Inclusion‐trail geometry of albite porphyroblasts in a fold structure in the Sambagawa Belt, central Shikoku, Japan

Abstract

The orientation of straight inclusion trails within albite porphyroblasts from basic schists has been measured around a north‐closure fold, in the Besshi district of the Sambagawa Belt, central Shikoku, Japan. The porphyroblasts are aligned with their longest dimension parallel to both the subhorizontal, east–west‐directed mineral lineation and to the fold axis. There is a systematic variation in inclusion‐trail geometry between the upper (northern) and lower (southern) fold limbs. The shear sense deduced from quartz c‐axis fabrics is top‐to‐the‐west in the upper limb and top‐to‐the‐east in the lower limb. Based on observed variations in porphyroblast inclusion trails, the structural history can be modelled as follows: (i) shear flow caused east–west stretching and folding of the metamorphic zonation; (ii) east–west ductile shear resulted in opposing senses of shear in the upper and lower limbs as the eclogite body situated in the core of the fold was extruded to the east.