Iron-bearing chlor-fluorapatites in crustal xenoliths from the Stromboli volcano (Aeolian Islands, Southern Italy): an indicator of fluid processes during contact metamorphism

Chlor-fluorapatite crystals, with an unusually high Fe content (up to 0.715 apfu), have been found in cordierite- and feldspar-bearing hornfels metapelitic xenoliths included in calcalkaline basaltic andesite lavas from the Stromboli volcano. These xenoliths originated in a mid-crustal (200-350 MPa) contact aureole (800 degrees C) surrounding the magmatic plumbing system of the volcano. The high and somewhat variable Cl, Fe, Na, and Mn content in the chlor-fluorapatite can be interpreted as CaCl2, FeCl2, FeCl3, NaCl, and MnCl2 hydrous chloride melts present in the calcalkaline basaltic andesite magma responsible for the metamorphism of the metapelites to hornfels facies in the contact aureole. During the partial melting and dehydration of the metapelites, H2O was strongly partitioned into the partial melt and subsequently into the calcalkaline basaltic andesite magma. This H2O-rich partial melt was partly replaced by hydrous chloride melts, which metasomatised the original fluorapatite to chlorfluorapatite with variable amounts of Cl, Fe, Na, and Mn. This scenario is supported by the patchy enrichment of Cl, Fe, Na, and Mn in the chlor-fluorapatite grains and the correspondingly low water activities present during the formation of the hornfels as confirmed by the absence of hydrous phases in the modal mineralogy of the xenoliths.