P–T Alpine metamorphic evolution of the Monte Rosa nappe along the Piedmont Zone boundary (Gressoney Valley, NW Italy)

Abstract

The pseudosection modelling of two chemical systems (both metabasic and metapelitic) allowed to reconstruct the exhumation P–T path followed by the southern slope of the Monte Rosa nappe (Upper Gressoney Valley) during the Alpine orogenesis. The metamorphic evolution of the polymetamorphic basement complex from the Monte Rosa nappe is marked by texturally distinct mineral assemblages, defining four Alpine metamorphic stages (M1 to M4) reflecting a sequence of different P–T conditions. Well preserved eclogitic boudins (M1 and M2 assemblages) were investigated to reconstruct the HP history related to the subduction phase, while re-equilibrated metapelites allowed to infer the P–T conditions attained during the development of the regional foliation (M3 and M4 assemblages). The HP stage (M1) occurred at 550–570°C and 24–27kbar and is characterised by the assemblage Omp+Grt+Lws+Phe+Qtz+Gln±Tlc in the eclogites. The M2 metamorphic stage consists of the assemblage Omp+Grt+Barr+Zo+Phe+Pg+Qtz and developed at 590–630°C and 14.5–16.5kbar suggesting a T increasing during decompression. After a further decompressional stage associated with a T decrease, the M3 tectono-metamorphic stage developed syn-kinematically with the main regional foliation S1. It is marked by the assemblage Phe+Pg+Grt+Chl+Ab/Olig+Hbl+Qtz+Rt/Ilm±Bt in metapelites and by Cam (blue-green)+Chl+Ab+Bt+Qtz+Rt in the re-equilibrated metabasite boudins. S1 developed during increasing T (from 550 to ca. 600°C) and sligthly increasing P (from 7 to 9kbar). Finally, the M4 assemblage grew as rims over the M3 minerals or overgrew the S1 regional foliation (albite porphyroblasts overgrowing the S1 foliation already defined by albite) and therefore can be considered as the final stage of the M3 stage. The P–T path proposed for the Monte Rosa nappe differs from previous works. In particular, we propose post-eclogitic decompression trajectory up to 7kbar, followed by a P–T increase that can be related to a second burial–exhumation cycle during Alpine orogenesis, as recently reported for other Alpine units.