Petrogenesis and evolution of internally zoned pegmatites: A tourmaline perspective from the Dakalasu deposit, Chinese Altai

Abstract

The role of fractional crystallization and partial melting on pegmatite is controversial. To constrain the evolution and petrogenesis of internally zoned pegmatites, we selected the Dakalasu No. 1 pegmatite dike, Chinese Altai, as a case, and used tourmaline as a probe. Tourmalines were developed along the boundary between pegmatite and the host biotite granite porphyry (Tur B_Ⅰ), at the border zone Ⅱ (Tur B_Ⅱ) and further at the intermediate zone (Tur IM), but absent in the central zone of the pegmatite. All the tourmalines belong to alkaline schorl series. Tur B_Ⅰ exhibits elevated concentrations of Mg (3.42–4.63 %) and V (104.2–263.4 ppm), coupled with low Al content at Y site, indicative of interaction between pegmatite and the host biotite granite. Tur B_Ⅱ and Tur IM have high ratios of Fe/Mg (5.00–6.70), but low Mg/(Mg + Fe) (0.13–0.18), suggestive of a magmatic origin. The contents of rare metals (Li, Be, Nb, Ta, and Sn) of tourmaline increase inward, whereas the contents of Mg, Ca, Sr, Ni, V, Cr and the ratio of Mg/(Mg + Fe) decrease, suggesting a sequential evolution from the border to the intermediate zone. All tourmalines contain low content of Li, indicating a limited fractional crystallization. Boron isotopes (δ¹¹B = −15.84 ‰ – −14.55 ‰) in tourmaline indicate that the pegmatite dike originates from the low-degree partial melting of metasedimentary rocks, rather than fractional differentiation of the host granite. We propose that tourmaline in zoned pegmatites can be employed as a pathfinder to explore the petrogenesis and evolution of pegmatite.