EXHUMATION OF ULTRAHIGH-PRESSURE METAMORPHIC OCEANIC-CRUST FROM LAGO-DI-CIGNANA, PIEMONTESE ZONE, WESTERN ALPS - THE STRUCTURAL RECORD IN METABASITES

The metamorphic and deformational history of coesite-bearing, ultrahig h-pressure metamorphic oceanic crust from Lago di Cignana, Valtournanc he, western Alps, as recorded along the decompression and cooling path , has been resolved to derive constraints on the physical state of the crust during exhumation. The following history is proposed: Eclogites derived from pillow basalts were deformed by dislocation creep oi pyr oxene at a depth of more than 80 km, in the stability field of coesite . The deformation was progressively localised into shear bands. Over t he first 40 km of subsequent exhumation, the rocks remained essentiall y unmodified. Any deformation must have been localised beyond the limi ts of the present outcrop. Retrogression first in a closed system star ted without concomitant deformation at a depth of about 40 km and temp eratures of 550 to 500 degrees C. Subsequently, the system size increa sed. Fluid infiltration, focussed along tensile fractures, is document ed by the formation of veins. Vein shapes and orientations indicate hi gh pore-fluid pressures and low differential stress. After cooling to about 350-450 degrees C at depths of 12 to 18 km, localised ductile de formation transformed the retrogressed eclogites into greenschists. Fo r that stage, the microstructure of the quartz veins indicates deforma tion by dislocation creep at falling temperatures and increasing diffe rential stress. Final cooling to below ca. 300 degrees C took place at depths of 7 to 12 km, as indicated by the density of fluid inclusions . Later deformation in the brittle field cannot be correlated with the P-T path. (C) 1997 Elsevier Science B.V.