Rb-Sr Chronology and Petrochemistry of Granitoids from the South-Eastern Part of the Singhbhum Craton, Orissa

The tonalitic suite of Kaptipada is high-Al₂O₃ type with a highly fractionated REE pattern suggesting its derivation by partial melting of amphibolite with hornblende as a residual phase. The homblende/biotite-bearing granites of Nilgiri and Ramapahari areas also occurring along the southeastern fringe,are comparatively K-rich than the lonalite-granodiorite suite of Kaptipada. Based on whole rock Rb-Sr isotopic analyses, lhe Kaptipada tonalite-granodiorite suite yields an age of 3275 ± 81 Ma with initial Sr ratio of O.70130± 0.00045; ... the Nilgiri granite, an age of 2366 ± 126Ma with initial Sr ratio of 0.72526 ± 0.00832 and Ramapahari granite, an age of 1895 ± 46 Ma wilh initial Sr ratio of 0.76521 ± 0.00427.

The temporal evolution of the tonalitic rocks of Kaptipada is indistinguishable from the earlier phases of Singhbhum and Bonai granitoids of the Singhbhum Craton. It is concluded that an extensive growth of tonalitic crust took place around 3.3 Ga in the region.