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

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 (<750 °C) estimated by the Ti-in-zircon thermometer for both I- and S-type granites suggest that S-type granites were likely derived from fluid-fluxed melting of the Triassic turbidites, favoring extraction of rare metals from staurolite, biotite, and muscovite in metaturbidites during anatexis. The coeval I-type granites provided the external heat and exotic volatiles, promoting staurolite and mica breakdown in the metaturbidites and thus facilitating the release of rare metals into the S-type leucogranitic melts. This suggests that although rare-metal pegmatite mineralization in Bailongshan is primarily related to S-type granites, I-type granites also played a contributing role. The formation of the coeval Late Triassic−earliest Jurassic barren I-type and fertile S-type granites in Bailongshan is attributed to final bidirectional subduction, slab rollback, and delamination of the Paleotethyan oceanic slab.