Crenulation cleavage and secondary compositional layering in greenschist facies semipelitic rocks (Lower Palaeozoic) of Sunnhordland region, SW Norway

Abstract

Greenschist facies semipelitic metamorphic rocks of Lower Palaeozoic age in Sunnhordland region, SW Norway contain a well-developed crenulation cleavage of the second generation. In the generation of the crenulation cleavage from a pre-existing fabric, gradations exist between discrete crenulation cleavage to zonal crenulation cleavage. The orientation of phyllosilicates within the cleavage septa is controlled primarily by the relative dominance of solution transfer of materials in discrete cleavage and by microfolding mechanism in zonal cleavage. In the former, new phyllosilicates may lie sub-parallel to the cleavage, but in the latter, phyllosilicate orientation may be variable, usually bimodal. The sites of discrete (solution) planes in variably tight folds, and the degree of development of zonal cleavage are dependent on the flattening strain. The secondary dominal character of the rock is a feature related to compositional differentiation through microfolding and essentially, solution transfer.

Key Words:

• Lower Palaeozoic rocks
• greenschist facies
• crenulation cleavage
• classification
• secondary compositional layering
• differentiated layering
• folding
• flattening
• strain
• microstructures
• stylolite
• pressure solution
• mica orientation
• cleavage septa
• microlithon
• nucleation
• kink-band
• rigid-body rotation
• chemical trends
• Sunnhordland region
• SW Norway
• N5932 NS950 E0515 E0545