Sr, Pb and Nd isotope studies and their bearing on the petrogenesis of the Jalor and Siwana complexes, Rajasthan, India

The Jalor and Siwana complexes belong to the Malani igneous suite, Rajasthan, India and are characterised by within - plate, A - type granites associated with coeval felsic and mafic volcanism. Sr, Pb and Nd isotopic compostions of the two complexes have been determined. Sr isotopic studies reveal an isochron age of 725±7 Ma for the felsic rocks of the Jalor Complex with an ,nitial ⁸⁷Sr/⁸⁶Sr ratio of 0.7062±0.0020. Three highly differentiated granites from Siwana plot on the same near-isochron, indicating that the complexes are coeval. The Pb and Nd isotopic ratios range between (²⁰⁶Pb/²⁰⁴Pb= 18.52-21.28., ²⁰⁷Pb/²⁰⁴Pb = 15.58-15.98, ²⁰⁸Pb/²⁰⁴Pb/²⁰⁴ Pb= 38.53.-40.8) and (¹⁴³Nd/ ¹⁴⁴Nd=0.5122-0.5127, ¹⁴⁷Sm/¹⁴⁴Nd=0.1260-0.157) respectively. Pb and Nd isotopic compositions of the Siwana samples show that the magma is mantle derived; for the Jalor complex the combined Sr, Nd and Pb data indicate primary mantle derivation with a variable degree of crustal contamination, with the assimilated crust being most likely of Archaean age.The petrogenesis of the two complexes is interpreted as an effect of fractionation of a primary mantle derived magma, simultaneous with assimilation of rocks from the lower crust. The data further document the presence of Archaean crust in the area.