Change in dominant mechanisms for phyllosilicate preferred orientation during cleavage development in the Kitakami slates of NE Japan

Abstract

Selected slates from the Kitakami Mountains of NE Japan were studied by means of finite strain measurement, X-ray texture goniometry, optical microscopy, back-scattered SEM, and X-ray fluorescence and diffraction analyses. Finite strain recorded by radiolarians in slates, although not equivalent to bulk strain, shows a positive correlation with strength of phyllosilicate basal plane fabrics. However, there is a possible difference between the fabric pattern and radiolarian strain ellipsoid shape as well as a difference in fabric pattern between chlorite and illite/mica. These relationships between phyllosilicate fabrics and finite strains suggest that strain estimation from phyllosilicate fabrics based on the March model may be unreliable unless their relationship is established in each case. Optical and back-scattered SEM studies revealed a change in microstructures in accordance with increasing fabric strength. In slates with weak fabric strength, cleavage domains are dark to opaque in appearance and characterized by a concentration of insoluble residues such as carbonaceous matter, sphene, ilmenite, rutile, epidote and apatite as well as phyllosilicates. Phyllosilicates in cleavage domains are oriented subparallel to cleavage, whereas those in intercleavage domains are variously oriented. As fabric strength increases, cleavage domains become transparent and composed mainly of phyllosilicates. Phyllosilicates become finer-grained and more uniform in grain size. They are well oriented in both cleavage and intercleavage domains. As fabric strength further increases, phyllosilicates tend to coarsen. These lines of microstructural evidence suggest that the dominant mechanism for phyllosilicate preferred orientation changes as cleavage develops from pressure solution transfer, through syntectonic crystallization—recrystallization, and possibly to oriented grain growth. Because fabric strength increases with increasing illite crystallinity, metamorphic grade (mainly temperature) must be one of the major factors which control the dominance of phyllosilicate preferred orientation mechanisms.