A possible new UHP unit in the Western Alps as revealed by ancient Roman quern-stones from Costigliole Saluzzo, Italy

Peculiar coesite + chloritoid + garnet ± glaucophane talcschists have been used to make at least four quern-stones, unearthed in the ruins of a villa rustica belonging to the Roman imperial period and located at Costigliole Saluzzo, Western Alps. The site of the villa rustica and the presence of coesite relics could suggest a possible provenance of these rocks from one of the already known ultra-high pressure (UHP) units of the Western Alps, i.e. either the Brossasco-Isasca Unit (BIU) of the southern Dora-Maira Massif or the Lago di Cignana Unit (LCU) of the Piemonte Zermatt-Saas Zone. However, similar talcschists have never been reported from these Units. Two samples of coesite-bearing, chloritoid + garnet ± glaucophane –talcschist collected from two different specimens of quern-stones have been petrologically investigated with the aim of defining their peak pressure–temperature (P-T) conditions. The stability field of the coesite + garnet + talc + chloritoid + glaucophane assemblage has been constrained using isochemical phase diagrams modelled in the MnNCFMASHO system; prograde P-T conditions have been additionally constrained using the yttrium-in-garnet (YAG) geothermometer. Thermodynamic modelling tightly constrains peak P-T conditions at 480-510°C, 27-31 kbar. The unusual Mg-rich composition of the talcschists suggests that they originated by Mg-metasomatism of either a continental crustal protolith (granodiorite or Fe-rich metapelite) or an oceanic crustal protolith (altered oceanic crust). A mechanism similar to that proposed for the well-known pyrope-bearing whiteschists of the UHP BIU, i.e. influx of antigorite-derived fluids along shear zones during subduction, can be envisaged. Although the field occurrence of these coesite + chloritoid + garnet ± glaucophane talcschists is still unknown, the obtained results clearly show that these rocks cannot belong to the UHP BIU, whose peak P-T conditions are at significantly higher T and P (730°C, 40-43 kbar). Therefore, this finding opens the challenging hypothesis of the existence of a further, still unmapped, UHP Unit in the western Alps that also experienced UHP metamorphism and fluid influx from underlying serpentinites during subduction. A detailed mapping and petrologic investigation of the tectono-metamorphic continental (i.e. southern Dora-Maira Massif) and oceanic (i.e. Monviso meta-ophiolite Complex) units in the proximities of Costigliole Saluzzo is required in order to further constrain the location and the dimension of this new UHP unit.