Coesite in garnet-quartzite of Orco Valley (Western Alps): an additional UHP unit in the records of deeply subducted meta-ophiolites

We report the occurrence of coesite in a white mica-garnet-bearing quartzite from the metasedimentary cover of the meta-ophiolites exposed in the Orco Valley, Western Alps (Italy). This discovery is an addition to the growing number of ultra-high-pressure (UHP) meta-ophiolite localities in this portion of the Alps, and it indicates that the hosting rock has reached depths exceeding the quartz-coesite transition (≥ 2.8 GPa, 80-100 km) during subduction. Here, the petrological and mineralogical observations on garnet-hosted inclusions of the sample are reported and used to qualitatively constrain the metamorphic evolution of Orco Valley, also in relation to the other UHP units. At the scale of the Alpine fossil subduction zone, the UHP evidence occurs locally and discontinuously along strike, with exposures that are patchy rather than continuous (e.g., Lago di Cignana, Ala Valley, Susa Valley, Lago Superiore); however, when compared, the different units show similar metamorphic and structural features, suggesting similar P-T evolutions. This finding supports the interpretation that UHP meta-ophiolites of the Western Alps represent remnants of a former level that underwent comparable conditions in the coesite stability field within the oceanic slab. The frequent new identification of coesite likely reflects both improvements in micro-analytical techniques and increasing attention to smaller isolated inclusions.