A microstructural study of Penrhyn Slate, of North Wales

Abstract

Optical microscopy of three orthogonal ultrathin petrographic sections, 5 to 10 μm, in conjunction with scanning electron microscopy (SEM ) were used in a microstructural analysis of the Penrhyn slates of North Wales. The cleavage domains of these slates are found to be more penetrative than the lenticular domains. Aspect ratios of quartz measured in the X-Z and Y-Z sections are found to be 2.5 and 1.8 respectively. A possible quartz c-axis fabric is observed. Assuming the fabric is not an artifact of measuring it represents extension in the Y direction which is characteristic of doubly plunging folds. Micas of the chlorite mica stacks which have formed kink band boundaries between the portion in the stack and the portion in the cleavage will recrystallize forming two discrete micas. Chlorite has grown in extension sites produced in the splitting of micas in the chlorite-mica stacks. Serrated kink band boundaries are developed in the chlorite of some chlorite stacks. Fluid-enhanced intergranular diffusional mechanisms are most likely responsible for the intimate intergrowths of quartz and mica. Quartz mica intergrowths, mica beards, are developed in a direction grossly parallel to the X axis of the finite strain ellipsoid. The aspect ratios of quartz and the symmetry of the mica beards indicate the orthorhombic symmetry of the finite strain ellipsoid.