Toshio Kutsukake

The garnet-bearing and cummingtonite-bearing hornblende-biotite quartz diorites occur as local lithologies of the Mitsuhashi Granite pluton in the Ryoke Belt, southwest Japan. High Fe/Mg ratio of the magma and also a low oxygen-fugacity favor the crystallization of garnet, which rarely coexists with the hornblende in granitoid rocks.

The Ryoke plutonism took place at the eastern margin of the Eurasian Continent in Cretaceous, before the opening of the Japan Sea. It belongs to the Cretaceous to Paleogene felsic magmatism, extensively occurred in the Inner Zone of the Southwest Japan, which constitutes a segment of the long magmatic belt ranging from Sikhote-Alin, through the southeastern Korea to Fujian, China. The older Ryoke granitic rocks represent a magmatic arc, related to the subduction of the oceanic plate underneath the Eurasian plate in that time. The younger and post-Ryoke granites, however, are not typical volcanic arc magmatism.

Cretaceous to Paleogene magmatlsm in the Chubu district is divided into five stages, based on the geological relations, petrographic characters and radiometric ages of the plutonic and volcanic rocks. The 1st stage ranges from 120 to 90 Ma in age. The Older Ryoke granitic plutonism occurred in the southern part of the Ryoke belt, almost contemporaneous with regional metamorphism. Main types of the plutonic rocks are tonalite and granodiorite free from magnetite, and distinguished by peraluminous character. Minor bodies of plutonic and volcanic rocks are also found in both the Mino and Hida belts. The ages of the 2 nd stage are 90 to 65 Ma. The distribution of this stage rocks occupy more than a half of the Cretaceous to Paleogene igneous rocks. Almost all of the Nohi Rhyolite and the Younger Ryoke granites belong to this stage. The component rocks are granite, granodiorite and tonalite in the plutonics, and rhyolite and rhyodacite with minor amount of dacite in the volcanics. They are generally lower in MgO and MgO/FeO ratio than the average of the Japanese granites. As for the opaque oxides, ilmenite alone is contained in small amount. The 3 rd stage ranges from 65 to 45 Ma. This stage magmatism is restricted within the northern and central part. Two different types, magnetite-free in the central part and magnetite-bearing in the northern part, are discerned. The former type show nearly the same chemical and petrographic characters as the component rocks of the 2nd stage. The latter is similar in chemical composition to the average Japanese granitic rocks, with the exception of the volcanic rocks of high K/Na ratio and high normative corundum. Of these rocks, the most predominant is felsic rocks. The 4 th stage ranges from 45 to 35 Ma. The present distribution of this stage rocks is confined to comparatively small areas. Mafic rocks predominate over granitic or rhyolitic ones. They are characterized by containing of abundant magnetites and high magnetic susceptibility. The 5 th stage rocks are found only in the Northern Japan Alps area. They consist of rhyolite and granophyre, characterized by the absence of potash feldspar in phenocryst. They unconformably overlie the rocks of the 4 th stage, however their precise ages have not yet been determined. The magmatism tends to become smaller in extension with descent of time and its southern limit (magmatic front) in each of the stages migrates northward in general. We recognized that the following three petrographic provinces exist in the Chubu district based on spatial and temporal distribution and petrological characters of the igneous rocks, the Tenryu-Mikawa, Nohi and Shirakawa-Northern Japan Alps. The Tenryu-Mikawa province is considered to have been formed during the 1st stage, while the formation of the Nohi and Shirakawa-Northern Japan Alps provinces was made in the 2 nd to 3 rd, and 3 rd to 5 th stases. resoectively.