Clues on the origin and differentiation of primitive arc magmas: the coarse grained, high-Mg basaltic enclaves of Capo Marargiu (Sardinia, Italy)

Recent models depict thermally mature magmatic systems as transcrustal crystal mushes where, from the base to the roof, primary magmas evolve towards silicic end-members through crystal fractionation. In these cumulitic piles, igneous distillation is variably overprinted by open system dynamics, with the result that plutonic and volcanic rocks show puzzling textures and hybrid bulk compositions. The first evolutionary stage of primary arc magmas occurs at the mantle-crust transition, where high pressure conditions may prevent water loss, thus promoting the fractionation of pyroxenitic to hornblenditic cumulates. Intriguingly, the average composition of continental crust is too felsic to be in equilibrium with the mantle, implying that (i) preserved mafic cumulates are too scarce than predicted according to mass balance principles, and (ii) high pressure cumulitic rocks and inclusions eventually exposed at Earth surface provide precious insights on the lower crustal evolution of arc basalts and picrites. The goal of this PhD project is to give a contribution to the scientific debate about arc magma evolution by investigating the basaltic to andesitic rocks from Capo Marargiu Volcanic District (i.e., CMVD). This is an Oligo-Miocene calc-alkaline complex located in north-western Sardinia (Italy) and characterised by the widespread occurrence of basaltic andesitic domes and dikes. One of these domes hosts abundant crystal-rich enclaves containing ~50% of millimetre- to centimetre-sized clinopyroxene and amphibole crystals with intriguing textures. Based on textural, geochemical, and experimental data, it is demonstrated that the CMVD plumbing system is a complex polybaric structure developed from the base of Sardinian crust to hypabyssal depths, where hydrous high-Mg basalts produce basaltic andesitic magmas by fractional crystallisation of high-pressure assemblages and assimilation of the Hercynian basement. Buoyant basaltic andesites undergo a second stage of differentiation in the shallow crust, leading the more evolved andesitic melts intruded as dikes. Enclave mineral texture and chemistry constrain the hybrid nature of these rocks due to the entrainment of lower crustal clinopyroxene + amphibole cumulates in a basaltic andesitic magma ascending towards shallow depths.