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

<p>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 &#949;Nd_&#160;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.</p> <p><strong>Keywords: Sr-Nd isotopes, Source magma, Sikkim Himalayas, Leucogranites</strong></p>