大和高原領家帯北縁部の地質

抄録

Geological and petrographical features of the district in question are generally presented and the northern boundary of the Ryoke metamorphic belt in the district is discussed. The original rocks of the metamorphics are mainly the Palaeozoic sediments and partly igneous rocks (diabase, gabbro et al.). The latter was intrude into the former before all granitic intrusions. The original Palaeozoic formations are chiefly composed of slate, sandstone and chert and divided into two formations: the upper and the lower apparently. The upper formation includes eruptive (basaltic and biotite andesitic) and calcareous rocks and is rich in chert at the uppermost horizon developed in the southern part of the district. Neoschwagerina (or Yabeina), Schwagerina sp. and Pseudodoliolina (?) are found in the limestone bed of the northern end of the district. In the northern part, the phenomena of the abnormal sedimentation is found frequently. The metamorphic formations, on the whole, form a long waveanticlinal structure trending from ESE-WNW to E-W. The metamorphic and granitic rocks are classified to two groups : the older and they ounger. The older metamorphic terrain, progressively from N to S, is divided into the zone of biotite-bearing slate, the zone of two mica phyllite, the zone of schistose hornfels and the zone of sillimanite gneiss. These were formed made in relation to the older granitic rocks. Generally speaking, the older granites are limited in the southern part of the district and concordant to the older metamorphics. The younger granites discordantly intrude to the older ones. As regard the geological structure and the lithofacies of the original rocks of the metamorphics and plutonic rocks, the southern part of the district is different from the northern part and their boundary is represented by the Kizugawa Tectonic Line formed at a certain age between the older and the younger plutonisms. The southern part is named the Ryoke zone proper and the northern part the Ryoke Northern Marginal zone.