The relationship of phyllosilicate orientation, X-ray diffraction intensity ratios, and c/ b fissility ratios in metasedimentary rocks of the Helvetic zone of the Swiss Alps and the Caledonides of Jämtland, central western Sweden

Abstract

Phyllosilicate preferred orientations, X-ray diffraction intensity ratios, and c/ b fissility ratios were determined from 23 metasedimentary rocks from the Helvetic zone of the Swiss Alps and from the Caledonides of Jämtland, Sweden. The relationships between these parameters of phyllosilicate orientation depend strongly on the lithology and the cleavage morphology. Differences in orientation direction and in degree of preferred orientation reflect both deformation intensities and lithologies. Whereas polyphase deformed, fine-grained samples generally have phyllosilicates that are oriented parallel to cleavage showing high orientation intensities, phyllosilicates in less deformed and/or coarse-grained rocks are oriented parallel to bedding with low orientation intensities. Weak orientation intensities in samples with phyllosilicates that are oriented parallel to bedding reflect compaction strain. The higher strain in samples with micas and chlorites that are parallel to cleavage is a combination of compaction and tectonic strain due to the development of crenulation cleavage.