2007 Volume 113 Issue 4 Pages 146-157
Slaty cleavage and chlorite-mica stacks in the Tertiary Taisyu Group, Tsushima Island, northwest of Kyusyu, have been studied by XRD, SEM and petrographic techniques. The intensity of cleavage development is classified into 3 classes; 0-, I- and II-classes. In the 0-class, cleavage can not be observed. The I- class is characterized by non-penetrative cleavage domains and pencil structure, and II- class by penetrative cleavage domains. Chlorite-mica stacks have generally tabular form with a long axis nearly parallel to bedding plane in the 0-class and show deformational features such as folding and kinking especially in the II- class. The thickness of the stacks increases from the 0-class to II- class: 9.4 μm in the 0-class; 12.7 μm in the I- class; 21.5 μm in the II- class. These data indicate that the shape change of the stacks accomplished by the chlorite and mica intergrowth during strain-controlled deformation and formation of slaty cleavage. Illite crystallinity values obtained from the typical samples of each class is 0.39-0.43Δ° 2θ in the 0-class; 0.36-0.40Δ° 2θ in the I- class; 0.35Δ° 2θ in the II- class, corresponding to diagenetic zone to lower anchizone.
Considering about the extensional setting of the Japan Sea, temperature of lower anchizone and thickness of the Taisyu Group, it is inferred that the geothermal gradient was relatively higher than the moderate one at the formation of slaty cleavage.
