Geological and petrological studies on the Tanzawa mountainland

Abstract

The Tanzawa mountainland consists mainly of a large amount of marine pyroclastics of the Miocene age. The central part of this area is occupied by a quartz diorite mass accompanied with various kinds of metamorphics around and within it. The present paper gives the general geology of the Tanzawa mountainland and its geological meaning in the Japanese islands. The metamorphics and the plutonics of the Tanzawa mountainland compose a plutonic zone, and the non-metamorphosed rocks of Neogene Misaka series are tectonically discordant with this plutonic zone. The quartz diorite mass including gabbroic parts is genetically related to some metamorphics. The metamorphics consist of green schist-amphibolite facies in the western contact with the plutonic mass, and hornblende hornfelsbiotite chlorite hornfels fades at the eastern contact. And both show an increasing of metamorphic grade towards the pultonic mass. Some high grade metamorphics and the other metamorphics are distinguished each other in their field occurrence and the petrographic observations. The low grade metamorphics and non-metamorphosed rocks are often separated by large faults and/or basic intrusions. It is pointed out that this mountainland covers not only a part of the Fossa Magna region and the Green Tuff area, but also the Shimanto geosynclinal area of the Mesozoic age. In comparison with ultrabasic and acidic plutonic rocks of the post Mesozoic age, this quartz diorite mass probably belongs to the basic intrusive zone with E-W direction of post Mesozoic igneous activities.