The Ernici-Roccamonfina Province

Abstract

The Ernici-Roccamonfina Province is sited on the western side of Central Apennines, between the Ancona-Anzio (or Olevano-Antrodoco) and the Ortona-Roccamonfina tectonic lines. Volcanic rocks exhibit a very wide range of compositions, from calcalkaline (CA) to Roman-type ultrapotassic (HKS) and kamafugitic (KAM). Both at Ernici and Roccamonfina, calcalkaline to potassic rocks were emplaced later than ultrapotassic volcanics. Abundances of incompatible elements and Sr-isotope ratios increase strongly from calcalkaline to ultrapotassic rocks (e.g. Th ~10 to 90 ppm, 87Sr/86Sr ~ 0.7067 to 0.7112), whereas Nd-Pb isotopes show an opposite trend. The Ernici volcanoes (about 0.6 to 0.3 Ma) consist of several monogenetic centres that erupted mafic CA to HKS and KAM pyroclastics and lavas. Roccamonfina (about 0.5 to 0.15 Ma) is a HKS stratovolcano with a large central caldera and post-caldera calcalkaline to potassic lava domes and flows. The ultrapotassic rocks from Ernici-Roccamonfina have compositions similar to volcanoes of the Roman Province, whereas calcalkaline to potassic rocks are geochemically akin to Campania. These variations reflect a heterogeneous mantle source, which was modified by two compositionally (and temporally?) distinct metasomatic events. An episode of metasomatism took place by addition of marly sediments from the Adriatic subducted lithosphere and was responsible for contamination of the mantle source of ultrapotassic magmas. This event was followed/accompanied by the arrival of new subduction-related material with distinct compositional features, which probably originated from the northern limb of the Southern Apennine subduction slab.