[S-MP09] Supercontinents and Crustal Evolution

[SMP09-P05] Nd isotope geochemistry of Archaean BIFs in the Chitradurga Schist Belt, Dharwar Craton, Southern India

*Kentaro KOINUMA¹, Madhusoodhan SATISH-KUMAR², Kaoru MISHIMA³, Yuichiro UENO³, Tomokazu HOKADA⁴ (1.Graduate School of Science and Technology, Niigata University, Niigata, Japan, 2.Department of Geology, Faculty of Science, Niigata University, Niigata, Japan, 3.Department of Earth & Planetary Sciences, Tokyo Institute of Technology, Tokyo, Japan, 4.National Institute of Polar Research, Tachikawa, Tokyo, Japan)

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 was supplied by 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, western 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 Sargur 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. 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. Bulk rock geochemistry results revealed that the BIFs contain only very less amount of Al2O3 or other oxides than SiO2 and Fe2O3. The majority of lower Chitradurga unit BIFs have low REE contents, LREEReferences
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