中国地方新生代玄武岩類の岩石学的並びに岩石化学的研究

Petrographical and Petrochemical Studies of the Cenozoic Basaltic Rocks in Chûgoku Province

鷹村 權

廣島大學地學研究報告

Geological report of the Hiroshima University

Basaltic rocks in Chûgoku Province can be grouped into the following nine districts; namely, 1) Hyôgo-Tottori Pref. district (mainly lava flows), 2) Okayama Pref. district (lava flows and domes), 3) Yokota district (mainly lava flows and partly domes), 4) Shimane Pref. district (almost lava flows), 5) Kibi plateau district (domes), 6) Hiba mountains district (mainly lava flows), 7) Sera plateau district (mainly domes), 8) Kammuri plateau district (lava flows), and 9) Yamaguchi Pref. district (mainly lava flows). Based on the geological studies and data of palaeomagnetism, it is revealed that the basaltic rocks of Miocene to Pliocene are distributed in Tottori Pref., the boundary district between Okayama and Tottori Prefs. and the Izumo-Iwami, district, and those of Late-Pliocene in Yakuno, Hamada, and Shinichi, and that other basaltic rocks belong to the Pleistocene.
According to the petrographical properties, basaltic rocks of respective districts show the following characters: 1) alkali series mainly in Hyôgo Pref. and calc-alkali series mainly in Tottori Pref., 2) calc-alkali series in the Seto Inland-Sea district and alkali series mainly in the Tsuyama basin and the Pref. boundary district, 3) alkali series, 4) almost alkali series except the calc-alkali series in the Omori formation, 5) alkali series, 6) alkali series, 7)
alkali series except the calc-alkali series in a part of the western area, 8) both alkali and calc-alkali series, and 9) alkali series mainly.
Xenoliths and xenocrysts in the basaltic rocks show distinct relationship to the rock series of host rocks. In the basalts of calc-alkali series, granitic xenoliths are general, while in those of alkali series are included olivine nodules, clinopyroxene nodules, kaersutite crystals, peridotite, pyroxenite, gabbro and anorthosite.
The author has analysed 294 samples of basalts and made such diagrams as FMA, Or-Ab-An, (K2O/Na2O)-SiO2, MgO/(Fe2O3+FeO)-SiO2, (Na2O+K2O)-Al2O3, etc. He has studied the degree of SiO2 saturation, alkali-calc index and SI. Based on these petro-chemical considerations, it is concluded that the character of the alkali series is not uniform throughout the Province, but that it becomes stronger from the east to the west, and that, in the direction N-S, it is most remarkable in the Chûgoku peneplain region. Specifically in the Tsuyama basin, the Kibi and the Sera plateaus, about half of the basalt samples is basanitic basalt undersaturated with respect to SiO2, which decreases down to 40-42 percent.
Basaltic rocks belonging to the alkali series in Chûgoku Province show the same characteristics as those in the Circum-Japan-Sea petrographic province.

日本語

廣島大學理學部地學教室

[ISSN] 0073-2303

[NCID] AN00213676