3:30 PM - 3:45 PM
[SMP14-07] Geological evolution of the Archean Chitradurga schist belt, Dharwar Craton, southern India
Geochemical characteristics of volcanic rocks and BIFs helped us to delineate the tectonic setting of the oceanic basin in which they have deposited. The major and trace element compositions of the samples from three units can be grouped into 2 types. The first type is characterized by flat REE pattern and spider diagram. The second group of rocks have enriched compositions of LIL, LREE and slightly depleted HREE than the first type. In addition, Nd isotope ratio is also different, the first group have near zero to positive Nd values compared to negative values for the second group. The geochemical and isotopic variations observed between the two types of volcanic rocks were cause by the difference in source magma genesis due to a difference in tectonic settings. The first type is related to a possible upwelling mantle plume. On the other hand, the second type can be related to an arc setting possibly associated with subduction.
Geochemical characteristics of chitradurga BIFs indicate that deposition was controlled by hydrothermal flux, however the epsilon Nd(T) values fluctuate from negative to positive values. The majority of epsilon Nd(T) values show only small variation between -1 and +2. The depositional environment can be modeled by a mixing between seawater with positive epsilon Nd values and hydrothermal flux derived from enriched mantle with negative epsilon Nd values, and the variations we observe depends on the hydrothermal flux from enriched mantle. This result is consistent with the REY characteristics, large positive Eu anomaly and low Y/Ho value, and suggest a deep sea hydrothermally controlled depositional environment. However, the BIFs associated with dolomite, have high Y/Ho ratio, positive Eu anomaly and broad range of Nd isotope ratio, which suggests that they were deposited in a shallow sea environment.
In addition, stromatolitic and massive carbonate rocks in the lower unit of Bababudan formation show large variation in carbon, oxygen, sulfur and strontium isotopic composition. Multiple sulfur isotope studies of pyrite in carbonate rocks from Bababudan Group show very large variation of sulfur isotopic composition, upto +19.4 per mil with negative cap delta 33S, whereas other sedimentary rocks show near 0 permil value. Based on the above results, we also discuss the changes observed in the atmospheric oxygen contents before the GOE.
Reference: Hokada, T. et al. 2013, Precambrian Research, 227, 99-119.