PRE-ALPINE METAMORPHIC EVOLUTION OF THE GNEISSES FROM THE VALPELLINE SERIES (WESTERN ALPS, ITALY)

The Valpelline series in the western Alps belongs to the Austroalpine domain which corresponds to the southern continental margin of the Tet hys during Jurassic times. In the Valpelline gneisses which are a part of the pre-Alpine basement, microscopic observations evidence two gen eration of garnets: Gr I consists of the centimeter-sized inclusion ri ch core, Gr II of the inclusion-poor rim of the centimeter garnets and millimeter garnets in the foliation plane. The inclusions in Gr I con sist of kyanite, alkali feldspar, plagioclase and rutile. Both rutile and kyanite have been observed in the matrix of the rock, Microstructu ral observations demonstrate that kyanite occurs before sillimanite re cording a pre-Alpine polymetamorphic evolution. Microstructural and pe trological investigations involve a progressive consisting of an early intermediate pressure granulitic stage recorded by rare relies of kya nite, rutile and alkali feldspar and the first generation of garnet. L ater the Valpelline gneisses have experienced an intensive reequilibra tion under HT amphibolite facies conditions, characterized by the asso ciation garnet II, sillimanite, biotite, cordierite and ilmenite and a ssociated with a main deformation stage. The earlier granulitic and th e amphibolitic stages are related to the pre-Alpine metamorphic evolut ion but, so far few radiometric ages are available in the Dent Blanche nappe, in this case this evolution can be related either to a late Va riscan or Permo-Jurassic period. The Alpine evolution s.s. occurred un der greenschists facies conditions characterized by chlorite, sericite , muscovite, clinozoisite and albite assemblage preferentially develop ed in narrow shear zones throughout the valley and in the contact zone between the Valpelline and the Arolla units.