Coesite in Alpine meta-ophiolites: hidden but widespread, and tectonically relevant

The occurrence of coesite, a Ultrahigh Pressure (UHP) index mineral,  in tectono-metamorphic belts is of paramount importance to pinpoint the depths attained during subduction. Such minerals are generally found as inclusions within garnets and are often the sole remnants of an UHP mineralogy in largely re-equilibrated rocks. UHP tectonometamorphic units in subducted oceanic lithosphere are of particular interest because they are natural laboratories to study element-exchange and fluid rock interactions occurring in a subducting slab at depths > 80 km. In this context, the meta-ophiolites of the Western Alps are a perfect case study, as they offer a continuous outcrop along the entire belt. Here, we focus on the UHP meta-ophiolites of the Internal Piedmont Zone (IPZ) in the Western Alps, where coesite inclusions in garnet have recently been found (Ghignone et al., 2023 and Boero, 2023). These localities lay on the same structural position of the Lago di Cignana Unit, wherein coesite was discovered in the early 90’s (Reinecke, 1991). In addition, these three UHP localities share similar metamorphic peak conditions and their PT estimates lie on the same metamorphic gradient (roughly 6°C/Km).

A targeted sampling campaign along the entire Western Alpine meta-ophiolitic belt allowed to better understand the distribution of coesite-bearing rocks. Metasediments (Grt-quartzite, Grt-Cld micaschist, Grt-calcschist) are the best lithotypes that preserved coesite, but also some meta-mafic lithotypes (eclogite, Grt-metabasite) contain it. Usually, garnets within metasediments are strongly zoned, whereas in meta-mafic lithotypes they have a more constant composition. Coesite was identified via µ-Raman spectroscopy, showing the typical vibrational modes of the phase (521, 427, 271 and 180 cm^-1), slightly shifted due to elastic residual strain. Coesite occur as pristine tiny crystals (<40 µm) entirely trapped in garnet, both isolated and clustered. Their shape varies from well-faceted to strongly anhedral with morphological evidence of resorption (i.e., lobed morphologies with rounded shapes and/or embayment). Bigger inclusions of quartz (>40 µm) present the typical features of re-equilibration after coesite (i.e., radial cracks, polycrystalline aggregates). 

Among the different UHP localities, the presence of coesite is limited to a specific garnet shell (e.g., core, mantle), identifying a specific moment of garnet growth in UHP metamorphic conditions. The other shells, contrarily, preserve inclusions of quartz. These differences allowed to reconstruct the prograde or retrograde evolution through a detailed inclusion study of their preserved elastic properties (i.e., elastic geobarometry).

Our results highlight that the entire IPZ eclogite-facies meta-ophiolites underwent UHP metamorphism in the coesite stability field. This suggests that a large volume of oceanic lithosphere was subducted at ca. 100 km depth and then returned to the surface. This is an important constrain to create  reliable tectonic models of  subduction and exhumation of the oceanic lithosphere in collisional subduction/accretionary systems.

 

Boero, F., 2023. Master Thesis, University of Turin. 115 pp.

Ghignone, S., Scaramuzzo, E., Bruno, M., Livio, F., 2023. Am Mineral, 108(7), 1368-1375.

Reinecke, T., 1991. Eur J Mineral, 3, 7-17.