Petrogenesis of granitic pegmatite veins: Perspectives from major element and B isotope in tourmalines, Chakabeishan, Northern Tibetan Plateau

•Tur-Be from the beryl-bearing pegmatite are magmatic in origin.•Tur-Li from the spodumene-bearing pegmatite are of both magmatic and hydrothermal origin.•Difference in core-to-rim B isotopic variation trend with δ11B decrease in Tur-Be and increase in Tur-Li.•δ11B decrease in Tur-Be results from degassing during its host pegmatitic melt evolution, whereas δ11B increase in Tur-Li related to fluid exsolution.•Pegmatites in CKBS deposit are products of protracted fractionation of granitic magmas.