Eastern Indian 3800-Million-Year-Old Crust and Early Mantle Differentiation

Samarium-neodymium data for nine granitic and tonalite gneisses occurring as remnants within the Singhbhum granite batholith in eastern India define an isochron of age 3775 $\pm $ 89 $\times $ 10$^{6}$ years with an initial $^{143}$Nd/$^{144}$Nd ratio of 0.50798 $\pm $ 0.00007. This age contrasts with the rubidium-strontium age of 3200 $\times $ 10$^{6}$ years for the same suite of rocks. On the basis of the new samarium-neodynium data, field data, and petrologic data, a scheme of evolution is proposed for the Archean crust in eastern India. The isotopic data provide evidence that parts of the earth's mantle were already differentiated with respect to the chondritic samarium-neodymium ratio 3800 $\times $ 10$^{6}$ years ago.