Insights into the genetic diversity and metallogenic environment of rare-metal pegmatites in Bailongshan, Karakorum (western China)

Abstract The closure of the Paleotethyan Ocean in the northern Tibetan Plateau formed a world-class rare-metal (Li, Be, Rb, Nb, and Ta) pegmatite metallogenic belt within the Dahongliutan-Hohxil-Songpan-Ganzi terrane of western China. Spodumene-rich pegmatites are hosted in the folded Triassic turbidites and often associated with Late Triassic–earliest Jurassic I-type and S-type granites. However, the parental affinity of these spodumene-rich pegmatites—whether from I-type or S-type granites—remains debated. To resolve this, we conducted geochronological and geochemical analyses of diorites, granodiorites, leucogranites, spodumene-free pegmatites, and spodumene-rich pegmatites from the Bailongshan ore field. Zircon U-Pb dating results show that the diorites intruded at 217.5–210.1 Ma, granodiorites at 212.3–205.0 Ma, and leucogranites at 215.1–196.2 Ma. Columbite U-Pb ages indicate emplacement of spodumene-free pegmatites at 220.9–196.2 Ma and spodumene-rich pegmatites at 213.9–194.7 Ma, suggesting synchronous magmatism during the Late Triassic–earliest Jurassic. Geochemically, hornblende-rich diorites and hornblende-bearing granodiorites show low SiO2, high MgO, metaluminous to weakly peraluminous signatures (Alumina Saturation Index [A/CNK] = 0.59–1.13), and calc-alkaline characteristics, and are typically I-type granites. Leucogranites containing muscovite, garnet, and tourmaline exhibit high SiO2, low MgO, strongly peraluminous signatures (A/CNK = 1.16–1.34), and high-K, calc-alkaline affinities. They are typically S-type granites, which show higher Rb/Sr and Rb/Ba ratios, indicating sedimentary rocks as their protoliths. Isotopically, S-type granites [87Sr/86Sri = 0.7111–0.7149; εNd(t) = −11.0 to −10.3; εHf(t) = −8.4 to −2.7] and pegmatites [87Sr/86Sri = 0.7182–0.7189; εNd(t) = −11.7 to −10.7; εHf(t) = −9.2 to −0.9] exhibit more enriched isotope compositions, similar to those of the wall-rock turbidites [87Sr/86Sri = 0.70803–0.7173; εNd(t) = −13.1 to −3.2], and are distinctly different from I-type granites [87Sr/86Sri = 0.7084–0.7098; εNd(t) = −6.8 to −6.3; εHf(t) = −6.2 to +0.9]. This implies that the pegmatites are derived from the S-type granites, not the I-type granites. The low temperatures (