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

摘要

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.