The Roman Province

Abstract

The Roman Magmatic Province (about 0.8–0.02 Ma) consists of about 900 km3 of dominant pyroclastic deposits and minor lavas building up the large volcanoes of Vulsini, Vico, Sabatini and Colli Albani (Alban Hills). Rocks range from potassic (K-trachybasalt to trachyte) to Roman-type ultrapotassic (leucite tephrite and leucitite to leucite phonolite) in composition. Felsic magmas were generated by fractional crystallisation of mafic melts. Evolution processes took place in large shallow-level magma chambers whose post-eruption collapse formed huge calderas and volcano-tectonic depressions. Potassic and ultrapotassic mafic magmas in the Roman Province display variable degrees of enrichment in incompatible elements. In contrast, radiogenic isotopic ratios are less variable (87Sr/86Sr ~ 0.7090 to 7110, 143Nd/144Nd ~ 0.5121, 206Pb/204Pb ~ 18.80, 176Hf/177Hf ~ 0.28258). Trace element and radiogenic isotope evidence suggests an origin of primary magmas in an anomalous but rather homogenous mantle source that has undergone variable degrees of partial melting at a pressure increasing from potassic to ultrapotassic magmas. Metasomatised phlogopite-bearing lherzolite or wehrlitic-clinopyroxenite mantle rocks are likely sources of Roman magmas. Mantle contamination was provided by carbonated pelites (marls), a process that took place during the Miocene to present subduction of the Adriatic continental plate beneath Northern Apennines.