Phyllosilicate preferred orientation in relation to strain path determination in the lower Paleozoic Stavelot-Venn Massif (Ardennes, Belgium)

Abstract

Phyllosilicate preferred orientation, measured by means of an X-ray pole figure goniometer, allows a quantitative exploration of the slaty cleavage development. The relationship between phyllosilicate preferred orientation and strain, as demonstrated in the March model, furthermore enables the application of phyllosilicate pole figures in a strain determination in slate belts. Both applications are illustrated in this paper, in which the results are presented of a crystallographic fabric analysis of argillites in the southern domain of the lower Paleozoic Stavelot-Venn Massif (Belgium). In these argillites the development of a phyllosilicate preferred orientation is primarily defined by the low-grade metamorphism and the deformation history. The metamorphism, causing recrystallisation to be active during cleavage development, assures the development of a high degree of preferred orientation. On the other hand, the deformation history defines the type of phyllosilicate pole figure pattern. Based on the distinction of different pole figure patterns, a possible strain path for the argillites in the southern domain of the Stavelot-Venn Massif is proposed. An axially symmetric pole figure pattern indicates a Caledonian cleavage development in a pure flattening regime. The Variscan cleavage development, on the other hand, is characterised by an additional extensional component in the cleavage plane. If the primary phyllosilicate fabric, due to a pre-deformational compaction or a Caledonian cleavage development, underwent a complete reorientation caused by syntectonic recrystallisation, a typical orthorhombic pole figure pattern is formed. Otherwise a transition pole figure pattern develops, reflecting the superposition of the Variscan cleavage development on a pre-existing Caledonian cleavage fabric. The general deformation history of the southern domain of the Stavelot-Venn Massif differs substantially from that described for the northern domain. Contrary to the northern domain the southern domain seems to have undergone considerable Variscan deformation.