This paper presents new data on the Alpine high-pressure evolution of
the Lanzo peridotite massif and its metasedimentary cover (Western
Alps). Field relations in the northwestern Lanzo massif show that the
serpentinized part is covered by a metasedimentary sequence including
local occurrences of metaophicarbonate, metabasalt, calcschist,
metaquartzite and gneiss. Remnants of granulite-facies mafic gneiss
associated with metaophicarbonate are found within the serpentinite.
Major element garnet zonation profiles in metaquartzite and mafic
gneiss were used to explore the contrasting origin of metasediments.
Bell-shaped Mn zoning in garnet from metaquartzite indicates a
one-stage Alpine metamorphic evolution, while inverse zoning in garnet
of the mafic gneiss shows that the protolith equilibrated under
high-grade metamorphic conditions in pre-Alpine times, similar to
kinzigites within the neighbouring Sesia zone. Phase equilibria in
metaquartzite and mafic gneiss point to 500 +/- 50 degrees C at
pressures of 0.9 to 1.3GPa for the Alpine metamorphic equilibration
that contrasts with the 550-620 degrees C and pressures in excess of
2.0GPa obtained for kyanite+talc+chloritoid and talc+chloritoid+garnet
domains in a metagabbro dike from the internal part of the Lanzo
massif. These different P-T trajectories can be explained by a much
stronger retrograde overprint at the basement-cover contact than in the
internal part of the Lanzo massif. (c) 2006 Elsevier B.V. All rights
reserved.