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[SGC46-10] Geochemical evolution of Late Archean volcanism in the Western Dharwar Craton, Southern India
Keywords:Geochemistry, Isotope, Archean, Plate techtonics
Unit A is characterized by flat REE pattern and spider diagram, whereas samples from unit B have enriched compositions of LILE, LREE and slightly depleted HREE than the first type. Unit C is similar to Unit A in most chemical signature, except for a spike in Pb. Nd, and Hf isotope ratios were also different for the three units, units A and C have positive εNd values in contrast to negative values for the unit B. Further Unit A has more depleted value than C. Pb isotope ratios are also different, however, in contrast to other isotope ratios, the 207Pb / 204Pb ratios of unit A are less enriched.
These geochemical characteristics are considered to reflect the differences in mantle source and tectonics setting. The units A and C have characteristics related to a possible upwelling mantle plume. Further, unit A has a higher 207Pb / 204Pb ratio than Unit C, it may be affected by recycling of crustal material. On the other hand, the unit B volcanism can be related to a volcanic arc setting accompanied by oceanic plate subduction. AFC modelling precludes the contamination of TTG crust for the observed enrichments. Therefore, the extremely enriched nature of unit B magma indicates the possibility that the mantle was originally enriched in addition to the input from subducting slab. Furthermore, geochemical discrimination diagrams suggest that units A, B and C have formed under different tectonic settings. The differences in tectonics setting are also complemented by Nd – Hf - Pb isotope data. In the WDC, the earliest komatiite greenstones were reported to have erupted at 3.2 Ga. The unit A in our study has also similar geochemical characteristics and a common source mantle is suggested. After the deposition of BIF unit B volcanic activity in a juvenile arc setting occurred. Lastly, volcanic rocks of the unit C erupted from a super plume which was not much affected by crustal recycling.
In summary, the geochemical characteristics shown by the volcanic rocks represented a drastic and sudden change in the tectonic setting, and represent a clear evidence for subduction and arc magmatism as early as Meso-Archaean.