| JaLCDOI | 10.18926/ESR/66844 |
|---|---|
| Title Alternative | Petrological characteristics of the stone chamber of Tobiotsuka Kofun, Okayama Prefecture |
| FullText URL | esr_030_001_012.pdf |
| Author | KANEKO, Takahiro| NOZAKA, Toshio| SEIKE, Akira| |
| Abstract | Tobiotsuka Kofun, a tumulus built on the Misu Hills in the Kofun period, has a horizontal stone chamber made of huge stone blocks with a width up to 2 meters or more. To specify the source of the stone blocks, we carried out the measurement of magnetic susceptibility, petrographic observation, and chemical analysis of minerals. The stones are amphibole-biotite granite with phenocrystic large grains of K-feldspar. The back-wall stone of the chamber has higher magnetic susceptibility than ceiling and side-wall stones, which probably results from a higher amount of magnetite formed by the alteration of biotite in the back-wall stone. Furthermore, the back-wall stone is different from ceiling stone in that it has lower XMg [Mg/(Mg + Fe) mole ratio], lower Al, Ti, and Na + K contents and higher Si contents of amphibole, higher XMg of biotite, and shows a tendency to have higher Na (albite component) contents at rims of plagioclase crystals and lower Ti contents of zircon. These characteristics of the back-wall stone are similar to those of granite exposed in the Koshinzan area about 2 km northeast of Tobiotsuka Kofun, whereas the ceiling and side-wall stones are similar to granite outcrops in the vicinity of Tobiotsuka Kofun, e.g., in the Midoriyama area. It is concluded that the quarry for the back-wall was located at a different place from that for the ceiling and side-wall stones. |
| Keywords | Tobiotsuka Kofun stone chamber granite |
| Publication Title | Okayama University Earth Science Report |
| Published Date | 2024-03-31 |
| Volume | volume30 |
| Issue | issue1 |
| Start Page | 1 |
| End Page | 12 |
| ISSN | 1340-7414 |
| language | Japanese |
| Copyright Holders | © 2024 by Okayama University Earth Science Reports Editorial Committee All Rights Reserved |
| File Version | publisher |
| JaLCDOI | 10.18926/ESR/42457 |
|---|---|
| Title Alternative | 西南日本の熱変成超苦鉄質岩体中のかんらん石と輝石の組成変化に関する覚書 |
| FullText URL | esr_017_001_005.pdf |
| Author | Nozaka, Toshio| |
| Abstract | This short article presents some diagrams showing the compositional variations of primary and metamorphic olivine, orthopyroxene and clinopyroxene in peridotites and serpentinites from thermally metamorphosed ultramafic complexes in SW Japan. In contrast to olivine, which shows a gradual change of chemical composition corresponding with metamorphic grade, orthopyroxene and clinopyroxene show clear differences in composition between primary and metamorphic phases. Compared with primary pyroxenes, even though their compositions could be variable depending on original rock composition, metamorphic orthopyroxene and metamorphic clinopyroxene is clearly deficient in Cr(2)O(3) and CaO, and in Cr(2)O(3) and Al(2)O(3), respectively. These characteristics are useful for the discrimination between the pyroxenes of different origin. |
| Keywords | chemical composition olivine orthopyroxene clinopyroxene metaperidotite |
| Publication Title | Okayama University Earth Science Report |
| Published Date | 2010-12-24 |
| Volume | volume17 |
| Issue | issue1 |
| Start Page | 1 |
| End Page | 5 |
| ISSN | 1340-7414 |
| language | English |
| Copyright Holders | © 2010 by Okayama University Earth Science Reports Editorial Committee All Rights Reserved |
| File Version | publisher |
| NAID | 120002753315 |
| JaLCDOI | 10.18926/ESR/13952 |
|---|---|
| FullText URL | ESR_1_1.pdf |
| Author | Nozaka, Toshio| Shibata, Tsugio| |
| Abstract | Ultramafic rocks exposed around Mt. Ohsa(= Ohsa-yama), Okayama prefecture, designated as "Ohsa-yama ultramafic body" all together, are one of the Alpine-type peridotites in the Sangun metamorphic belt. They are intensely serpentinized and locally suffered contact metamorphism by younger granitic intrusions. In a por-tion of the ohsa-yama body where it has been affected by the contact metamorphism, the constituent minerals, texture and structure of primary ultramafic rocks have been locally preserved. Petrographic studies revead that the primary ultramafic rocks of the ohsa-yama body consist domimantly of dunite and harzburgite possessing no obvious layering, and their constituent minerals are similar in composition to those of the Tari-Misaka and Ashidachi ultramafic bodies. These features indicate that unlike the Ochiai-Hokubo body, the Ohsa-yama ultra-mafic body belongs to the "massive group" of the Arai's (1980) classification. |
