Multiphase evolution of a Li-pegmatite field from the Tashisayi area, Altyn Tagh, NW China: insights from a petrological, geochemical, and geochronological study

Abstract

The Tashisayi Li deposit was newly discovered in the eastern part of the Tashisayi batholith, located in the Altyn Tagh region of Northwest China. A Li-rich composite pegmatite-aplite dyke (γ02) displays superimposed relationships among different Li-bearing phases, including lepidolite-albite-quartz pegmatite (LAQ), spodumene-albite-quartz pegmatite (SAQ), and aplite. The timing and conditions of magmatism and Li mineralization in the Tashisayi remain enigmatic. The study involved field observations, U–(Th)–Pb dating of columbite-group minerals (CGM), zircon, and monazite, and geochemical analyses of CGM and quartz. U–Pb dating of CGM of the γ02 dyke revealed formation ages of 471.6±3.5 Ma (LAQ), 439.6±5.0 Ma (SAQ), and 416.3±4.8 Ma (aplite). Zircon U-Pb and monazite U-(Th)-Pb dating of biotite granite, pegmatitic aplite, and muscovite granite yielded ages of ca. 473 Ma, 439 Ma, and 425 Ma, respectively. The dating results indicate that the rare-metal pegmatites and granites in the Tashisayi area were emplaced during various periods from the Early Ordovician to Early Devonian, consistent with other rare-metal deposits in the Tugeman region. The textural and geochemical analyses on the CGM and quartz reveal that the LAQ, SAQ and aplite crystallized from highly evolved magmas under water-poor and relatively low temperature conditions, experiencing distinct evolution trend and forming processes. Additionally, both LAQ and SAQ were influenced by fluid or magma activities and the pegmatitic melt forming LAQ could enrich both Li and Sn. Extensive tectonic events in the Altyn Tagh Orogen, including ocean basin closure and continental collisions, promote the development of Li-rich granitic magmas. Thus, we argue that the multiple magmatic and Li mineralization events in the Tashisayi area are most likely originated from the melting of Proterozoic crustal materials, and the process was controlled by tectonic interactions between the Central Altyn, Southern Altyn, Northern Altyn, and Eastern Kunlun blocks.