ABSTRACT

U-Pb dating of zircons in a pelitic gneiss from the Oki metamorphic rocks gives an upper intercept age of 1.96 ± 0.04 Ga on the concordia diagram and an average Pb-Pb age of 1.96 ± 0.04 Ga. Rb-Sr biotite ages vary with crystal size and indicate that large biotite crystals are zoned in age. The relation between ages and crystal sizes reveals two different stages of crystallization. The age of the first stage was estimated to be about 180 Ma, which represents the time of the main phase of metamorphism. The second stage was estimated to have occurred at 161 ± 2 Ma, which represents the final phase of metamorphism. The U-Pb and Pb-Pb zircon ages represent the time of prominent plutonic activity in the provenance from which the clastic precursor of the pelitic gneiss was derived.