Subduction and exhumation of an Ultra-High Pressure oceanic slab in the Western Alps, new insights from the Lago Superiore Unit

Ultra-high pressure (UHP) units sample the deepest portion of a subduction zone that returned to the surface, escaping their fate of disappearing deep into the mantle. Several mechanisms have been proposed for the exhumation of UHP units in collisional orogens but the topic remains still controversial and poorly understood. The models invoked for the exhumation of UHP units generally require a positive buoyancy as trigger of exhumation. However, Ghignone et al. (2023) reported for the first time the occurrence of a slice tens of kilometres in length of oceanic slab, i.e., the Lago Superiore Unit (LSU), that reached UHP depth. This latter represents a portion of the former Alpine Tethys oceanic lithosphere now accreted within the Western Alpine collisional system (Ghignone et al., 2023). In this contribution we present new insights on the subduction-accretionary processes preserved in the UHP Lago Superiore Unit. Our study is based on i) a new structural map of the LSU considering new data, ii) structural and kinematic field data, and iii) new prograde and retrograde P-T estimations calculated combining quartz-in-garnet elastic thermobarometry with Zr-in rutile and Ti-in-quartz thermometry. Our new integrated kinematic and thermobarometric model suggests that the primary process driving the exhumation of the UHP Lago Superiore Unit was the progressive extraction of a composite metamorphic wedge. Final extension as revealed by thermobarometric constrain, allowed the exhumation of the Lago Suepriroe Unit at shallow crustal levels. Ghignone, S., Scaramuzzo, E., Bruno, M., Livio, F., 2023. Am Mineral, 108(7), 1368-1375.