Geochemistry, depositional environment and tectonic setting of the BIF's of the Late Archaean Chitradurga Schist Belt, India

Abstract

Banded iron formations (BIF's) of the Chitradurga Schist Belt (CSB) occur in association with (1) arenite-stromatolitic carbonate-argillite, (2) volcanic-volcaniclastic and (3) greywacke-argillite volcanic suites. Cherty oxide-, carbonate- and sulphide-facies BIF's are exposed within 15 km from west to east. In addition, shales (Sh) and shaly banded iron formation (SBIF) occur with all the three types. Cherty carbonate-, oxide- and sulphide-facies BIF's show close similarities in their majorand trace-element constituents except in their CO 2, S and Ce contents. Pure cherty BIF's are depleted in ΣREE, Zr, Hf, Cr, Ni, Co, Nb, Ta, V, Sc and Y, and are generally enriched in La and have strong positive Eu anomalies with respect to SBIF. Many samples of cherty oxide- and carbonate-facies BIF's exhibit negative Ce anomalies. However, sulphide-facies BIF's are devoid of negative Ce anomalies, and indicate that in this case oxidation of Ce 3+ to Ce 4+ has not taken place. Simultaneous increase of LREE-HREE, HfZr, CrZr, RbSr, VSc, NbTa, NdFe, and several other diagnostic pairs, suggest that the variation in the BIF composition is governed by the type and quantity of the clastic component. Available data indicate that FeO and SiO 2 from the BIF's were mainly provided by the hydrothermal waters added at the vent sites of the Archaean Mid-Oceanic Ridges (AMOR) or at some unidentified off-the-shelf volcanically active environment. As a consequence of thermal and chemical potential differences and upwelling, the FeO- and SiO 2-enriched ocean water was transported to the shallow-shelf environment, where it reacted with photosynthetic O 2 to precipitate Fe-oxides and -hydroxides. Upon burial in shallow shelf regions, the oxides and hydroxides locally reacted with the organic carbon to produce Fe-carbonates. Primary carbonates were also precipitated in the basin along with other types of BIF's. In the deeper part of the ocean, Fe-sulphides were precipitated in a reducing environment near vent sites. Terrigenous and volcaniclastic debris were delivered to the basin during transgression and volcanic activity. It is therefore proposed that the compositional characteristics of the BIF's from the CSB are dependent on the availability of dissolved CO 2, O 2, particulate carbon and clastic input in the different parts of the basin. Facies changes are related to the compositional layering of the ocean and its transgressioe-regressive cycles. A mini plate model with increased ridge-trench length, shelf area and hydrothermal water flux of higher exit temperature combined with the advanced stages of biological activity explains the genesis of BIF's found in greenstone belts.