Abstract
Macroscopic fractures developed in the Late Cretaceous Toki Granite in the Tono district, central Japan, were analyzed in the field and under the microscope. The fractures are characterized by their morphology (open or closed, brittle and/or plastic deformation) and filled with several kinds of materials. The fracture-filling materials are authigenic minerals (iron-oxide, sericite, quartz, chlorite and epidote) and/or pulverized grains derived from the host granite (quartz, feldspar, biotite, etc.). Sericite, quartz, chlorite and epidote are hydrothermal minerals related to the activity of granitic magma. Iron-oxide, however, is considered to have been formed in a later stage of fracture filling on the basis of the occurrence.All the fractures are primarily formed as open and/or shear fractures by brittle failure, although some of them also show microstructures caused by plastic deformation, such as dynamically recrystallized quartz. This type of fractures are interpreted to have originated from brittle fractures which were subsequently hydrolytically weakened in the presence of hydrothermal solution and deformed plastically.
