PORPHYROBLAST NON-ROTATION DURING CRUSTAL EXTENSION IN THE VARISCAN LYS-CAILLAOUAS MASSIF, PYRENEES

The dominant foliation (S-2) in the Variscan Lys-Caillaouas massif (Central Pyrenees, France/Spain) formed in a subvertical orientation during crustal thickening (D-2) Subsequent non-coaxial crustal extension produced a subhorizontal crenulation cleavage (S-3), whereby S-2 was folded and rotated. Andalusite, staurolite, cordierite and biotite porphyroblasts grew very early during this extension(D-3) and included S-2 as straight, curved and weakly crenulated inclusion trails. These inclusion trails exhibit a strong subvertical preferred orientation, which is independent of the local magnitude of D-3 Strain, the dip angle of the external main foliation (S-2), Or the aspect ratio and shape-orientation of porphyroblasts. This indicates that porphyroblasts did not rotate in the D-3 deformation field and hence, preserve the orientation of S-2 at the time it was trapped in the porphyroblast, relative to the bulk D-3 how plane. Porphyroblasts could maintain a stable orientation in the D-3 flow due to complete accommodation of the bulk flow vorticity by heterogeneous shear strain. The lack of vorticity of local volumes of zero (shear) strain, such as porphyroblasts, was balanced by concentration of shear strain in planar zones anastomosing around them. It is likely that S-3 maintained a subhorizontal orientation throughout the D-3 crustal extension, which would imply that porphyroblasts also remained stationary relative to the earth's surface during D-3, whereas S-2 in the deforming matrix experienced shear-induced rotation. A late brittle-ductile folding event (D-4), involving differential rigid block rotations, dispersed the orientation of inclusion trails to some degree. These late rotations could be corrected for by artificial 'unfolding' of the F-4 folds.