First evidence of UHP in the Lago Superiore Unit (Monviso, Western Alps)

The first occurrence of ultra-high-pressure (UHP) metamorphism in the Western Alps was documented by Chopin in 1984 (Chopin, 1984) with the discovery of coesite in the southern Dora Maira massif. Since then, just one additional UHP terrain was discovered until the end of the 90’s. In recent times, new occurrences of coesite have been reported in different units of the Western Alpine belt, widening the distribution of UHP terrains, with important tectonic implications. Here, we report the first discovery of coesite in the meta-ophiolitic suite of the Monviso Massif, in the northern Lago Superiore Unit (LSU). Previous petrographic studies and thermodynamic modelling in the area suggested that these alpine units may have experienced UHP metamorphism, but no direct evidences (i.e., coesite occurrence) have been reported to date.  The presence of coesite is demonstrated by µ-Raman analyses. The Raman spectra show the typical peaks of coesite, slightly shifted towards higher wavenumbers. The main peak is located at 522 cm^–1, and the secondary peaks at 426, 270 and 178 cm^–1. Coesite inclusions consist of intact single crystals (10-60 µm) hosted by garnet, without evidence for re-equilibration features. Typical coesite-related features such as radial cracks in garnet host mineral and palisade texture are present in large polycrystalline quartz inclusions (>80 µm). Peak metamorphic conditions have been constrained through different techniques (detailed garnet inclusion analysis, elastic geobarometry and thermodynamic modelling).

The occurrence of UHP terrains along the Western Alps is becoming more common than expected. Our results, alongside with the novel evidence for UHP in the Western Alps, will lead to new tectonic models for the subduction and exhumation of UHP terrains, constraining the evolution of subduction-accretionary systems.

Chopin, C., 1984, Coesite and pure pyrope in high-grade blueschists of the western Alps: a first record and some consequences: Contributions to Mineralogy and Petrology, v. 86, p. 107–118, https://doi.org/10.1007/BF00381838.