Report
Physical properties of Neogene granitic rocks in Japan
2021 Volume 72 Issue 1 Pages 81-94
Details
2021 Volume 72 Issue 1 Pages 81-94
Petrophysical measurements such as density, porosity, magnetic susceptibility and Natural
Remanent Magnetization (NRM) of Jurassic, Cretaceous and Paleogene granitic rocks which constitute the Japanese Islands had been conducted to clarify their physical properties. Similar measurements have been conducted for Neogene granitic rocks in Japan this time. Neogene granitic rocks in Japan are distributed widely from the backbone mountains of Hokkaido, the northernmost big island to Yakushima and Ishigaki Islands in the southern island chains. However, most plutons are relatively small except the Kofu, Tanzawa and Yakushima Granites with some outcropping areas and it is not easy to collect samples from every small pluton. Therefore, samples of Neogene granitic rocks were collected mainly from the Kofu and Kai-Komagatake Granites, and the Chichibu and Tanzawa Granites in the outer zone of Southwest Japan and measured in this study. The total number of measurements amounted to 210.
The results of the measurements were classified and summarized as in six areas: 1) Echigo-Yuzawa, Wada Pass and Nasu-Dake Granites, 2) Chichibu and Tanzawa Granites, 3) Kofu and Kai-Komagatake Granites, 4) Cape Shiono and Cape Muroto Granites, 5) Southern Kyushu (Satsuma Peninsula and Osumi Peninsula) Granites and 6) Yaksushima Granites.
The mean density increases from the Southern Kyushu Granites (2.62 g/cm3=103 kg/m3) to the
Yakushima, Kofu and Kai-Komagatake, Echigo-Yuzawa, Wada Pass and Nasu-Dake, Chichibu and Tanzawa, Cape Shiono and Cape Muroto Granites (2.96 g/cm3) in this order. The mean porosity is almost inversely proportional to its mean density and ranges from 0.29 % to 1.94 %.
The mean magnetic susceptibility indicates 2 × 10-4 and 5 × 10-4 (SI) for the Southern Kyushu and Yakushima Granites, 5 × 10-3 for the Cape Shiono Granites, 10-2 for the Cape Muroto Granites, 2 × 10-2 and 3 × 10-3 for the Kofu and Kai-Komagatake Granites, 3 × 10-2 for the Echigo-Yuzawa, Wada Pass and Nasu-Dake Granites and 4 × 10-2 for the Chichibu and Tanzawa Granites.
On a basis of petrophysical properties and their lithology, granitic rocks of the Southern Kyushu and Yakushima Granites are defined as paramagnetic to weak magnetic. The Cape Shiono and Cape Muroto Granites are paramagnetic to weak magnetic and medium magnetic, respectively. Those of the Kofu and Kai-Komagatake Granites are high magnetic and medium magnetic, respectively. Both the Echigo-Yuzawa, Wada Pass and Nasu-Dake Granites, and the Chichibu and Tanzawa Granites indicate high magnetic. No relationship between the density and NRM is observed except the samples of the Tanzawa Granites. Königsberger ratio (Qn) of the rock samples shows less than 0.4 except the samples of the Yakushima Granites, and the Cape Shiono and Cape Muroto Granites.
These results are summarized in Table 1 (physical properties of Neogene granitic rocks in Japan).
