Contrasting roles of detrital and authigenic phyllosilicates during slaty cleavage development

Eight samples from a prograde slate sequence from Central Wales ranging from the diagenetic zone to the epizone, were studied by X-ray texture goniometry, SEM and TEM. X-ray pole figure data imply that, in the four lower-grade samples, the crystallographic preferred orientations of both mica and chlorite change from parallel to bedding, to intermediate to bedding and cleavage, to parallel to cleavage orientation with increasing grade. There is no difference in the degree of preferred orientation between mica and chlorite in individual samples. The sample with an intermediate preferred orientation has the lowest degree of preferred orientation. In the four higher-grade samples, the preferred orientations of mica are primarily in the cleavage orientation, whereas those of chlorite are largely parallel to bedding. Observations by SEM and TEM show that the samples have two populations of phyllosilicates: (1) large (tens of mu m) chlorite-mica stacks of detrital origin, dominated by chlorite in a matrix of (2) authigenic fine-grained (tens of nm) mica and chlorite with dominant illite (muscovite). Preferred orientation measurements of chlorite primarily reflect the chlorite in the stacks. However, preferred orientation measurements of mica are dominated by the mica in the stacks for four lower-grade samples, but by mica in the fine-grained matrix with (001)parallel to cleavage for the four higher-grade samples. These observations collectively show that: (1) mechanical rotation of detrital grains was the dominant reorientation mechanism of cleavage formation in the lower-grade samples: (2) in the higher-grade samples, the preferred orientation of matrix mica and chlorite transformed from bedding-parallel to cleavage-parallel primarily by dissolution and neo-crystallization. Copyright (C) 1996 Elsevier Science Ltd