Abstract
The distribution of granitic rocks occupies 12 percent of the Japanese Islands, but the physical properties of the granitic rocks such as density, porosity, natural remanent magnetization (NRM), and ultrasonic velocity have only been reported for small limited areas with an exception of magnetic susceptibility for the regional areas. The aim of this study is to clarify the physical properties of granitic rocks in the Japanese Islands by the systematic measurements under the same precision and to try to discuss the characteristics of each exposure area and the differentiation process of magma. Samples were collected at about 570 outcrops from four areas (Kitakami Mountains, Taiheizan,Kurikoma-Narugo and Murakami areas) in the northern Tohoku region. All the samples were measured on density, magnetic susceptibility and NRM. The results are summarized as follows: 1.Density The mean density of these areas varies from 2.66 to 2.77(g/cm3 =103 kg/m3), the lowest one is Murakami area, the second Kurikoma-Narugo area, the third Taiheizan area and the highest is Kitakami Mountains. 2.Porosity There is no large difference between the four areas. The variation range is about 0.4～0.5(%). 3.Magnetic susceptibility All the samples can be sectioned within a zone bounded by two straight lines, in a density magnetic susceptibility chart: The samples in these four areas lie in each characteristic zone. Magnetic susceptibilities near along the lower line are subject to the content of paramagnetic rock-forming iron minerals in granitic rocks. 4.Natural remanent magnetization (NRM) The relation between the density and NRM shows a pattern similar to that between density and magnetic susceptibility but less correlative. Königsberger ratio (Qn) of 70 percent or more granitic rocks shows less than 0.4 in this region. Consequently, NRM is negligible for a case of analysing magnetic anomaly in the field.
