Upper–greenschist facies intragrain deformation of albite in mylonitic meta–pegmatite and the influence of crystallographic anisotropy on microstructure formation

Abstract

We studied the deformation behaviour of albite from Permian meta–pegmatite in Cretaceous upper–greenschist facies shear zones from the Austroalpine Matsch Unit in the Eastern Alps (Italy). Sodium–feldspars from these rocks provide excellent natural examples for studying mechanisms of intragranular deformation under mid–crustal conditions in grains with different angular relations between their (010) planes and the kinematic frame. The studied rocks were deformed at c. 500 °C in localized shear zones with well characterized top–W shear kinematics supposedly during the Cretaceous upper–greenschist facies tectonometamorphic event. Microstructural and chemical data suggest that crystallographic anisotropies in albite exert a strong control on microstructure formation and that albite primarily deformed by a combination of brittle fracturing, dissolution–precipitation and incipient crystal plasticity as a function of the orientation of the crystallographic anisotropy relative to the supposed shortening direction. Dissolution along discontinuities forming stylolites perpendicular to the shortening direction is associated with the precipitation of fine–grained albite with some compositional variability (Ab96–98 and Ab89–91) in cracks. New albite precipitates form aggregates with straight segments of high angle grain boundaries, nearly 120° dihedral angles and only a poor or no orientation relation to the hosting clast. Intragranular kinking is related to continuous lattice rotation of up to 15° by a misorientation axis close to albite [100] and the formation of subgrain boundaries with maximum misorientations of 7°. Synthetic microshear zones supposedly nucleated on pre–existing cracks, and are associated with formation of subgrain boundaries in shortening quadrants and cracks together with precipitates of potassium feldspar in extensional quadrants adjacent to the microshear zone. New microstructural and textural data from mylonitic Permian meta–pegmatites document various closely linked crystal plastic and brittle deformation mechanisms and highlight the role of crystallographic anisotropies and their orientation with respect to the kinematic frame in microstructure formation.