The Intra-Apennine Province

Abstract

The Intra-Apennine Province (IAP) consists of various small outcrops of pyroclastic rocks plus a few lavas at San Venanzo and Cupaello (Umbria). Pyroclastic rocks often contain silicate and carbonate material; silicate fraction is melilititic to phonolitic in composition. Lavas have olivine melilitite and kalsilite melilitite compositions. Overall, lavas and silicate pyroclasts exhibit an ultrapotassic kamafugitic affinity, high enrichments in LILE (e.g. Cs, Rb, LREE) and low abundance of Ta and Nb. The rocks of the Intra-Apennine Province have very similar Sr–Nd–Pb isotope ratios and incompatible trace element signatures to the mafic rocks of the nearby Roman Province, suggesting a close genetic relationship and geodynamic significance. These compositions are best explained by assuming a magma origin in an anomalous upper mantle that was enriched in incompatible elements by subducted marly sediments. Interaction between kamafugitic melts and sedimentary carbonates probably occurred during magma ascent. This process generated an increase of δ18O ‰ and CaO content of magmas, but had little effects on incompatible trace element ratios and radiogenic isotope signatures. The carbonate-rich rocks occurring in the IAP are mixtures of carbonate and silicate components. The very nature (i.e. magmatic vs. secondary, and mantle-derived vs. crust-derived) of these carbonates has been much debated and, perhaps, different types of calcite occur in the single rocks.