Petrogenesis of Oligocene Granitoids and its Implications on Beryllium Mineralization at Pagele, Nianqingtanggula Mountain (Central Tibet)

The newly discovered Oligocene granitoids (33.1–28.7 Ma) at Pagele are magmatic rocks related to beryllium mineralization during the India–Asia late-collisional stage. This discovery provides an ideal example to study the late-collisional orogeny and beryllium prospecting in the Lhasa terrane. The Oligocene granitoids include porphyritic granodiorite, Stage I, II and III granites, and granitic pegmatite. Geochemical analysis shows that the porphyritic granodiorite is characterized by high SiO₂, K₂O, total ΣREE contents, and (La/Yb)_N ratios; while the latter two by higher SiO₂, lower ΣREE and (La/Yb)_N ratios. Notably, the granitic pegmatite has extremely high Y/Ho, low K/Rb and Zr/Hf, and distinct REE tetrad effect (1.14–1.21). This study suggests that the porphyritic granodiorite may be derived from partial melting of beryllium-rich materials composed of Lhasa ancient crust (70%–80%) and enriched Lhasa lithospheric mantle (20%–30%) under the tearing subduction of Indian slab. The three-stage granites and granitic pegmatite, which contain higher beryllium contents or beryls, were likely generated by highly fractionation of the porphyritic granodioritic magma or other homologous magma. Considering the possible genetic and spatial link between Indian slab tear and rifts, we suggest that highly-fractionated granites in rifting systems represent important Be prospecting targets in the Lhasa terrane.