Caratterizzazione petrografica delle litologie oceaniche associate alle mineralizzazioni a solfuri massivi nella Zona Piemontese del Colle della Borra (Val Soana, Alpi Occidentali)

The study area is located at Colle della Borra (2.578 m), within the Piemonte Zone, where new detailed observations on the geological context and on lithologies associated with massive sulphide mineralization were made. The meta-ophiolite exposed at Colle della Borra, consisting of abundant calcschists and micaschists with metabasites, quartz-ites, marbles, chloriteschists and metasomatic metagabbros, have long been considered to be of Combin type, belonging to the greenschist-facies Colle della Rosa-Dondena tectonic Unit (CRD), interposed between two eclogitic-facies ophiolitic units. In this work, garnet-chloritoid-, garnet-titanite-bearing phengitic leaden micas-chists and lawsonite-bearing calcschists preserving an old schistosity, have been investi-gated in detail. The high-pressure tectono-metamorphic evolution of meta-ophiolites in the study area has been qualitatively constrained based on the comparison with analogous parageneses in Grt-Cld-bearing micaschists along the Western Alps. This comparison suggests the presence of relict eclogitic-facies metamorphic imprint in this area. Three main metamorphic stages have been recognized: 1) D1 phase which developed during prograde path of the Alpine subduction producing the S1 foliation (S1 relict foliation is documented by the occurrence of rootless isoclinal folds); 2) the D2 phase which started at the metamorphic HP peak producing the S2 foliation and then developed during exhu-mation under blueschist‐ to greenschist-facies conditions (S2 pervasive plane-parallel fo-liation is marked by eclogite‐facies mineral assemblages partly retrogressed; 3) D3 (with-out foliation) which represents the late stage that locally developed an incipient crenula-tion cleavage; all the D2 structures were overprinted by D3 larger folds. Considering the abundant relicts of lawsonite completely replaced by zoisite + phengite (partly recrystallized into Fe-epidote and paragonite) in the calcschists, an origin from fluid‐rock interaction and Ca‐metasomatism in the subduction environment is in-ferred for this lithology. The presence of lawsonite relicts is a clear evidence of the intense process of Ca-metasomatism along the subduction channel during prograde evolution. The lawsonite breakdown is testified by Ca-rich garnet rims in the micaschists. The ophiolitic sequence shows both a continental affinity (calcschists and micas-chists) and an oceanic affinity (presence of VMS mineralisation and metabasites) and can be interpreted to reflect an original ocean-continent-transition (OCT) zone. The sequence probably derived from a slow-ultraslow-spreading-ridge segment affected by hydrother-mal fluid circulation (testified by presence of VMS deposit) and by intense tectonic ac-tivity, near a continental margin. The presence of high-pressure mineral assemblages in the meta-ophiolites of the Colle della Borra sequence suggests they do not represent a greenschist-facies CRD slice, but rather the top portion of the eclogite metasedimentary cover of the Zermatt-Saas oph-iolites, which are exposed about 150 meters downstream towards Campiglia. Conse-quently, the sequence of Colle della Borra calls for reconsidering the tectono-metamor-phic evolution of the entire CRD and of all ophiolitic units described in the Upper Soana valley (Piamprato and Campiglia valleys). This study marks the starting point for further studies in this area, which could help to overcome the limits imposed by mineralogical associations. A modern approach to the study of ophiolites in this area has to combine the geological data with both the deformation distribution, the paleogeographic reconstruc-tion and the metasomatic processes that occurred during subduction.