Calcite-Bearing Foiditic Lavas Of Colli Albani Volcanic District (Central Italy): New Petrographic And Geochemical Data.

8. The Colli Albani volcanic district, near the city of Rome, represents one of the most peculiar volcanic districts on the Earth because of its liquid line of descent characterized by differentiated K-foiditic magmas. Field, geochemical and experimental studies have demonstrated that such a differentiation trend is mainly due to magma-carbonate interaction. One impressive evidence of this process is the occurrence of magmatic calcite in the lava flows groundmass. Despite many studies have investigated petrological and geochemical features of the Colli Albani magmas, several questions remain unanswered. One of these concerns the 87Sr/86Sr and 143Nd/144Nd ratio trends observed in products representative of the whole Colli Albani eruptive history. Another open question concerns the role of fluorine in the magmatic system and its effects on phase relationships. Starting from these questions we have started a petrographic and geochemical study on lava flows emplaced during the whole Colli Albani activity with the aim to better understand the processes of genesis and evolution of magmas. Here we present preliminary petrographic and geochemical data on those lava flows. In particular, the oldest lava flow studied (≥560 ka) is a lithic fragment in the Trigoria Tor de’ Cenci pyroclastic flow that has the highest 87Sr/86Sr ratio (0.711196) among all those studied. Young lava flows present low 87Sr/86Sr (0.709879) and high 143Nd/144Nd ratios and some of these are characterized by the occurrence of strontian fluoro-magnesiohastingsite in the groundmass and lack of calcite, probably linked to changes in fluorine and CO2 activity. We interpret the variations in fluorine activity and isotope values as due to changes in the metasomatic component (i.e. phlogopite) of the mantle source.