Deformation path of amygdules in basic schists in Itoshima area, northern Kyushu, Japan

A deformation path of amygdules in basic schists that underwent contact metamorphism in Itoshima area, northern Kyushu, Japan, has been revealed using shape and internal microstructure of the amygdules. The amygdules are filled mainly with plagioclase that was recrystallized during the contact metamorphism. Crystal size of the plagioclase decreases with increasing aspect ratio of the amygdules. Some amygdules with low-aspect ratio contain plagioclase showing flat twin boundaries. This suggests that such amygdules were undeformed after peak of the contact metamorphism and preserved the strain accumulated until then, which probably includes prograde stage of the contact metamorphism and former regional Renge metamorphism. In contrast, the amygdules with high-aspect ratio underwent superimposition of deformation taking place after peak of the contact metamorphism. The strain analysis shows the constriction types for the former amygdules while the plane-strain types for the latter. This reveals a deformation path of the amygdules such that the constrictional strain before the peak of contact metamorphism was partially overprinted by later flattening strain, showing the apparent plane-strain types.