Phase composition in volcanic rocks from South Tuscany magmatic area (Central Italy): inferences on conditions of magma crystallization

Microprobe analyses of olivine, pyroxene, mica and feldspar phenocrysts in trachybasalts, olivine latites and latites from Radicofani, Mts. Cimini and in minettes from Montecatini val di Cecina and Orciatico (Tuscan magmatic area) are reported. Olivines are more strongly magnesian in Mts. Cimini samples (Fo = 90-93 mol%) than in the Radicofani samples (Fo = 68-84 mol%) while in Montecatini val di Cecina and Orciatico rocks olivine is completely transformed. Clinopyroxenes have variable compositions ranging from endiopside to diopsidesalite and augite in the Radicofani trachybasalts and from endiopside to augite in the Mts. Cimini rocks. Minette cpx have strongly different compositions being low-iron endiopside in the Orciatico samples, and augite in the Montecatini val di Cecina rocks. All the analyzed cpx have high Al contents and invariably low calculated ferric iron. Ca-tschermak molecule is the main component of all cpx whereas Ca-Ti-tschermak molecule is important only in Orciatico cpx. Mts. Cimini micas are biotites, while minette micas are phlogopites. The latter are strongly zoned in Ti, Mg and Cr and these elements display significantly different abundances in the micas of the two minette outcrops. Such bimodalism has been recognized to be common in micas from world-wide distributed minettes. Substitution mechanism 2 Mg(VI) = Ti(VI)-(VI) appear to dominate with respect to Mg2Si = Ti2Al. In Radicofani and Mts. Cimini samples feldspar phenocrysts are both plagioclase (An = 86-52 mol%) and K-feldspar (Or = 85-73 mol%), while in the minettes only K-feldspar (Or = 79-77 mol%) is present. The obtained mineral chemistry data joined with experimental studies allow to elucidate equilibrium/disequilibrium relationships among phases and to di scuss evolutionary processes of South Tuscany volcanics. In general, crystal fractionation accompanied either by mixing between differently evolved magmas at Radicofani or by assimilation of crustal material at Mrs. Cimini appears to be the main evolutionary processes. For minettes different conditions in their crystallization history are recognized: the Orciatico rocks have mica compositions indicating crystallization at low pressure under subvolcanic conditions, whereas for Montecatini in val di Cecina rocks a polybaric crystallization is evidenced.