Fluorophlogopite and F-rich phases in limestone clasts from the Campanian Ignimbrite quarried at Fiano (southern Italy): mineralogical, geochemical and volcanological insights.

Fluorine-rich metamorphosed xenoliths associated to the distal Campanian Ignimbrite (CI) tephra (Campania region, southern Italy) have long attracted the interest of the mineralogical community (i.e. Scacchi, 1890; Zambonini, 1919; Masi & Turi, 1972; Balassone et al., 2002). These rocks derive from the Mesozoic carbonate lithotypes of the Campanian Apennine, embedded in the pyroclastic flow and presently resting at the bottom of the tuff formations. The sedimentary protoliths suffered from the action of hot, volatile-rich pyroclastic flow, and consequently were affected by thermal metamorphism to various degrees. Their peculiarity is the occurrence of F- and Mg-bearing phases, with an ubiquitous presence of neoformed fluorite. The occurrence at Fiano quarries (the so-called “Tufare”) is the classical locality for these rock xenoliths. Similar rocks were also found in the CI of the Caserta area, and in the Latium region (Colli Albani). This research aims both at a crystal chemical study of fluorophlogopite occurring in the Fiano xenoliths, and at a geochemical and volcanological survey, to constrain the petrogenesis processes related to this rare F-, Mg-rich assemblage and to CI. The analysed samples belong to two lithotypes: mica-bearing clasts and variably metamorphosed carbonate blocks. The former lithotype is represented by abundant fluorite, followed by fluorophogopite, F-rich chondrodite, fluoborite, diopside and (Fe,Mg)-oxides. Minor to trace contents of calcite, humite, tremolite, and grossular also occur. Carbonate rocks can show calcite only, or also trace amounts of fluorite. The Fiano micas composition approaches that of the Vesuvius micas from the 1872 eruption (Balassone et al., 2013). The micas belong to the 1M polytype and have crystal chemical features typical of fluorophlogopites i.e., low c lattice parameter (~ 10.13 Å), (~ 2.060 Å) and (~ 3.135 Å) distances. New data on minor to ultratrace elements amounts found in the studied clasts, together with Carbon and Oxygen isotope data will be reported and interpreted in a petrogenetic and volcanological frame. Balassone G., Franco E., Mattia C.A., Petti C. & Puliti R. 2002. Re-examination of fluosiderite, an unknown mineral from southern Italy: equal to fluorine-rich chondrodite. Eur. J. Min., 14, 151-155. Balassone G., Scordari F., Lacalamita M., Schingaro E., Mormone A., Piochi M., Petti C. & Mondillo N. 2013. Trioctahedral micas in xenolithic ejecta from recent volcanism of the Somma-Vesuvius (Italy): crystal chemistry and genetic inferences. Lithos, 160-161, 84-97. Masi U. & Turi B. 1972. Frazionamento isotopico del carbonio e dell'ossigeno negli inclusi calcarei metamorfosati del "Tufo grigio campano" Auct. di Fiano (Salerno). Per. Min., 41, 291-310. Scacchi A. 1890. La regione vulcanica fluorifera della Campania. Mem. R. Com. Geol. It., I, 1-48. Zambonini F. 1919. Il tufo pipernoide della Campania e i suoi minerali. Mem. Descr. Carta Geol. It., 7, 130 pp.