Petrogenesis of the Chakabeishan pegmatites, North Qaidam Terrane: Implications for Indosinian lithium mineralization in the northern Tibetan Plateau

Pegmatite-type Li deposit is a critical metal, which has been attracting extensive attention. However, its petrogenesis and Li-mineralization mechanism remain under debate. Recently, four types of Late Triassic (222−212 Ma) pegmatites are recognized in the Chakabeishan area of Markam–Yajiang–Karakoram (MYK) giant Li ore belt in northern Tibet: tourmaline-, garnet-. spodumene-, and lepidolite-bearing pegmatites. The tourmaline-bearing pegmatites show uniform Nd isotopic compositions (εNd(t) = −13.8 to −14.5), closed to those of the metasedimentary wall rocks (εNd(t) = −15.2 to −17.8), but significantly lower than those of the surrounding Middle Triassic (∼ 244 Ma) syenogranites (εNd(t) = −4.7 to −5.5) that are typical I-type granites. These data, together with their higher Sm/Nd ratios, lower Y and LREE contents relative to the metasedimentary wall rocks, and lack of biotite, demonstrate that the tourmaline-bearing pegmatites were formed by the disequilibrium melting of the metasedimentary wall rocks involving water-absent muscovite dehydration melting with monazite in the residue. From the tourmaline-bearing pegmatites through the garnet-bearing pegmatites to the spodumene- and lepidolite-bearing pegmatites, gradually increasing whole-rock LiBe contents as well as decreasing K/Rb ratios and increasing Cs contents in the K-feldspar and muscovite suggest that they were generated by continuous evolution of magmas similar to the tourmaline-bearing pegmatites. In addition, mineralogical observation and variable Nb and Ta contents in muscovite indicate that the Chakabeishan pegmatites experienced a three-stage evolution process, i.e., magmatic stage for tourmaline- and garnet-bearing pegmatites, coexisting aqueous and silicic magma to hydrothermal transition stage for spodumene-bearing pegmatites, and hydrothermal stage for lepidolite-bearing pegmatites. Taking into account regional geological constraints and our data, we suggest that the low-degree partial melting of clay-rich sedimentary rocks initially control the Li enrichment. The subsequent exsolution and accumulation of a fluid phase during magmatic evolution process occurred in the Li-poor and Li-rich pegmatites, respectively, which is responsible for the further Li mineralization.