Origin of the Azubai Be-mineralized pegmatites, Xinjiang Altay: constraints from Li–Hf isotopes

Abstract The genetic mechanism of rare-metal pegmatites – whether they form primarily by extreme fractionation of granitic magmas or direct crustal anatexis – remains highly controversial. The Azubai Be deposit in the Xinjiang Altay provides an ideal case to address this controversy, as its Be-mineralized pegmatites show a significant age gap (∼170 Ma) with the spatially associated Halong S-type granite, ruling out a simple fractional crystallization origin. This study presents an integrated Hf–Li isotopic and geochemical investigation of the Azubai Be-mineralized pegmatites, the Halong granite, and the potential metasedimentary source (Kulumuti Group). The Azubai Be-mineralized pegmatites exhibit homogeneous, positive ε Hf ( t ) values (−1.2 to +3.4) and δ 7 Li values (+2.5 to +3.8 ‰), which contrast sharply with the broad and predominantly negative isotopic signatures of the Kulumuti Group metasedimentary rocks ( ε Hf ( t ): −4.3 to +4.6; δ 7 Li: −8.6 to −0.4 ‰), precluding a direct anatectic origin. In contrast, the Azubai Be-mineralized pegmatites isotopic compositions show significant overlap with those of the Halong granite ( ε Hf ( t ) = −2.8 to +3.4; δ 7 Li = +0.3 to +3.7 ‰). These Hf–Li isotopic results provide robust support for the recently proposed “two-stage model,” which entails: (1) Early Devonian formation of the Halong granite via crustal anatexis, and (2) Triassic post-orogenic extensional setting remelting of this pre-existing, Be-enriched granite, leading to the generation of the Azubai Be-mineralized pegmatites. This model reconciles the apparent paradox of spatial association without a direct genetic-temporal link and underscores the role of crustal recycling through reactivation of pre-existing granites in rare-metal metallogeny.