Transition from ultrapotassic kamafugitic to sub-alkaline magmas: Sr, Nd, and Pb isotope, trace element and 40Ar-39Ar age data from the Middle Latin Valley volcanic field, Roman Magmatic Province, Central Italy

The Middle Latin Valley volcanic field forms part of the Roman Magmatic Province and includes Pleistocene monogenetic volcanism characterised by the emplacement of small lava flows and minor pyroclastic ejecta and flows. The absence of a main volcanic edifice and of a large, shallow-level magma reservoir allows the eruption of primitive magmas. Geochemical and petrological data suggest that at least four different types of mafic parental magmas are present within the volcanic field: i) melilite-bearing ultrapotassic (kamafugitic), ii) plagioclase-bearing and -free leucititic (HKS), iii) shoshonitic and iv) sub-alkaline. 40Ar-39Ar dating reveals diachronous emplacement of mafic magmas with different levels of K-enrichment; the kamafugitic lavas are the oldest and the sub-alkaline lavas the youngest. Incompatible trace element contents strictly follow K2O, but overall the groups of rocks show similar trace element fractionation, with high field strength elements less enriched than large ion lithophile elements. Despite a restricted range in MgO and SiO2 contents, the Middle Latin Valley volcanic rocks have highly variable Sr, Nd and Pb isotopic compositions. The sub-alkaline rocks have the lowest 87Sr/86Sr and the highest 143Nd/144Nd, while the kamafugitic rocks have the highest 87Sr/86Sr and the lowest 143Nd/144Nd. Intermediate isotopic compositions between these two end-members are shown by leucitites/plagioleucitites and shoshonites. A clear, time-dependent trend of isotopic variation is observed. This also holds true for Pb isotope compositions with shoshonitic and sub-alkaline rocks showing the most radiogenic signatures, and the kamafugitic rocks the least radiogenic signatures. The overall geochemical characteristics of the magmas can be reconciled in terms of a model involving recycling of marly shales within the upper mantle; this overprinted previously pervasive metasomatism related to melts derived from altered oceanic basalts. The crustal derived (shale) end-member is considered to have been concentrated within a metasomatic vein network within the lithosphere, whilst the fluid-metasomatised end-member occurs within the surrounding mantle. Early partial melting of veins produced strongly undersaturated melilite-bearing ultrapotassic magma (Kamafugitic). The progressive exhaustion of the veined mantle increased the contribution of the surrounding mantle to magma production, explaining the decrease of K2O with time in the mafic magmas and the geochemical and isotopic transition from leucititic/plagio-leucititic to shoshonitic and sub-alkaline magmas, the latter being the youngest products erupted.