MONAZITE GEOCHRONOLOGY AND RARE EARTH ELEMENT VARIATIONS IN LEUCOGRANITES FROM SIKKIM, EASTERN HIMALAYAS

The Higher Himalayan Sequences (HHS) in North Sikkim incorporates leucogranite bodies, emplaced into migmatitic gneisses in the form of sills and dykes. The leucogranites are divided into two varieties: Two mica leucogranites (2mg) and Tourmaline leucogranites (Tg) both of which host zircon and monazite as accessory minerals. In this study, monazite grains, that occur as anhedral to subhedral in shape, were analyzed for U-Th-Pb isotopic ratios and rare earth elements (REE) abundances using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS). In 2mg, the monazite grains yielded ²⁰⁶Pb/²³⁸U intercept dates of 14.24, 14.85, and 15.44 Ma whilst in Tg, the monazite grains yielded ²⁰⁶Pb/²³⁸U intercept dates of 13.51, 13.99, and 14.63 Ma. The chondrite-normalized REE patterns of monazites for 2mg and Tg are strongly enriched in light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs) with marked negative Eu anomaly (Eu/Eu* of 0.01– 0.08), and (Eu/Eu* of 0.01– 0.05) respectively with little variation between the different aged regions of the grains. The negative Eu anomaly is interpreted as co-crystallization of monazite with feldspar during melt crystallization, where Eu preferentially partitions into feldspar. The monazite Eu anomaly is significantly influenced by changes in Eu valence induced by changes in oxygen fugacity (Holder et al., 2020). In comparison to 2mg, Tg has a wide range of thorium and comparatively restricted uranium contents. The monazite saturation temperatures (Montel, 1993) are 730-839°C and 690°-727°C for 2mg and Tg respectively, broadly similar to leucogranite temperature obtained across the Himalayan orogen. The U-Pb ages of monazite range from ~14-15 Ma, suggesting that leucogranite emplacement was coeval with the Middle Miocene movement along the South Tibetan Detachment System (STDS), and it has been suggested by Kellett et al. (2013) that the activity time along the STDS in the north of Sikkim ranges from 24-13 Ma.

Keywords: Monazite, Geochronology, Leucogranites, Sikkim