Sm-Nd isotope systematics of Indian shales constrain the growth of continental crust: Implication for supercontinent cycle and mantle plume activity

This study reports the Sr-Nd isotopic composition of Indian shale samples of Archean to Recent age from different litho-units of Archean cratons and Proterozoic basins. Combining our measured Sr-Nd isotopic ratios with literature data, we calculated the Nd model ages of these shales and estimated the growth rate of the Indian continental crust, assuming that the Nd model age of clastic sediments represents an average of existing continental crustal sources. Unlike the widely variable 87Sr/86Sr ratio (0.76 ± 0.12, 1 S.d) and 87Rb/86Sr (2.53 ± 1.09) in the Indian shales, the average present-day 143Nd/144Nd (0.51170 ± 0.00028; εNd(0) = −18.8 ± 5.4) and 147Sm/144Nd (0.111 ± 0.009) isotopic ratios of Indian shales (n = 54) estimated in this study are comparable to that of the average global clastic sediments (mostly shales), North American shales, and Ganga River sediments. However, the Nd isotopic compositions of these shales (εNd(0) ≈ −19) are slightly different from those of the Upper Continental Crust (UCC), loess, and average river/aeolian particulates (εNd(0) ≈ −10), despite having similar Sm/Nd ratios (0.17−0.19). The striking uniformity of Sm/Nd ratios and 143Nd/144Nd in contemporary UCC-derived erosional products like the Ganga River sediments suggests that the isotopic compositions of average Indian shale might be representative of Indian UCC. Using a dimensionless erosional parameter K, the growth of the Indian shield was modeled considering the effect of preferential erosion, which leads to less contribution from older shield areas. We thus develop a two-stage crustal growth model that assumes K = 1 for the Archean (no erosion) and K = 6 for the post-Archean to Recent. Our results suggest that ∼80% of the continental crust was formed by the end of Archean with the mean age of the continent of 2.42 Ga, which is different from the Australian shield. This study represents a first-ever estimate of the growth rate of the Indian shield that manifests a rapid growth in the Archean and a long-term, slow and progressive growth in the post-Archean comprising short periods of rapid crustal growth, which could be correlated with the supercontinent cycle and mantle plume activities forming large igneous provinces (LIPs).