STRUCTURAL AND COMPOSITIONAL FEATURES OF THE MT. MUSINÈ PERIDOTITES (LANZO MASSIF, WESTERN ALPS, ITALY)

Abstract

Present knowledge indicates that the ophiolitic peridotites of the Lanzo South Massif were exhumed from spinel-facies conditions to shallow oceanic levels during the rifting stage of the Jurassic Ligurian Tethys. During exhumation, most of these peridotites were diffusely percolated and impregnated by asthenospheric melts. Field, structural and petrological-geochemical investigations, in the frame of a multidisciplinary investigation, have been dedicated to the southern flank of Monte Musine (Lanzo South, Western Alps), where the extreme structural and lithological variability and the mutual relationships between different rock types (i.e. petrogenetic processes) allow to reconstruct some steps of the composite structural and petrological evolution of this peridotite massif. The oldest structural elements recognizable in the field are represented by pyroxenite bands and by spinel-facies foliation, which are almost parallel in any outcrop. This feature essentially supports the presence of a long-time constant strain field at spinel facies conditions. Consistently, the percolating melt increments, which produced the plagioclase-rich impregnation of the spinel peridotites, utilized most probably the spinel-facies foliation plane to migrate by porous flow. A strong deformation event developed along shear zones, following the plagioclase impregnation, forming plagioclase peridotite tectonites which were subsequently used by silica-undersaturated asthenospheric melts, i.e. aggregated MORB-type melts, to migrate upwards along these structural discontinuities. These pyroxene-undersaturated melts interacted with the plagioclase tectonite mineral assemblage, dissolving plagioclase and clinopyroxene, and forming concordant harzburgite bands or, later, dissolving plagioclase and both pyroxenes, forming concordant dunite bands. Subsequent melt-related events, i.e. the formation of decametre- to hectometre-wide replacive dunite bodies, and intrusion events, i.e. the intrusion of MORB gabbroic dikes, were driven by a different strain field strongly discordant with the previous foliation and with the trend of the shear zones and the concordant replacive bands. Thus, the section of mantle lithosphere of Monte Musine record a composite scenario of melt-related and tectonic-metamorphic processes occurred during its exhumation to shallow oceanic levels of the Jurassic Ligurian Tethys.