Nature of the Ryoke Regional Metamorphism and Plutonism

Abstract

Regional metamorphism and the older granitic intrusions in the Ryôke zone took place in a term between late Jurassic to early Cretaceous. On the other hand the younger granitic intrusions did in late Cretaceous. The Ryôke regional metamorphism and plutonism occurred not in accompaniement with the subsidence of the Honshû geosyncline but in rather non-subsidence region. They do not belong to the so-called axial plutono-metamorphic zone of the orogenic belt. The metamorphism is characterized by both low solid- and low water-vapour pressures, and has a nature very similar to contact metamorphism around granite pluton. The older granitic intrusions bear the intimate connection with the metamorphism and acted on as main heat-source of the latter. They have distinct geochemical and mineralogical characters from other Cretaceous granitic rocks in the inner zone of southwest Japan. This feature is resulted from the fact that they took activities in the Ryôke zone, a special zone. It is still to be elucidated that the colossal heat-energy, sufficent to high-temperature metamorphism and formation of vast granitic magmas, is concentrated to the long narrow Ryôke zone. It must be a phenomenon to be explained in regard to the geological developments of the Japanese Islands since the Honshû orogeny to the late-Mesozoic acid igneous activities. It becomes to be generally accepted that the Ryôke regional metamorphism and plutonism belong to the late-Mesozoic acid igneous activities as a whole. However, the mutual relations to the Honshû orogeny have also to be paid attention to.