熊野酸性岩類北岩体の花崗斑岩と溶結凝灰岩および基盤岩との接触部における物性の特徴

抄録

The Kumano Acidic Rocks is a large-scale igneous complex which consists of rhyolite lava, welded tuff, granite porphyry and granite, and the complex was rapidly cooled in Middle Miocene. The granite porphyry occurs as a ring (arcuate) dike and a sub-horizontal sheet intrusion. The ring dike distributing on the north of Owase, intrudes into the Cretaceous sedimentary rocks. The sheet intrusion distributing between Owase and Kumano, intrudes into the welded tuff. To investigate the relation between solidifying condition of igneous rock and the physical rock properties, dry and wet densities, porosity, ultrasonic velocity, magnetic susceptibility and natural remanent magnetization have been measured on 21 samples from the outcrop of the northern body of the Kumano Acidic Rocks and 55 samples from the Ichiura drilling core. The granite porphyry of the ring dike shows higher density, lower porosity, and higher ultrasonic velocity than those of the sheet intrusion and it has thicker chilled margin than granite porphyry of the sheet intrusion. These indicate that the ring dike experienced stronger cooling than the sheet intrusion. In the Ichiura core, the deeper welded tuff shows the lower porosity, the higher density, the higher ultrasonic velocity and the higher natural remanent magnetization, indicating welding compaction. At the granite porphyry and the welded tuff contact zone, density, porosity, ultrasonic velocity and natural remanent magnetization show systematic trends with distance from the contact. These trends are probably associated with the chilled margin of the granite porphyry and the recrystallization of the welded tuff. Ultrasonic velocities of the welded tuff and the granite porphyry decrease with increasing porosity. Although welded tuff and granite porphyry cannot be distinguishable by single physical property because of their overlapped variation range, the dependence of ultrasonic velocity on porosity for the welded tuff is different from that of the granite porphyry and hence the ultrasonic velocity-porosity relationship can be used to identify rock type.