Geochronology of weakly metamorphosed Jurassic accretionary complex (the Kuga Group) in eastern Yamaguchi Prefecture, southwest Japan

Weakly metamorphosed Jurassic accretionary complex called the Kuga Group in eastern Yamaguchi Prefecture is composed mainly of olistostromal complex with matrices of mudstone The northern part of this group is separated from the southern part by a north-dipping fault. Reconstructed primary stratigraphy of these rocks is identical to that of modern trench sediments. Although the age of matrices of mudstone has not been fixed, it is considered that the Northern Unit formed as an accretionary complex probably at early Middle Jurassic subduction zone. The rocks in the Northern Unit have undergone a high pressure type metamorphism of the pumpellyite-actinolite facies. K-Ar ages of recrystallized white micas from 18 phyllitic pelites in the Northern Unit concentrate in the range of 170-150 Ma (middle Middle to early Late Jurassic). -from English summary