Mineralogical and petrological constraints and tectonic implications of a new coesite-bearing unit from the Alpine Tethys oceanic slab (Susa Valley, Western Alps)

Abstract

Worldwide, Ultrahigh Pressure (UHP) oceanic units are rare and to date only three were recognized: Tianshan (China), and Lago di Cignana and Lago Superiore Unit (Western Alps, Italy). The UHP oceanic units represent the only geological object directly exhumed from mantle depth and they record fundamental information about processes occurring in the deepest portions of the subduction interface.In this work, we describe the occurrence of a new UHP oceanic unit within the Western Alps (mid Susa Valley, Internal Piedmont Zone). We here report the finding of a UHP index mineral, i.e., coesite and we provide a detailed study of garnet inclusions in metapelites and metabasites part of the meta-sedimentary cover of the meta-ophiolites cropping out in the mid Susa Valley. The samples were investigated via optical microscope, Raman spectroscopy, Elastic Geothermometry and classic thermometry (Zr-in-rutile) in order to constrain the P-T evolution of the area.The finding of a new UHP unit in the Internal Piedmont Zone, together with the already described Lago di Cignana and Lago Superiore units, points towards the possible existence of a UHP oceanic slice that reached a similar peak-P (i.e., the return point) along the same subduction gradient. This slice was then removed from the slab and dismembered during exhumation, and it is now exposed as coesite-bearing units juxtaposed with lower pressure eclogite-facies ophiolites. Moreover, the large occurrence of coesite along the entire Internal Piedmont Zone significantly increases the extension of the UHP oceanic units. Hence, the model of a localized non-lithostatic pressure is at the state of the art difficult to apply to the oceanic units of the Western Alps.