Variscan tectonometamorphic evolution of the eastern Lys-Caillaouas massif, Central Pyrenees - evidence for late orogenic extension prior to peak metamorphism

Abstract

Four deformation phases have been distinguished in the Cambro-Ordovician metasediments of the eastern Lys-Caillaouas massif. D1 only affected rocks of a stratigraphic unit below and Ordovician metaconglomerate horizon and is of pre-Variscan age. During D2 tight folds with steep E-W trending axial plane foliations S2 have formed, indicating N-S shortening. D3 consists of: (i) porphyroblastrotation, (ii) formation of gently dipping crenulation cleavages S3 and (iii) transposition of S2 to S3 in highest grade metamorphic rocks. D3 is interpreted as vertical shortening and horizontal extention. Both D2 and D3 are of Variscan age. D4 includes various deformation phenomena of unknown age, postdating peak metamorphism. Metamorphism started at the end of D2 and continued until after D3. Three phases have been distinguished: plurifacial regional metamorphism Ml and M3 and contact metamorphism M2 around the Lys-Caillaouas porphyritic biotite granite. This study shows that Variscan N-S shortening in the eastern Lys-Caillaouas massif was followed by metamorphism and crustal extension. Flat-lying structures overprinted steep structures during the extensional phase. Peak metamorphism was reached afterwards.