Petrology of an apatite-rich layer in the Finero phlogopite–peridotite, Italian Western Alps; implications for evolution of a metasomatising agent

We discovered an apatite-rich layer in the Finero phlogopite–peridotite body, Western Alps. Fine-grained apatite grains are concentrated, up to 10 modal percent locally, to form a lens within or adjacent to a thin layer, <1 cm in thickness, consisting of very fine-grained minerals of olivine, orthopyroxene, spinel, amphibole, phlogopite, sulfide minerals and apatite with small amount of carbonate and clinopyroxene. In the thin layer, apatite-rich part is richer in amphibole and carbonate minerals than apatite-poor part, suggesting that an alkali- and phosphate-rich carbonatitic metasomatising agent locally existed in the thin layer. On the other hand, film-shaped orthopyroxenes occur at grain boundaries of olivine in the thin layer. The film-shaped orthopyroxenes are distinctively low in Al2O3, Cr2O3 and CaO contents compared with those of the host peridotite, suggesting the consumption of a SiO2-rich metasomatising agent as the thin layer formed. These two distinctive metasomatising agents would be caused by the immiscibility of a homogeneous metasomatising agent which was evolved to be a high CO2/H2O with the result that hydrous minerals and other metasomatised minerals were formed by interaction between a parent metasomatising agent and host peridotite in the late stage of exhumation of the Finero complex. The parent metasomatising agent is probably derived from a subducted slab, possibly containing small amount of sediments.