New insights on the petrology of the ancient activity of the Roccamonfina volcano (central-southern Italy)

Roccamonfina is an extinct stratovolcano of the Italian peninsula where leucite-bearing ultrapotassic products are generally considered to predate leucite-free shoshonitic products. This relationship is largely based on studies conducted on lavas, while pyroclastic successions have been generally overlooked due to their poor preservation. Here we report a detailed petrological investigation of the juvenile products from pyroclastic deposits of the oldest stages of the volcano activity (i.e. >350 ka) cropping out on the eastern side of the edifice. Most of the samples are represented by crystal-poor pumice clasts with few clinopyroxene and sanidine microphenocrysts set into a brownish, glassy groundmass with sparse, variably altered to fresh leucite crystals. A smaller number of samples are relatively crystal-rich and feature clinopyroxene, altered leucite and biotite phenocrysts plus variable amounts of sanidine and plagioclase, depending on the rock composition. Glass compositions range from phonotephritic to phonolitic, in line with the compositions of the oldest Roccamonfina lavas. In addition, leucite-free juvenile clasts were also observed. These clasts are typically crystal-poor, and featuring only rare alkali feldspar and clinopyroxene phenocrysts set into a trachytic groundmass whose compositions are in line with that of the most recent shoshonitic products. Interestingly, leucite-bearing and leucite-free samples display comparable Sr (0.70900-0.70939 vs. 0.70872-0.70913) and Nd (0.51217-0.51220 vs. 0.51220-0.51224) isotope ratios. These preliminary results indicate that the ancient Roccamonfina activity was significantly more complex than previously thought, featuring at least two different magma series possibly generated by different partial melting degrees of the same mantle source.