start-ver=1.4
cd-journal=joma
no-vol=16
cd-vols=
no-issue=1
article-no=
start-page=9
end-page=28
dt-received=
dt-revised=
dt-accepted=
dt-pub-year=2009
dt-pub=20091225
dt-online=
en-article=
kn-article=
en-subject=
kn-subject=
en-title=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
kn-title=Sr–Nd isotopic compositions of Paleoproterozoic metavolcanic rocks from the southern Ashanti volcanic belt, Ghana
en-subtitle=
kn-subtitle=
en-abstract=
kn-abstract=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.
en-copyright=
kn-copyright=

en-aut-name=DampareSamuel
en-aut-sei=Dampare
en-aut-mei=Samuel
kn-aut-name=ダムパリサムエル
kn-aut-sei=ダムパリ
kn-aut-mei=サムエル
aut-affil-num=1
ORCID=

en-aut-name=ShibataTsugio
en-aut-sei=Shibata
en-aut-mei=Tsugio
kn-aut-name=柴田次夫
kn-aut-sei=柴田
kn-aut-mei=次夫
aut-affil-num=2
ORCID=

en-aut-name=AsieduDaniel
en-aut-sei=Asiedu
en-aut-mei=Daniel
kn-aut-name=アシードダニエル
kn-aut-sei=アシード
kn-aut-mei=ダニエル
aut-affil-num=3
ORCID=

en-aut-name=OkanoOsamu
en-aut-sei=Okano
en-aut-mei=Osamu
kn-aut-name=岡野修
kn-aut-sei=岡野
kn-aut-mei=修
aut-affil-num=4
ORCID=

en-aut-name=ManuJohnson
en-aut-sei=Manu
en-aut-mei=Johnson
kn-aut-name=マーヌジョンソン
kn-aut-sei=マーヌ
kn-aut-mei=ジョンソン
aut-affil-num=5
ORCID=

en-aut-name=SakyiPatrick
en-aut-sei=Sakyi
en-aut-mei=Patrick
kn-aut-name=サチパトリック
kn-aut-sei=サチ
kn-aut-mei=パトリック
aut-affil-num=6
ORCID=

affil-num=1
en-affil=
kn-affil=岡山大学

affil-num=2
en-affil=
kn-affil=岡山大学

affil-num=3
en-affil=
kn-affil=Department of Earth Science, University of Ghana

affil-num=4
en-affil=
kn-affil=岡山大学

affil-num=5
en-affil=
kn-affil=Department of Earth Science, University of Ghana

affil-num=6
en-affil=
kn-affil=Department of Earth Science, University of Ghana
en-keyword=Sr–Nd isotopes
kn-keyword=Sr–Nd isotopes
en-keyword=petrogenesis
kn-keyword=petrogenesis
en-keyword=tectonic setting
kn-keyword=tectonic setting
en-keyword=Birimian metavolcanics
kn-keyword=Birimian metavolcanics
en-keyword=Ashanti volcanic belt
kn-keyword=Ashanti volcanic belt
END