紀伊半島西部の白亜紀四万十帯付加コンプレックスのFT年代

Zircon fission track ages were measured for sandstones from the Hanazono, Yukawa, Miyama and Ryujin complexes of the Cretaceous Shimanto belt in the western part of the Kii region. The obtained FT age and frequency distribution show various patterns for each complex, probably due to a variety of thermal affect during accretion and exhumation process. Individual zircon grains from the Yukawa complex (YK-2) retain FT ages older than the depositional age, probably reflecting their provenance ages. This indicates that the Yukawa complex has not been heated above zircon partial annealing zone (PAZ) after deposition. The sample from the Hanazono complex (HZ-8) shows a peak of age slightly younger than the depositional period with small number of grains retaining older ages, reflecting age resetting caused by thermal affect during accretion process. This type of age spectrum is similar to those of samples from the Miyama (MY-24) and the Ryujin complex (RJ-7). FT age frequency distribution of the Ryujin complex showing bimodal, suggesting a variety of degree of age resetting and heating up to temperature around upper limit of partial annealing zone (PAZ) of FT in zircon.