Clast Rotation and the Origin of Thick Ultramylonites: the El Pichao Shear Zone (Sierra de Quilmes), NW Argentina

The El Pichao shear zone in the Sierra de Quilmes, NW Argentina is a 3-7km thick, ductile shear zone between high grade migmatites and low grade metamorphic sequences of an exhumed basement. In the low grade metasediments deformation fabrics vary from protomylonite via mylonite to ultramylonite. These two sequences are separated by a granitic intrusion and intensely intruded by pegmatite dykes. The shear zone overprints all three main rock sequences. The ultramylonite fabrics can reach extreme thicknesses of 1km, which has been seldom observed elsewhere. Thus, the El Pichao Shear Zone provides a unique opportunity to understand differences in strain accommodation at the mylonite-ultramylonite transition, and the nature of thick ultramlyonite sequences. The mylonite sequences contain an anisotropic matrix comprising Bt + Qtz + Ms + Plg + Kfs, with coarse Qtz ribbons, mica bands and feldspar porphyroclasts up to 5 cm in diameter. Qtz ribbons have undergone grain boundary migration recrystallization and have been folded around feldspar clasts. Feldspar clasts have been variably rotated, demonstrated by the orientation of dynamically recrystallized material around the clasts. Three types of deformation behaviour occur simultaneously amongst the feldspar clasts: (i) brittle fracturing, (ii) partial recrystallization, (iii) complete recrystallization. The ultramylonite sequences contain a fine-grained equigranular matrix of Qtz + Ms + Bt + Plg + Kfs with feldspar porphyroclasts. Rotation-induced strain accommodation is demonstrated by the presence of δ-style porphyroclasts and the orientation of micas around clast boundaries. Qtz ribbons or strong S-C fabrics are lacking and the matrix tends to be homogeneous with only weak foliation defined by the preferred orientation of micas. The bulk connectivity of phases in the matrix decreases toward ultramylonite, which suggests a homogenization of the matrix between mylonite and ultramylonite. Quartz c-axis data demonstrates a systematic variation in the accommodation of strain in quartz between the mylonite/ultramylonite sequences, and within different structural domains of the mylonites. This data suggests that the slip systems of quartz change as a result of the homogenization process between mylonite and ultramylonite. The data also supports our current arguments that quartz slip systems are only relevant to the structural domains from which they are derived, and cannot be used to make kinematic assumptions for the whole rock. We argue based on petrographic features that the formation of thick ultramlyonites can occur where strain is high enough to instigate intense clast rotation. The homogenization of the originally banded mylonite results from continual rotation of clasts, which disaggregated the anisotropic matrix and thus inhibited strain localisation. The relative rotation of clasts in the matrix was a function of their vorticity and geometry, which may have influenced the variable deformation behaviours of feldspars in the mylonites. Strain softening at the clast matrix interface may have also played a role in increasing the vorticity of clasts and promoting rotation-induced strain accommodation.