Elemental and boron isotopic variations of tourmalines from the Miocene leucogranite-pegmatite in Kuju, eastern Himalaya: Implications for the evolution of magmatic melts

The high-differentiated leucogranites widely distributed in the Himalayas are closely related to rare metal mineralization. The elements and boron isotopes of tourmaline in magmatic rocks can effectively trace magma evolution, which is significant for rare metal migration and enrichment. This study focuses on the Miocene leucogranite and pegmatite in Kuju (Kuqu), eastern Himalaya. It presents a comprehensive analysis of the geological characteristics and monazite U(-Th)-Pb chronology to determine the temporal and spatial evolution relationship between muscovite granite and pegmatite. Subsequently, this paper uses electron microprobe and LA(MC) -ICP-MS to investigate the elemental geochemical and boron isotope characteristics of agglomerated allotriomorphic tourmaline (Tur-GI) and veined self-shaped tourmaline (Tur-GII) in muscovite granite, as well as agglomerated self-shaped tourmaline (Tur-P) in pegmatite. The study aims to determine the origin of tourmaline and its implications for rare metal mineralization. The muscovite granite and pegmatite have similar Pb-208/(232) Th weighted average ages of 16. 8Ma and 15. 8Ma, respectively. The pegmatite is the product of the highly differentiated evolution of muscovite granite. The Fe/ (Mg + Fe) and Na/ (Na + Ca) ratios of the three types of tourmalines vary from 0.64 to 0.91 and 0.93 to 0. 96, respectively, with compositions similar to black tourmaline. The black tourmaline with alkaline groups found in the leucogranite and pegmatite in Kuju is crystallized directly from the magmatic melt. The trace element content is mainly controlled by the composition of the magmatic melt and the contamination of the local surrounding rock contamination. The boron isotope composition of tourmaline ranges from -13. 7% to 7. 6%, with higher delta B-11 values observed for later crystallization series. A correlation exists between the delta B-11 value of tourmaline and other geochemical ratios or trace element contents, which further reflects the continuous evolution process of magma. Single magmatic crystallization differentiation can promote the rare metal (e. g., Li) enrichment to some extent.