JaLCDOI 10.18926/ESR/19849
タイトル(別表記) ガーナ,アシャンティ火山帯南部に分布する古原生代変火山岩類のSr-Nd 同位体組成
フルテキストURL 016_009_028.pdf
著者 ダムパリ サムエル| 柴田 次夫| アシード ダニエル| 岡野 修| マーヌ ジョンソン| サチ パトリック|
抄録 Neodymium (Nd) and strontium (Sr) isotopic data are presented for Paleoproterozoic metavolcanic rocks in the southern part of the Ashanti volcanic belt of Ghana. The metavolcanic rocks are predominantly basalts/basaltic andesites and andesites with minor dacites. Two types of basalts/basaltic andesites (B/A), Type I and Type II, have been identified. The Type I B/A are stratigraphically overlain by the Type II B/A, followed by the andesites and the dacites. The analyzed volcanic rocks commonly have low initial (87)Sr/(86)Sr ratios consistent with previous studies on Paleoproterozoic rocks from the West African craton. The LREE-depleted, tholeiitic Type I B/A exhibit back-arc basin geochemical signatures and show high positive epsilon Nd (i.e., ε(Nd) (2.1 Ga) = +3.89 to +7.21), which suggest a long term depleted source and also indicate that they were produced in an entirely oceanic environment devoid of influence of continental crust. The isotope signatures are thus consistent with the previously published trace element data of the Type I basalts/basaltic andesites in suggesting that their parent magma was generated from a depleted mantle. The Type I B/A have Nd model ages (T(DM2)) of 1.83–2.09 Ga similar to their formation ages, suggesting that they were juvenile at their time of formation. The andesites and the Type II B/A andesites show LREE-enriched patterns and exhibit characteristics of subduction zone-related magmas, and show initial ε(Nd) (2.1Ga) values of –1.15 to + 1.35 and Nd model ages (T(DM2)) of 2.32–2.58 Ga. The LREE-enriched dacitic porphyry also exhibits characteristics of subduction zone-related magmas, and have initial ε(Nd) (2.1Ga) value of –2.24 and Nd model ages (T(DM2)) of 2.64 Ga. The Nd isotopic data confirms the juvenile character of the Birimian crust, but also suggests some contributions of a pre-Birimian crustal material (or Archean?) in the genesis of some of the metavolcanic rocks. Our isotopic result is consistent with the island arc complex model which views Paleoproteozoic terranes of West Africa in the context of subduction–accretion processes.
キーワード Sr–Nd isotopes petrogenesis tectonic setting Birimian metavolcanics Ashanti volcanic belt
出版物タイトル Okayama University Earth Science Report
発行日 2009-12-25
16巻
1号
開始ページ 9
終了ページ 28
ISSN 1340-7414
言語 English
著作権者 岡山大学地球科学研究報告編集委員会
論文のバージョン publisher
NAID 120002308985
JaLCDOI 10.18926/ESR/19850
タイトル(別表記) 岡山県に分布する白亜紀前期羽山層砕屑物質の供給源:砕屑粒の鉱物化学組成とモード組成から導かれる制約
フルテキストURL 016_029_042.pdf
著者 アシード ダニエル| 鈴木 茂之| 柴田 次夫|
抄録 Petrographic and phase chemistry studies of detrital grains were carried out on sandstones from the Lower Cretaceous Hayama Formation, Inner Zone of Southwest Japan, to determine their provenance and the tectonic setting during the early Cretaceous. The results of the modal mineralogy suggest that the Hayama Formation has magmatic arc provenance and that deposition of the sediments took place in the back-arc areas with detritus mostly derived from the magmatic arc and rifted continental margins. The chemical compositions of chromian spinel, chlorite and sphene indicate that significant proportions of the detrital grains were derived from mafic and/or ultramafic sources. The source areas are the mafic and ophiolitic rocks in the Sangun-Renge and Akiyoshi terranes and the felsic volcanic rocks probably from either the Akiyoshi terrane or a source not presently exposed in southwest Japan. However, minor amounts of the detritus were derived from the basement rocks; i.e., carbonates and siliciclastic rocks of the Akiyoshi terrane and the metamorphosed mafic rocks of the Chizu terrane.
