The Journal of the Geological Society of Japan

Geology of the Hayami Volcanic Area : with Special Reference to the History of the Volcanic Activities

Taro KASAMA

1953 Volume 59 Issue 692 Pages 161-172
Published: May 25, 1953
Released on J-STAGE: April 11, 2008

DOIhttps://doi.org/10.5575/geosoc.59.161

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The Hayami volcahic area is situated in the eastern central Kyusyu, comprising such volcanoes as Yuhu, Turumi, etc., and the famous Beppu Hot Springs., Geologically, this area is covered by various sorts of volcanic rocks in association with the Cenozoic sedimentaries, unconformably overlying on the basement rocks., The basement rocks consist of granites and mica-schists of so-called Ryoke Metamorphics, and are exposed only along the valley at the foot of the mountains., A features of the granite are shown in Table 1., The Cenozoic sedimentaries are divisible into the following two groups by a distinct unconformity : the lower is the Nozuharu group with the "Older Volcanics, " and the upper is the Oita group with the "Younger Volcanics., " Stratigraphical details are shown in Table 2 & 3., To elucidate an outline of the geologic history of the volcanic activities, discussions are attended to clarify the stratigraphical relation of the lavas and pyroclastic rocks to the sedimentary units at first., The following succession of volcanic activities were obtained: 1., The activities of the Mukainoharu volcanic-group (pyroxene-andesite, dacite, etc., ) in the Nozuharu group (Mio-Pliocene)., 2., A period of the non-volcanic activity., 3., The activities of the Ozika-Kanagoe volcanics-group (pyroxene-andesite, plagioliparite, etc., ) in the Oita group (Plio-Pleistocene)., 4., The activities of the Yuhu-Turumi volcanics-group (hornblende-andesite, pyroxene-andesite, etc., ) followed from the Ozika-Kanagoe volcanics-group., Stratigraphical setting and the petrological features of these volcanic units show that the Hayami is not simple but composite volcanic area., In other words, here the Mio-Pliocene volcanic zone "Setouti" is crossed by the Pleistocene volcanic zone "San'in" (Daisen).,

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Geotectonics of the Pacific Concerning the Japanese Island : I., The Fossa Magna, the Shichito and the Ogasawara Salients

Shingo EHARA

1953 Volume 59 Issue 692 Pages 173-200
Published: May 25, 1953
Released on J-STAGE: April 11, 2008

DOIhttps://doi.org/10.5575/geosoc.59.173

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On the Gneisses and Migmatites of the Central Hidaka Metamorphic Zone

Koshiro KIZAKI

1953 Volume 59 Issue 692 Pages 203-215
Published: May 25, 1953
Released on J-STAGE: April 11, 2008

DOIhttps://doi.org/10.5575/geosoc.59.203

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Various kinds of Metamorphic rocks which are regarded as the product of the Hidaka Orogeny, constitute a prominent extended zone along the Hidaka mountain range., Around the upper course of the River Satsunai which is in the central part of the zone, argillaceous metamorphic rocks are widely developed., Their many rock facies are distinguishable and each facies is zonally arranged with N-S trend., In the argillaceous metamorphic rocks of this area, the writer has descriminated from east to west across the metamorphic zone, five different zones, viz., 1., hornfels zone, 2., banded gneiss zone, 3., coarse granitic migmatite zone, 4., breccia like quartz dioritic migmatite zone and 5., Plagioclase porphyroblast biotite gneiss zone., In this paper their chief petrographical characteristics are described and some considerations about their chemical compositions are also offered., As to the chemical compositions of these gneisses, the following features are conspicuous : 1) In the banded gneisses, K₂O, Al₂O₃ contents are greater than in the rocks of any other facies., 2) Biotite-gneiss containing plagioclase porphyroblast has the highest Na₂O/K₂O, and contrasts with the banded gneiss which has high K₂O and low Na₂O contents., 3) K₂O is comparatively more abundant also in the coarse granitic migmatite, this abundance may be due to the presence of much orthoclase in the coarse granitic migmatite., 4) In the breccia like quartz dioritic migmatite (agmatitic migmatite), the containing hornfels is more basic than is the hornfels of the hornfels zone., From the petrographical view-point, the rock forming biotites were studied., Their chemical compositions are varied corresponding to the differences in the bulk composition of the host rocks., Their relation is given in the Al₂O -FeO -MgO Diagram., (Fig., 13)

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