The Paleo-Tethys tectonic zone has been recognized as a world-class rare-metal (Li-Rb-Be-Nb-Ta) pegmatite belt. Previous studies indicate that the rare-metal pegmatite mineralization is related to the Late Triassic–Early Jurassic granitoids. However, it remains debated which granites, among the various coeval I-, A- and S-type granitoids in the tectonic belt, are responsible for the rare-metal pegmatite mineralization. We address these questions through a systematic geochemical study of the Bailongshan granite complex, which is composed of both biotite granites and two-mica granites and is related to the largest Li deposit in this zone. The similarities in Sr–Nd–Hf–O isotopic compositions between the two-mica granites (I_Sr=0.7176 to 0.7183, εNd(t)= − 10.7 to − 10.1, εHf(t)= − 14.12 to − 4.58, δ¹⁸O = 10.11 to 13.46‰) and rare-metal pegmatites (I_Sr=0.7181 to 0.7189, εNd(t)= − 11.72 to − 10.68, εHf(t)= − 12.15 to − 5.37, δ¹⁸O = 10.37 to 12.37‰), both showing affinity with sedimentary source, provide convincing evidence that the rare-metal pegmatites were derived from the two-mica granites. The differences in these parameters between the two-mica granites and the biotite granites (I_Sr=0.7083 to 0.7086, εNd(t)= − 5.9 to − 5.7, εHf(t)= − 6.64 to − 1.50, δ¹⁸O = 7.27 to 9.36‰, characteristic of I-type granites) indicate that they were derived from different sources. Trace element modeling indicates that the pegmatites were produced via extremely high fractional crystallization (> 90%) of the two-mica granites, which is also supported by the difference in δ⁷Li values between the two-mica granites (-0.6 to 0.5‰) and pegmatites (2.04 to 4.94‰). Comparison of the geochemical data between the two-mica granites and metasedimentary rocks in the area suggests that the rare metals in the mineralizing magmas were most likely derived from the partial melting of metapelites of the Triassic Bayanharshan Group. The relatively high temperatures (771 to 830 °C) estimated from the Ti-in-zircon thermometer for the two-mica granites favor extraction of rare metals from both biotite and muscovite in the source rocks during the partial melting. The results of this study, together with published data of Late Triassic to Early Jurassic granitoids in the Paleo-Tethys tectonic zone, indicate that the rare-metal pegmatite mineralization is related to S-type granites, but not all S-type granites are fertile. The combination of rare-metal-rich source rocks (metapelites), high temperatures due to an external heat source favoring the release of rare metals from the source rocks, and high degrees of fractional crystallization facilitating further enrichment of rare-metals in the pegmatite magmas, is critical for the rare-metal mineralization.

古特提斯构造带被认为是世界级的稀有金属（Li-Rb-Be-Nb-Ta）伟晶岩带。前期研究表明，稀有金属伟晶岩矿化与晚三叠世—早侏罗世花岗岩类有关。然而，在构造带中同时代的各种I型、A型和S型花岗岩中，哪种花岗岩是稀有金属伟晶岩成矿的原因仍存在争议。我们通过对白龙山花岗岩杂岩进行系统的地球化学研究来解决这些问题，该杂岩由黑云母花岗岩和二云母花岗岩组成，与该地区最大的锂矿床有关。双云母花岗岩Sr-Nd-Hf-O同位素组成的相似性（I _Sr =0.7176至0.7183，εNd(t)=−10.7至−10.1，εHf(t)=−14.12至−4.58， ^δ18 O = 10.11 至 13.46‰) 和稀有金属伟晶岩 (I _Sr =0.7181 至 0.7189, εNd(t)= − 11.72 至 − 10.68, εHf(t)= − 12.15 至 − 5.37, δ ¹⁸ O = 10.37 至 12.37‰ ），两者都显示出与沉积源的亲和力，为稀有金属伟晶岩源自双云母花岗岩提供了令人信服的证据。二云母花岗岩与黑云母花岗岩这些参数的差异(I _Sr =0.7083 至 0.7086, εNd(t)= − 5.9 至 − 5.7, εHf(t)= − 6.64 至 − 1.50, δ ¹⁸ O = 7.27至9.36‰（I型花岗岩的特征）表明它们来自不同的来源。微量元素模拟表明，伟晶岩是通过二云母花岗岩极高的分异结晶（> 90%）产生的，这也得到了二云母花岗岩之间δ ⁷ Li值差异（-0.6至0.5）的支持。 ‰）和伟晶岩（2.04至4.94‰）。 对比该区二云母花岗岩与变沉积岩地球化学数据表明，成矿岩浆中的稀有金属很可能来源于三叠纪巴颜喀拉山群变泥岩的部分熔融。根据锆石钛温度计估算的双云母花岗岩相对较高的温度（771 至 830 °C）有利于在部分熔融过程中从源岩中的黑云母和白云母中提取稀有金属。本研究结果结合已发表的古特提斯构造带晚三叠世至早侏罗世花岗岩类资料​​表明，稀有金属伟晶岩矿化与S型花岗岩有关，但并非所有S型花岗岩都是肥沃的。富含稀有金属的烃源岩（变岩）、外部热源产生的高温有利于稀有金属从烃源岩中释放出来，以及高程度的分异结晶有利于伟晶岩浆中稀有金属的进一步富集，对于稀有金属矿化至关重要。