| Keywords | petrography dunite harzburgite massive ultramafic body |
| Publication Title | Okayama University Earth Science Report |
| Published Date | 1994-09-20 |
| Volume | volume1 |
| Issue | issue1 |
| Start Page | 1 |
| End Page | 8 |
| ISSN | 1340-7414 |
| language | English |
| Copyright Holders | 岡山大学地球科学研究報告編集委員会 |
| File Version | publisher |
| NAID | 120002310594 |
| JaLCDOI | 10.18926/ESR/13938 |
|---|---|
| FullText URL | ESR_2_1.pdf |
| Author | Nozaka, Toshio| Shibata, Tsugio| |
| Abstract | The Ohsa-yama ultramafic body, which consists of several types of serpentinized peridotites, underwent contact metamorphism caused by a Cretaceous granite intrusion ; this metamorphism resulted in the formation of contact aureole, 1.5-2.0 km wide, around the granite intrusion, and produced progressive mineral changes in metaserpentinites toward the contact between the Ohsa-yama body and the granite intrusion. On the basis of analysis of mineral paragenetic relations, the Ohsa-yama ultramafic body can be divided into three zones with progressive changes in mineral assemblages as follows : Zone Ⅰ : serpentine ± chlorite ± brucite Zone Ⅱ : olivine + talc ± tremolite ± chlorite Zone Ⅲ : olivine + orthopyroxene ± tremolite ± spinel Zone Ⅰ corresponds to the parts unaffected by the thermal event, and Zones Ⅱ and Ⅲ correspond to the thermally metamorphosed parts of the Ohsa-yama body. The results obtained in this study are generally consistent with those of the previous studies on metamorphic peridotites from the Sangun and Muzuru zones. |
| Keywords | peridotite serpentinite thermal metamorphism contact metamorphism |
| Publication Title | Okayama University Earth Science Report |
| Published Date | 1995-09-20 |
| Volume | volume2 |
| Issue | issue1 |
| Start Page | 1 |
| End Page | 12 |
| ISSN | 1340-7414 |
| language | English |
| Copyright Holders | 岡山大学地球科学研究報告編集委員会 |
| File Version | publisher |
| NAID | 120002310611 |
| JaLCDOI | 10.18926/ESR/13917 |
|---|---|
| Title Alternative | Ferroglaucophane schist from the Ohsa-yama srea, Okayama Prefecture, Japan |
| FullText URL | ESR_4_33.pdf |
| Author | Nozaka, Toshio| |
| Abstract | A ferroglaucophane schist occurs in fault contact with serpentinites at the Ohsa-yama area, Southwest Japan. It consists of albite, ferroglaucophane and actinolite with small amounts of stilpnomelane, sphene, apatite and K-feldspar. Petrological studies reveal that the schist was originally igneous rock of felsic or intermediate composition and has suffered two stages of metamorphism : ferroglaucophane has been formed at the first stage and actinolite at the second stage. In a basic schist from the same outcrop that the ferroglaucophane schist occurs, tremolitic amphibole instead of alkali amphibole has been formed along with chlorite, phengite and albite. Such a difference in mineral association between the schists is ascribed to a difference in whole rock composition, particu-larly in Fe/Mg ratio. Some of the schists from th Ohsa-yama area characteristically contain glaucophane or ferroglaucophane and have low Fe2O3/FeO ratios, forming a striking contrast to the crossite schists that commonly occur in the so-called "Sangun metamorphic terrane". These facts suggest that the alkali amphiboles of the Ohsa-yama schists were formed under higher P/T and more reducing conditions than those of the regional metamrphic rocks. On the other hand, development of the second-stage actinolite in the ferroglaucophane schist is consistent with the meta-morphic parageneses of greenschists from adjacent areas. Consequently the ferroglaucophane schist is considered one of the tectonic blocks that were captured by mobile serpentinites and have suffered the regional metamor-phism after the emplacement of the serpentinites into the present geologic position. |
| Keywords | ferroglaucophane petrology tectonic block serpentinite high P/T metamorphism, Ohsa-yama Sangun metamorphic terrane Southwest Japan |
| Publication Title | Okayama University Earth Science Report |
| Published Date | 1997-09-20 |
| Volume | volume4 |
| Issue | issue1 |
| Start Page | 33 |
| End Page | 40 |
| ISSN | 1340-7414 |
| language | Japanese |
| Copyright Holders | 岡山大学地球科学研究報告編集委員会 |
| File Version | publisher |
| NAID | 120002310538 |