キーワード modal analysis sandstone mineral chemistry provenance Hayama Formation
出版物タイトル Okayama University Earth Science Report
発行日 2009-12-25
16巻
1号
開始ページ 29
終了ページ 42
ISSN 1340-7414
言語 English
著作権者 岡山大学地球科学研究報告編集委員会
論文のバージョン publisher
NAID 120002309000
JaLCDOI 10.18926/ESR/13853
タイトル(別表記) ガーナ,南部アシャンティ帯の原生代プリンスタウン・グラニトイド岩体に関する主成分地球化学
フルテキストURL ESR_12_15.pdf
著者 Samuel Dampare| 柴田 次夫| Daniel Asiedu| Shiloh Osae|
抄録 The Pateoproterozoic metavolcanic rocks of the southern Ashanti greenstone belt of Ghana are intruded by three major suites of granitoids, locally called Prince's Town, Dixcove and Ketan plutons. The Prince's Town pluton is the largest intrusive body in the Axim area, and tends to separate the Axim volcanic branch from the Cape Three Points branch. The Pluton consists of granitic to dioritic rocks, which are generally massive but occasionally display alignment of ferromagnesian minerals. The rocks contain mainly plagioclase, K-feldspar, quartz, amphibole, biotite and opaques. The feldspars are mostly sericitized and saussuritized, and alteration of amphibole and biotite to epidote and chlorite is common. Accessory minerals include apatite, sphene and zircon. The geochemical data indicate that the rocks are tonalitic to granodioritic in composoition, metaluminous (ASI<1) and have I-type characteristics. The granitoids have the SiO2 content of 63-70% ; total iron, as Fe2 O3 of 3.10-5.80% ; (Na2O+K2O) content of 5.01-6.96% and Na2O/K2O ratios from 1.34 to 2.70 ; and are characterized by Mg# ranging from 53 to 48. The Fe*(=FeOtot/FeOtot+MgO) and modified alkali-lime index (MALI) of the rocks indicate that the Prince's Town pluton is dominantly magnesian and calcic in nature. Higher values in molar CaO/(MgO+FeOtot) coupled with low molar AI2O3/(MgO+FeOtot) may suggest their derivation from partial melting of metabasaltic to metatonalitic sourcc, with a possible contribution from metagreywacke, but preclude any contribution from metapelitic sources. The Birimaian metavolcanic rocks are the likely source material candidate for the rocks. CIPW norm calculations yielded a crystallization temperature of~650-685℃ and a pressure of 4-7kb for the rocks, suggesting a lower crustal souree. The Prince's Town plutonic rocks also show characteristice of plutons emplaced in a volcanic are tectonic setting environment. This observation is largely consistent with previous studies conducted on granitoids from other parts of the southern Ashanti greenstone belt c and the belt-type granitoids of Ghana as a whole.
キーワード Geochemistry tectonic setting granitoids Birimian Ghana
出版物タイトル Okayama University Earth Science Report
発行日 2005-12-31
12巻
1号
開始ページ 15
終了ページ 30
ISSN 1340-7414
言語 Japanese
著作権者 岡山大学地球科学研究報告編集委員会
論文のバージョン publisher
NAID 120002310649
JaLCDOI 10.18926/ESR/13851
タイトル(別表記) ガーナ, ビルム・ダイヤモンド産地のアクァティア地域に分布する沖積層の重鉱物分析
フルテキストURL ESR_12_7.pdf
著者 Samuel Dampare| Daniel Asiedu| Bruce Banoeng-yakubo| 柴田 次夫|
抄録 Heavy mineral analysis was carried out on Pliocene to Recent alluvial sediments from the Birim diamondiferous field od Ghana. The main objective of the study was to examine the mineral composition of heavy fractions in order to identify : (1) the heavy mineral assenblage that occur in the sediments, (2) particular diamond indicator minerals associated with the diamonds, and (3) the provenance of the alluvial sediments. The heavy minerals are essentially composed of staurolite, ilmenite and magnetite in varying proportions, with trace amounts of leucoxene, rutile, garnet and zircon. The heavy mineral assenblage and chemical composition of ilmenite and gamet suggest their derivation from phyllites and schists which reflect directly the composition of the basement rocks developed in the study area. The absence of diamond indicatior minerals such as apatite, pyrope garmet, chromian spinel, and picroilmenite in the heavy fraction is unlikely to be due to their destruction during intense weathering and/or diagenesis but rarher their non-occurrence in the area.
キーワード Heavy minerals provenance sediments diamonds Ghana
出版物タイトル Okayama University Earth Science Report
発行日 2005-12-31
12巻
1号
開始ページ 7
終了ページ 14
ISSN 1340-7414
言語 English
著作権者 岡山大学地球科学研究報告編集委員会
論文のバージョン publisher
NAID 120002310657
著者 山本 悦世| 岩﨑 志保| 髙田 浩司| 宇田川 武久| 能城 修一| 柴田 次夫|
発行日 2003-03-31
出版物タイトル 岡山大学構内遺跡発掘調査報告
16巻
資料タイプ 研究報告書
著者 Daniel K. Asiedu| 鈴木 茂之| 柴田 次夫|
発行日 1996-09-20
出版物タイトル Okayama University Earth Science Report
3巻
1号
資料タイプ 紀要論文
著者 野坂 俊夫| 柴田 次夫|
発行日 1995-09-20
出版物タイトル Okayama University Earth Science Report
2巻
1号
資料タイプ 紀要論文
著者 野坂 俊夫| 柴田 次夫|
発行日 1994-09-20
出版物タイトル Okayama University Earth Science Report
1巻
1号
資料タイプ 紀要論文
著者 李 青一| 柴田 次夫| 光野 千春|
発行日 1984-03-25
出版物タイトル 岡山大学温泉研究所報告
54巻
資料タイプ 紀要論文