Mylonitic Foliation Developed by Heterogeneous Pure Shear under High-Grade Conditions in Quartzofeldspathic Rocks (Chímparra Gneiss Formation, Cabo Ortegal Complex, NW Spain)

Abstract

The quartzofeldspathic Chímparra Gneiss Formation contains S-type mylonites developed under high pressure-high temperature metamorphic conditions. These mylonites may appear either as an homogeneous planar foliation that obliterates the previous textures and structures of the rock, or as an inhomogeneous planar foliation forming anastomosing shear bands. In the anastomosing areas, quasi-lenticular pods of migmatitic gneiss have been partially preserved from later deformation and show a previous migmatitic texture. Within these pods, the migmatitic banding has been symmetrically folded and the geometry of the folds indicates a progressive flattening towards the edges of the pods. The mylonitic foliation is parallel to the axial planes of these folds, and causes an intersection lineation parallel to the fold axis; however, a mineral lineation is absent. Kinematic criteria deduced from the geometry of polycrystalline quartz aggregates, extensional crenulation cleavages (ECCs), and quartz c-axis fabrics do not indicate any consistent preferred sense of movement. Both macro-and microstructural analyses suggest an inhomogeneous pure shear or flattening as the main deformation process related to the development of the mylonitic foliation.