Origin of a carbonate-bearing fluorapatite from Tertiary volcanics of the Veneto Volcanic Province, Italy

We present chemical and mineralogical data on a megacryst of a unique carbonate-bearing fluorapatite from altered Tertiary volcanics of the Veneto Volcanic Province (VVP) in the western Lessini Mountain range (Veneto, northern Italy). The cm-sized specimen was identified and characterized by scanning electron microscope (SEM), X-ray powder diffraction (XRPD), micro-Raman spectroscopy and electron probe microanalyses. Major and trace elements of the carbonate-bearing fluorapatite are consistent with the crystallization at depth from a nelsonitic melt or an evolved alkaline melt derived from a mantle source metasomatized by carbonate-rich fluids. The Sr and Nd isotopic composition fits with the lavas and xenoliths from the VVP showing a DM-HIMU affinity with addition of a crustal, possibly carbonate, component. Our data are in agreement with a recent geodynamic model for the hybridization of the VVP mantle triggered by breakdown of carbonates within the subducting Tethyan oceanic slab. Cronstedtite, chabazite-Ca, calcite associated with reaction rims of amphibole and secondary carbonate-rich fluorapatite within the megacryst originated from low temperature hydrothermal alteration of the volcanics. Cronstedtite is the first occurrence in the VVP area.