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

Abstract

The petrogenesis of regionally zoned granitic pegmatite veins remains debated. Because of the economic significance, we carried out a study on the Chakabeishan (CKBS) pegmatite-type Li-Be deposit, eastern North Qaidam Tectonic Belt, Northern Tibetan Plateau, by means of in-situ major element and B isotope compositions of tourmalines in the beryl-bearing and spodumene-bearing pegmatite veins. Tourmalines (Tur-Be) from the beryl-bearing pegmatite are homogeneous schorl with low Mg/(Mg + Fe), high Na/ (Na + Ca) and YAl, suggesting that they are of magmatic origin. Two generations of tourmalines (TurLi) from the spodumene-bearing pegmatite are identified: (i) the crystal cores (mostly elbaite and Lirich schorl with subordinate schorl) are consistent with being of magmatic origin crystallized at the magmatic stage; (ii) the crystal rims (schorl) are best understood as the overgrowth at the later hydrothermal stage. Tur-Be and Tur-Li show an obvious difference in core-to-rim B isotopic variation trend with d11B decrease in Tur-Be and increase in Tur-Li. The core-to-rim d11B decrease in Tur-Be results from degassing during its host pegmatitic melt evolution, whereas the core-to-rim d11B increase in Tur-Li is related to fluid exsolution. The estimated d11B values for the initial melts of the beryl-bearing and spodumenebearing pegmatites are -10.46% and -10.78%, respectively, indicating that they most likely originate from protracted fractional crystallization/differentiation of granitic intrusions rather than partial melting of metapelite. Both Mg/(Mg + Fe) ratios and Li abundances in the cores of Tur-Be are lower than those of Tur-Li, suggesting that Tur-Li crystallizes from chemically more evolved melts. (c) 2023 China University of Geosciences (Beijing) and Peking University. Published by Elsevier B.V. on behalf of China University of Geosciences (Beijing). This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).