Fluorophlogopite-bearing and carbonate metamorphosed xenoliths from the Campanian Ignimbrite (Fiano, southern Italy): Crystal chemical, geochemical and volcanological insights

Fluorine-, boron-, and magnesium-rich metamorphosed xenoliths occur in the Campanian Ignimbrite (CI) deposits at Fiano(southern Italy), at about 50 km northeast of the sourced volcanic area. These rocks originated from Mesozoic limestones of the Campanian Apennines, embedded in a fluid flow. The studied Fiano xenoliths consist of ten fluorophlogopite-bearing calc-silicate rocks and five carbonate xenoliths, characterized by combining mineralogical analyses with whole rock and stable isotope data. The micaceous xenoliths are composed of abundant idiomorphic fluorophlogopite, widespread fluorite, F-rich chondrodite, fluoborite, diopside, Fe(Mg)-oxides, calcite, humite, K-bearing fluoro-richterite and grossular. Out of the five mica-free xenoliths, two are calcite marbles, containing very subordinate fluorite and hematite, and three are weakly meamorphism carbonates, composed of calcite only. The crystal structure and chemical composition of fluorophlogopite approach those of the end member. The Fiano xenoliths are enriched in trace elements with respect to the primary limestones. Comparison between the REE patterns of the Fiano xenoliths and those of both CI and Somma-Vesuvius marble and carbonate xenoliths shows that the Fiano pattern on the whole both overlaps that of Somma-Vesuvius marble and carbonate xenoliths, and yet reproduces the trend of CI rocks. δ13C and δ18O values depict the same trend of depletion in the heavy isotopes observed in the Somma-Vesuvius nodules, related to thermometamorphism. Trace element distribution, paragenesis, stable isotope geochemistry and data modelling point to infiltration of steam enriched in F, B, Mg, As into carbonate rocks at temperature of ∼ 300- 450°C during the emplacement of the CI.