Mineralogy and Petrology of Mg-rich calc-alkalic, potassic, and ultrapotassic associated rocks: the Middle Latin Valley monogenetic volcanoes, Roman Magmatic Province, Southern Italy

In the Middle Latin Valley volcanism developed during Late Pleistocene, with eruption of high-Mg magmas characterised by low phenocryst content. Small volumes of magma arrived to surface rapidly favoured by an extensional post-collision regime, and thus small monogenetic centers aligned along two main tectonic trends have been formed. The absence of a large magmatic reservoir allowed the occurrence of a fairly large amount of mafic, strongly primitive, volcanic rocks with a fairly large enrichment in potassium. They range in composition from plagioclase-free ultrapotassic melilite-bearing (kamafugites) to ultrapotassic leucitites and plagioclase leucitites, to trachybasalts (shoshonites), and sub-alkaline calc-alkaline basaltic rocks. The composition of minerals, from the four different groups of rock, is reported. On the basis of Fe-Mg distribution between olivine crystals and whole rock, equilibrium crystallisation has been argued. Clinopyroxene shows the typical trend of alkaline potassic and ultrapotassic rocks, ranging in composition from diopside to hedenbergite. Melilite-bearing ultrapotassic rocks are found beside feldspar-bearing ultrapotassic and potassic primitive rocks. Magnesian chromite is the main spinel and it has been found enclosed in olivine core. Olivine-spinel pairs indicate that the magmas from which they crystallised were in equilibrium with a strongly depleted mantle source. An origin from a common mantle source, in terms of peridotitic component is inferred for the different varieties of ultrapotassic to sub-alkaline primitive rocks. This source was characterised by different metasomatic enrichment arranged in a network of veins, possibly within the lithospheric upper mantle.