Strontium-Neodymium isotopic compositions of Higher Himalayan leucogranites and granitic gneisses from Sikkim Himalayas, India

The Himalayan leucogranites are important for deciphering the role of crustal thickening and anatexis during the tectono-thermal evolution of the Himalayas. The Higher Himalayan Sequences (HHS) in Sikkim are intruded by Miocene leucogranites which comprise two-mica-bearing (2mg) and tourmaline-bearing leucogranites (Tg) both of which are peraluminous in nature, and are characterized by high ⁸⁷Sr/⁸⁶Sr ratios and low ¹⁴³Nd/¹⁴⁴Nd ratios, typical of crustal values. In the case of 2mg, the average (⁸⁷Sr/⁸⁶Sr) ratio is 0.792020, and the average (¹⁴³Nd/¹⁴⁴Nd) ratio is 0.511764 whilst for Tg, the average (⁸⁷Sr/⁸⁶Sr) and (¹⁴³Nd/¹⁴⁴Nd) ratios are 0.772874 and 0.511912 respectively. In comparison to 2mg, Tg has lower average ratios of (⁸⁷Sr/⁸⁶Sr) and slightly elevated average ratios of (¹⁴³Nd/¹⁴⁴Nd). The Sr-Nd isotope compositions for the granitic gneisses from the HHS have an average (⁸⁷Sr/⁸⁶Sr) ratio of 0.764026 and an average (¹⁴³Nd/¹⁴⁴Nd) ratio of 0. 511959. The present day average εNd  values for 2mg, Tg, and granite gneiss are -17.1, -14.2, and -13.2 respectively. The granite gneiss samples all lie within the field defined by the HHS across the Himalayan orogen as do the Tg samples suggesting a source from pelitic rocks within the HHS. The 2mg samples are somewhat evolved towards more radiogenic Sr isotope ratios and slightly lower neodymium ratios suggesting either a source in metagreywacke rocks of the HHC or, alternatively, reflect minor melt contributions from anatexis of the Lesser Himalayan formations.

Keywords: Sr-Nd isotopes, Source magma, Sikkim Himalayas, Leucogranites