Petrological outline of an area near Sparone (Orco Valley, Western Italian Alps)

Several rocktypes and their metamorphic mineral growth are described from an area on the western border of the Sesia-Lanzo Zone in the Orco Valley (NW Italy). It is argued that in some rocks (garnet-rich gneisses and micaceous gneisses) pre-Alpine metamorphic minerals are present, in other rocks (carbonate-bearing schists, albite-chlorite gneisses) such minerals are rare or absent. For the latter rocks it is therefore difficult to establish whether they are strongly retrograded Alpine basement rocks, or rocks belonging to the suite of ophiolitic schistes lustrés. The two possibilities are discussed.