TECTONO-METAMORPHIC EVOLUTION OF THE ROIGNAIS-VERSOYEN UNIT (VALAISANDOMAIN, FRANCE)

The Roignais-Versoyen Unit (Western Alps, France) offers the best pres erved example of ultramafic-mafic rocks affected by eclogitic metamorp hism in the Valaisan domain during the Alpine orogenesis. The Roignais -Versoyen unit is composed of the Valaisan flysch and the igneous sedi mentary pile of the Versoyen (Versoyen complex). The Versoyen complex shows three metamorphic stages which are characterized by: (1) A high- pressure low-temperature (HP-LT) eclogitic stage giving rise to the de velopment of omphacite +/- Fe-garnet +/- glaucophane 1 +/- zoisite +/- rutile +/- quartz in the metagabbros and Fe-garnet +/- jadeite +/- ch loritoid in the metasediments. (2) A blueschist stage producing the de velopment of glaucophane 2 +/- phengite +/- zoisite-clinozoisite in th e metagabbros and glaucophane 2 +/- zoisite-clinozoisite +/- lawsonite in the metasediments. (3) A greenschist stage corresponding to the gr owth of actinolite +/- albite +/- chlorite +/- Mn-garnet +/- stilpnome lane +/- quartz +/- tourmaline and of prehnite +/- pumpellyite. Petrog raphical study allows to estimate the P-T condition of the eclogitic s tage and the retromorphic evolution under blueschist and greenschist f acies conditions. New structural data allow to precise a part of the t ectonic evolution of the Roignais-Versoyen unit. Stretching lineation, extensional crenulation cleavage and drag folds verging to the SE ind icate a top to the: SE shearing under greenschist facies conditions. T he location of the deformation allows to interpret the Versoyen comple x as a normal ductile shear zone. In the Versoyen complex, earlier (un der pre-greenschist facies conditions) deformation has been observed: the distribution of the textures within the laccoliths indicates a suc cession of normal and inverted limbs. This succession is explained by the presence of isoclinal folds verging to the NW. We outline the rela tionships between the post-eclogitic evolution and the exhumation proc esses and we discuss the significance of the eclogitic facies metamorp hism in the metamorphic zoneography of the Western Alps.