Evolution of slaty cleavage in relation to diagenesis and metamorphism: a study from the Hunsrückschiefer

Abstract

Detailed studies of the Hunsrückschiefer (Rheinisches Schiefergebirge), collected from the Katzenberg Mine, southeast of Mayen (West Germany), indicate that the slaty cleavage fabric is represented by a variety of morphologic features. These include anastomosing thin films of layer silicates separating narrow lenses of rock matrix, the preferred dimensional orientation of flat grains of quartz, feldspar, and calcite occurring in the rock matrix bounded by films of layer-silicate minerals, and beardlike growth of chlorite, muscovite, and other minerals on those surfaces of quartz and other host minerals that make a high angle with the cleavage direction. Other features that help to define cleavage in slates are "seamlike" bunches of muscovite crowded with opaque minerals in metasiltstone layers and silty dikelike bodies in metaclaystone layers. There are also some veins of quartz-chlorite-calcite parallel to the direction of cleavage. Evidence of mobilization of silty and clayey materials along cleavage planes and the presence of silty dikes along the cleavage planes suggest that the cleavage-forming deformation began during the prelithification stage of the rocks. The deformation at this stage, besides developing seams of muscovite, also caused formation of weak anistropy by mechanical rotation of detrital micas to the planes of cleavage. The earliest feature to develop following lithification and early diagenesis is the beardlike growth of chlorite, muscovite, and rutile on different host minerals. The layer-silicate films appeared in the rocks following growth of chlorite-muscovite porphyroblasts, in the earliest metamorphism of the rocks. It is argued that these films evolved through the development of fractures (which also pass through the chlorite-muscovite porphyroblasts), coupled with re-crystallization following Riecke's principle. Further readjustments in the rocks caused flattening of minerals like quartz, feldspar, and calcite through solution. The veins of quartz-chlorite-calcite were the last to form in the rocks.