18:15 〜 19:30
[SGL42-P08] 南インドダールワールクラトン中の縞状鉄鉱層を用いた地球化学的研究
キーワード：縞状鉄鉱層, ダールワールクラトン, 太古台, Nd同位体
Banded iron formations (BIF) are marine chemical sediment rocks precipitated mostly in Archaean and early Paleoproterozoic between 2.7Ga and 2.3Ga. This time interval record profound changes in the redox state of the oceans and atmosphere, such as the Great Oxidation Event (GOE). We present here the geochemical data obtained from 3.0 Ga banded iron formation (BIF) in the Chitradurga Schist Belt, Dharwar Craton, Southern India. This region exposes the Archaean strata predominated by supracrustal greenstone belts, stratigraphically overlying the Peninsular gneiss. Chitradurga schist belt comprises of three important BIF layers distributed in the Bababudan and Chitradurga groups. We present here the salient geochemical characteristics and strontium and neodymium isotope results of the BIFs and discuss the depositional environment.
BIF contain very low content of Al2O3 (<1wt.% except 1 sample) indicating less detrital components. The PAAS-normalized REY patterns shows positive La and Eu anomaly, low concentration of rare earth element, depletion of light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs). These features differ with other Archaean BIFs in terms of lack of positive Y anomaly. The large positive Eu anomalies in Archean BIF of Chitradurga schist belt attribute to high-T hydrothermal fluid fluxes (>250⁰C), while the negative Ce anomaly reflects the lack of significant oxidizing agents.
Sr isotopic composition of BIF shows large variations suggesting post depositional alterations, whereas Nd isotope rations gave consistent information. Most of the samples show εNd(3000Ma) in the range of +2 to +4 and TDM model age in accordance with sedimentation age. The εNd(3000Ma) of depleted mantle is about +4, which suggests that most of the Chitradurga BIFs were deposited in an environment strongly affected by input from a depleted mantle. However samples with different REY pattern show higher εNd(3000Ma) between +6 and +14 and their TDM model age are not equal to the sedimentation age. The geochemical results thus suggest that the BIFs in the Chitradurga schist belt were deposited near possible ridges affected by hydrothermal activities.
BIF contain very low content of Al2O3 (<1wt.% except 1 sample) indicating less detrital components. The PAAS-normalized REY patterns shows positive La and Eu anomaly, low concentration of rare earth element, depletion of light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs). These features differ with other Archaean BIFs in terms of lack of positive Y anomaly. The large positive Eu anomalies in Archean BIF of Chitradurga schist belt attribute to high-T hydrothermal fluid fluxes (>250⁰C), while the negative Ce anomaly reflects the lack of significant oxidizing agents.
Sr isotopic composition of BIF shows large variations suggesting post depositional alterations, whereas Nd isotope rations gave consistent information. Most of the samples show εNd(3000Ma) in the range of +2 to +4 and TDM model age in accordance with sedimentation age. The εNd(3000Ma) of depleted mantle is about +4, which suggests that most of the Chitradurga BIFs were deposited in an environment strongly affected by input from a depleted mantle. However samples with different REY pattern show higher εNd(3000Ma) between +6 and +14 and their TDM model age are not equal to the sedimentation age. The geochemical results thus suggest that the BIFs in the Chitradurga schist belt were deposited near possible ridges affected by hydrothermal activities.