Detailed study of garnet inclusions from the UHP meta-ophiolites of the Western Alps: new petrological constraints on the deep history of a subduction zone

The occurrence of Ultrahigh Pressure (UHP) index minerals (i.e., coesite, microdiamond, overall) in tectono-metamorphic belts is of paramount importance to attest the depths attained during subduction of the lithosphere. Recently, many studies focuses on the importance of the inclusions stored in host garnet that preserve information on the trapping conditions (i.e., elastic properties). Detailed inclusions study allows to recognize UHP mineral phases that escaped re-equilibration. Locally, these minerals are no more present in rock matrix, making inclusions the unique evidence of early rock evolution. In this work, we perform a detailed inclusion study through a multi-technique approach to obtain new constraints on the peak metamorphic conditions of a deeply subducted oceanic slab (> 100km).

We focused on two tectono-metamorphic units outcropping in the Western Alps, characterized by a metamorphic peak in UHP conditions. These units belong to the meta-ophiolites of the Internal Piedmont Zone (IPZ) and are located in (i) the Susa Valley (Ghignone et al., 2021), and (ii) Pellice Valley (Colle del Baracun; Ghignone et al., 2023). The units consist of oceanic lithosphere (serpentinite, metagabbro) and metasedimentary cover (micaschist, calcschist). For each unit we selected three samples of Grt-bearing metasediments, making a detailed characterization of the garnet inclusions with optical microscope and micro-Raman spectroscopy. Measured Raman spectra allowed to identify coesite in the Susa Valley (previously unrecognized). We applied Raman-based elastic geothermobarometry (Angel et al., 2015) on quartz and zircon inclusions in garnet as well as Zr-in-rutile thermometry to define the metamorphic path of the units. Furthermore, we combined the distribution of inclusions in zoned garnet with multispectral compositional maps (SEM-EDS), for obtaining a detailed mineral assemblage related to each garnet growth stage.

Our results highlight substantial differences of inclusion mineralogy and their microstructural position within the garnet shells between the two studied units. The IPZ in the Susa Valley recorded (i) the peak-PT under UHP conditions, (ii) a second event under HP eclogitic conditions, and (iii) a late re-equilibration event under LP conditions. Instead, the IPZ in the Pellice Valley recorded i) an early event corresponding to the prograde stage, ii) a peak pressure in UHP conditions, iii) a late re-equilibration event under LP conditions. Despite these differences, the two studied meta-ophiolitic units show similar metamorphic evolutions down to UHP conditions. This evolution follows a similar gradient to that of the coesite-bearing Lago di Cignana unit (e.g., Groppo et al., 2009). This may point out that in the Western Alps three major ophiolite slices underwent UHP metamorphism, thus suggesting that a large volume of oceanic lithosphere was subducted at ca. 100 km depth.

Angel, R., J., Nimis, P., Mazzucchelli, M., L., Alvaro, M., Nestola, F. (2015). J. Metamorph. Geol. 33 (8), 801-813. [10.1111/jmg.12138]

Ghignone, S., Borghi, A., Balestro, G., Castelli, D., Gattiglio, M., Groppo, C. (2021). J. Metamorph. Geol. 39 (4), 391–416. [10.1111/jmg.12574]

Ghignone, S., Scaramuzzo, E., Bruno, M., Franz, A. L. (2023). Am. Mineral. 108, 1368–1375. [10.2138/am-2022-8621]

Groppo, C., Beltrando, M., Compagnoni, R. (2009). J. Metamorph. Geol., 27(3), 207-231. [10.1111/j.1525-1314.2009.00814.x]