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 a two stage volcanic activity, divided by volcano- tectonic collapses. Ultrapotassic leucite-bearing rocks are confined in the pre-caldera stage and display geochemical characteristics very similar to those of other volcanoes of the Roman Province. Shoshonitic rocks have been poured out 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, Roccamonfina shoshonitic rocks show differences with respect to shoshonites from Northern Roman Province, and recall closely those of the Neapolitan volcanoes. Eventually, in the very last phases of volcanic activity, sub-alkaline magmas are found both as enclaves in trachytic domes, and as final lavas vented by a fracture within the Monte Santa Croce dome. Ultrapotassic rocks that built up the pre-caldera composite volcano are plagioclase-bearing leucitites and, similarly to other Roman Provinces volcanoes, have by high levels of incompatible trace elements with an orogenic signature having troughs at Ba, Ta, Nb, Ta, and Ti, and peaks at Cs, K, Th, U, and Pb. Initial values of 87Sr/86Sr range from 0.709261 to 0.709987, of 143Nd/144Nd range from 0.512134 to 512195, of 206Pb/204Pb range from 18.788 to 18.851, of 207Pb/204Pb range from 15.685 to 15.701, and of 208Pb/204Pb range from 39.048 to 39.076. Shoshonites show a larger differentiation degree but, as a whole, lower levels of incompatible trace elements with similar pattern of incompatible trace element contents with respect to earlier ultrapotassic leucite-bearing rocks. On the other hand, shoshonitic rocks have Sr, Nd, and Pb isotopes consistently different than pre-caldera ultrapotassic leucite-bearing rocks. 87Sr/86Sr ranges from 0.706604 to 0.708332, 143Nd/144Nd ranges from 0.512179 to 0.512382, 206Pb/204Pb ranges from 18.838 to 19.153, 207Pb/204Pb ranges from 15.661 to 15.694, and 208Pb/204Pb ranges from 38.999 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. The genesis of this uncommon association is investigated in details. It is argued that ultrapotassic magmas have been generated in a modified lithospheric mantle after crustal- derived metasomatism. Interaction between metasomatic agent and lithospheric upper mantle produced a low-melting metasomatised veined network. Pure partial melting of vein mineralogy produced pre-caldera leucite-bearing ultrapotassic magmas. Leucite-free magmas have been generated further melting, at higher temperature, of the same meltasomatic mantle previously depleted by the ectraction of ultrapotassic rocks. We suggest that a key role in this second episode of melting is played by little but significant addition of astenospheric upper mantle material from foreland mantle, flowing through the corner of the Ionian present day subducted slab during its roll-back