of the pre-caldera Roccamonfina volcano. On the basis of new trace element and Sr–Nd–Pb isotope data, we show that the Roccamonfina shoshonitic rocks are distinct from shoshonites of the Northern Roman Province, but are very similar to those of the Neapolitan volcanoes. The last phases of volcanic activity erupted sub-alkaline magmas as enclaves in trachytic domes, and as lavas within the Monte Santa Croce dome. Ultrapotassic rocks of the pre-caldera composite volcano are plagioclase-bearing leucitites characterised by high levels of incompatible trace elements with an orogenic signature having troughs at Ba, Ta, Nb, and Ti, and peaks at Cs, K, Th, U, and Pb. Initial values of 87 Sr/ 86 Sr range from 0.70926 to 0.70999, 143 Nd/ 144 Nd ranges from 0.51213 to 0.51217, while the lead isotope rations vary between 18.788–18.851 for 206 Pb/ 204 Pb, 15.685–15.701 for 207 Pb/ 204 Pb, and 39.048–39.076 for 208 Pb/ 204 Pb. Shoshonites show a similar pattern of trace element depletions and enrichments to the earlier ultrapotassic leucite-bearing rocks but have a larger degree of differentiation and lower concentrations of incompatible trace elements. On the other hand, shoshonitic rocks have Sr, Nd, and Pb isotopes consistently different than pre-caldera ultrapotassic leucite-bearing rocks. 87 Sr/ 86 Sr ranges from 0.70665 to 0.70745, 143 Nd/ 144 Nd ranges from 0.51234 to 0.51238, 206 Pb/ 204 Pb ranges from 18.924 to 19.153, 207 Pb/ 204 Pb ranges from 15.661 to 15.694, and 208 Pb/ 204 Pb ranges from 39.084 to 39.212. High-K calc-alkaline samples have intermediate isotopic values between ultrapotassic plagioclase leucitites and shoshonites, but the lowest levels of incompatible trace element contents. It is argued that ultrapotassic magmas were generated in a modified lithospheric mantle after crustal-derived metasomatism. Interaction between the metasomatic agent and lithospheric upper mantle produced a low-melting point metasomatised veined network. The partial melting of the veins alone pr