Mineralogy and petrology of associated Mg-rich ultrapotassic, shoshonitic, and calc-alkaline rocks: The middle Latin Valley monogenetic volcanos, Roman Magmatic Province, southern Italy

In the Middle Latin Valley, in the Roman Magmatic Province, southern Italy, volcanism developed during the Late Pleistocene, with eruption of high-Mg magmas characterized by a low content of phenocrysts. Small volumes of magma, transported to the surface rapidly, favored by an extensional post-collision regime, led to small monogenetic centers aligned along two main tectonic trends. 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 calc-alkaline basaltic rocks (subalkaline). The composition of minerals from the four different groups of rock is reported. The Fe-Mg distribution between olivine crystals and whole rocks indicates equilibrium crystallization. The 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. Magnesiochromite is the main spinel; it has been found enclosed in olivine cores. Olivine-spinel pairs indicate that the magmas from which they crystallized were in equilibrium with a strongly depleted mantle source. An origin from a common mantle source, in terms of a peridotitic component, is inferred for the different varieties of ultrapotassic to subalkaline primitive rocks. This source was characterized by different degrees of metasomatic enrichment arranged in a network of veins, possibly within the lithospheric upper mantle.