Formation of rare-element pegmatites in the Chinese Altai: Contribution of two-stage melting

Abstract

With increasing discoveries of rare-element pegmatites in metamorphic-migmatitic belts, the role of partial melting of metamorphic protoliths in the formation of pegmatites has attracted significant attention. A previously proposed two-stage melting model for metamorphic sedimentary rocks and subsequent granitic rocks can explain the formation of Li-mineralized pegmatitic magmas, although systematic geologic examples are still missing. Here, we present systematic geochemical data from lithium-cesium-tantalum (LCT) pegmatites (with ages of 400−150 Ma), S-type granitoids (500−350 Ma), and metasedimentary rocks of the Habahe group (HGMRs, 600−450 Ma) in the Chinese Altai orogenic belt, providing evidence of a two-stage melting process occurring in nature. Zircon Hf-O isotope analyses and whole-rock geochemistry suggest pegmatite formation in the Permian−Jurassic (280−150 Ma) as a result of the anatexis of S-type granitoids, which originally crystallized from partial melts of HGMRs. The content of rare elements increases progressively from HGMRs to S-type granitoids to LCT pegmatites, aligning with the modeled melting process. The two-stage melting of the source HGMR protolith resulted in significant extraction of rare elements, with the maximum enrichment factors for Li, Be, Rb, Cs, Nb, and Ta reaching 20, 31, 31, 120, 45, and 44, respectively. This pronounced metal enrichment led to the formation of rare-element mineral assemblages of spodumene, pollucite, and columbite group minerals. This study confirms that multistage crustal melting can be efficient in the enrichment and mineralization of rare elements.