Oxygen Isotopic composition of Higher Himalayan Leucogranites from the Sikkim Himalaya, India

The Higher Himalayas in Sikkim consist of two-mica leucogranites (2mg), tourmaline leucogranites (Tg), and pegmatites. The leucogranites in North Sikkim intrude the Higher Himalayan Sequences (HHS). In this study, we present the first systematic dataset of whole-rock oxygen isotopic compositions for Higher Himalayan leucogranites from Sikkim, providing insights into their magmatic sources and evolution. Oxygen isotope measurement was accomplished using bulk fluorination and isotope ratio mass-spectrometry, and the oxygen isotope ratios (δ¹⁸O) were measured relative to Vienna Standard Mean Ocean Water (VSMOW). The analyses were calibrated against international standards NBS-28 (quartz). The two-mica leucogranites (7 samples) are characterized by biotite and muscovite, exhibit a mean δ¹⁸O_W.R value of 9.6 ± 1.7‰, whilst tourmaline leucogranites (3 samples), characterized by the presence of tourmaline, yield a mean δ¹⁸O_W.R value of 11.6 ± 3.9‰. The variations in δ¹⁸O values possibly reflect the originally distinct δ¹⁸O signatures of the source sediments, which were moderated by diffusive exchange during diagenesis and metamorphism (France-Lanord et al., 1988). The higher δ¹⁸O values observed in leucogranite samples may be attributed to the pelite-rich sediments, and the lower δ¹⁸O values can result from metagreywacke source or due to the presence of epidotized calc-silicates.