Origin of the Pebbles of Plutonic and Metamorphic Rocks in the Neogene Ogawamachi Group in the Kanto Mountains

Abstract

The basal part (Gotanda conglomerate) of the Ogawamachi Group (Middle Miocene) comprises large pebbles, sometimes large boulders, of pultonic and metamorphic rocks, from which the following rock-types are discriminated under the microscope: granite, granodiorite, banded quartz diorite, diorite, amphibole schist, cordierite-biotite gneiss and biotite quartzo-feldspathic gneiss. Of these pebbles,cordierite-biotite gneiss is safely identified with the gneiss of the cordierite zone of the Ryoke belt and granitic rocks are mostly similar to some Ryoke granites. Moreover, TAKEI and KOIKE (1977) have found a small outcrop of the gneiss to the east of Ogawa-machi. From these facts, it is certain that the Ryoke belt was widely exposed to the north of the Kanto mountains before the Middle Miocene. In the northern Kanto mountains, the pelitic metamorphic pebbles in the Cretaceous system are exclusively biotite-schist. On the contrary, they are cordirite-biotite gneiss in the Middle Miocene series. Accordingly, the Ryoke belt in this area would continue to be raised at the time from the Cretaceous to the Middle Miocene. Then, the belt was changed into a subsiding zone during a time of the Miocene, and was largely covered by the Miocene sediments.