変成岩岩石学の現状と将来への展望 アルプスと西南日本 二つの代表的造山帯の年代学的特徴

Abstract

There are two types of orogenic belts : the continent-continent collision type such as Alps and the subduction related (Cordilleran) type of southwestern Japan. Their metamorphic rocks have contrasting geochronological characteristics. Alpine metamorphic rocks have recorded three major metamorphic event : 1) the Cretaceous ultra-high and related high P/ T metamorphism in continental material such as the Sesia-Lanzo and the Pennitic Internal Crystalline Massifs, 2) the Eocene high P/T metamorphism in the ophiolites and calc-schists of the Mesozoic Tethys and 3) medium P/T metamorphism, in which both types of high P/ T metamorphic rocks were variably reset by Oligocene thermal events. However, almost all geochronological data reported from the Alpine metamorphic belt show mixed ages which are due to the mixture of minerals formed in the three metamorphic events, requiring a petrological and chronological re-scrutiny by both expert petrologist and geochronologist with a conscientious strategy. On the contrary, the southwest Japan has several metamorphic belts in which the rocks record generally mono-metamorphism, suggesting a simple history in comparison with Alpine rocks. However, recent study has revealed a complex history of the low P/ T metamorphic belt : the low P/ T metamorphism took place in the pre-dating accretionary complexes which have suffered the subduction related metamorphism or in the geologic unit consisting of the predating high grade metamorphic rocks. This gave us the discordant age relation similar to that in Alps. Thus, orogenic belts are so heterogeneous in geochronology, indicating that the belts have recorded the complicated history of earth crust. Conversely, the advanced geochronology and petrology will make it possible to peruse the complicated history of earth crust.