日本地球惑星科学連合2015年大会

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インターナショナルセッション(ポスター発表)

セッション記号 S (固体地球科学) » S-MP 岩石学・鉱物学

[S-MP09] Supercontinents and Crustal Evolution

2015年5月27日(水) 18:15 〜 19:30 コンベンションホール (2F)

コンビーナ:*Madhusoodhan Satish-Kumar(Department of Geology, Faculty of Science, Niigata University)、Yasuhito Osanai(Division of Evolution of Earth Environments, Faculty of Social and Cultural Studies, Kyushu University)、Geoffrey H. Grantham(Council for Geoscience, P/Bag X112, Pretoria, SOUTH AFRICA)、Sajeev Krishnan(Centre for Earth Sciences, Indian Institute of Science, Bangalore 560 012, INDIA)、Tomokazu Hokada(National Institute of Polar Research)

18:15 〜 19:30

[SMP09-P05] 南インドダールワールクラトンチトラドルガシストベルトにおける太古代の縞状鉄鉱層を用いた地球化学的研究

*鯉沼 健太郎1サティッシュクマール マドスーダン2三島 郁3上野 雄一郎3外田 智千4 (1.新潟大学自然科学研究科、2.新潟大学理学部、3.東京工業大学地球惑星科学専攻、4.国立極地研究所)

Banded Iron Formations (BIFs) are successive layers of fine grade quartz and iron minerals which consist mainly of hematite, magnetite, and siderite. They are chemically precipitated in the sea and formed mostly in the Archaean and early Paleoproterozoic, and therefore record the information of the ancient oceans. It is believed that the iron source was supplied hydrothermal vents (Bekker et al., 2010), whereas silica was either sourced from hydrothermal vents (Steinhofel et al., 2010) or continental weathering (Hamade et al., 2003). In addition to the origin of BIF, the oxidation-reduction state of the seawater can be constrained by the characteristics of trace element, rare earth element and isotope geochemistry. We have studied the geochemical characteristics, in particular the Nd isotopes of BIFs in the Chitradurga schist belt, Dharwar craton, Southern India.
The Chitradurga Schist Belt belongs to the Dharwar Supergroup that overlies the basement Peninsular Gneiss (~3.0 Ga) with enclaves of Surgur group (3.3~3.1 Ga). The Dharwar Supergroup is subdivided into two groups, the Bababudan Group and the Chitradurga group. Hokada et al., (2013) suggested that the oldest depositional age of Bababudan Group and lower unit of Chitradurga group is around 3.14 Ga and 3.22?2.92 Ga and the youngest depositional age of upper unit of Chitradurga group is between 2.68 Ga and 2.63 Ga. In the lower Chitradurga unit is metamorphosed to the biotite-muscovite grade whereas the upper unit is chlorite-muscovite grade. Three major BIF layers occur in the Chitradurga Schist Belt, which belongs to the Bababudan Group, lower Chitradurga unit and upper Chitradurga unit. We compare the geochemical features of these three layers in this presentation.
The Chitradurga BIFs are mostly composed of quartz, magnetite and hematite and rarely contain siderite, pyrite, and carbonate minerals. Their compositions are almost SiO2 and Fe2O3 and include few each other composition. The following data is the lower Chitradurga unit BIF. The majority BIFs in this unit show low REE contents, LREEReferences
Bau and Moller., 1993 Geochimica et Cosmochimica Acta 57, 2239-2249
Bekker et al., 2010 Economic Geology 105, 467-508.
Hokada et al., 2013 Precambrian Research 227, 99-119
Hamade et al., 2003 Geology 31, 35-38
Steinhofel et al., 2010 Geochimica et Cosmochimica Acta 74, 2677-2696