山口県ほうべん山花こう岩体の岩石記載・全岩化学組成及び帯磁率

Abstract

Petrographic character, magnetic susceptibility and whole rock chemistry are examined on the Hobenzan granitic complex, located in Yamaguchi Prefecture, Southwest Japan. The granitic complex intrudes along the Saigatao Tectonic Line which divides the Sangun metamorphic rocks from “non-metamorphosed” Paleozoic formation. Graphite in the pelitic schist within about 200meters from the complex disappeared because of thermal metamorphism. The granitic complex, belonging to the calc-alkaline rock series, consists of tonalite, biotite-hornblende granodiorite, hornblende-biotite granite and biotite granite. Rocks of the complex are higher in K2O and lower in FeO and Na2O than the average Japanese granitoids (Aramaki et al., 1972). Normative Q-Or-Ab system shows that the complex has intruded into the high level of a crust (0.5kb±), and this is in harmony with the petrographic characters of the complex. Magnetic susceptibility, ranging from 5 to 557×10-6emu/g, displays lower values in rocks of a southern marginal part of the complex. The magnetic susceptibility, Fe-Ti oxide and sulfide mineral assemblages and TiO2-FeO-Fe2O3 relation of rocks all indicate that rocks of the southern marginal part were formed under lower fO2 conditions. The granodioritic magma was partly reduced by graphite from the pelitic schists, and a part of ferric iron in the rocks near the Sangun metamorphic rocks changed into ferrous iron.