Lu-Lu Hao

Recycling of molybdenum isotopes in continental subduction zones remains debated. In this contribution, we re‐visit the Mo isotope compositions of the Sailipu post‐collisional ultrapotassic rocks in the Himalaya‐southern Tibet orogen. These ultrapotassic rocks have very varying δ^98/95Mo values of −0.66 to −0.07‰ and Mo/Ce ratios of 0.0008–0.005, which are lower than those of mid‐ocean ridge basalts (MORB; δ^98/95Mo = −0.20 ± 0.06‰, and Mo/Ce = 0.03) and oceanic subduction‐related (i.e., mantle source involving fluids, residual slab, or oceanic sediments) magmatic rocks (e.g., modern arc lavas, Cenozoic OIB‐type basalts in eastern China and the central Mariana Trough basalts in the back‐arc basin, syn‐collisional andesitic rocks in southern Tibet). Combined with the light Mo isotopes of the Himalayan schists and gneisses, we suggest that the light Mo isotopic signature of the Sailipu ultrapotassic rocks is derived from subducted Indian continental crust. This is consistent with the extremely low δ¹¹B (−17.4 to −9.7‰) and B/Nb (0.16–1) values and enriched Sr‐Nd‐Pb isotopes of the Sailipu ultrapotassic rocks. Thus, this study reveals the recycling of light Mo‐B isotopes during continental subduction and demonstrates that Mo‐B isotopes can effectively distinguish between continental and oceanic subduction.

Adakitic rocks at continental collisional zones have important implications for understanding the mechanism of crustal reworking. The Himalayan–Tibetan orogen, built by India–Asia collision and Indian continental plate subduction, is one of the most prominent Cenozoic continent–continent collision zones, and Cenozoic post-collisional adakitic rocks widely occur in the Lhasa block of southern Tibet. Numerous studies have suggested that the adakitic granitoids in the eastern Lhasa block were derived from partial melting of a juvenile crust and post-collisional mantle-derived ultrapotassic magmas significantly contributed to this crustal reworking by energy and mass transfer. However, the genesis of adakitic rocks in the western Lhasa block remains highly debated, hindering our understanding of crustal reworking in the whole Lhasa block. Here, we report zircon U–Pb age and whole-rock major, trace elemental and Sr–Nd–B isotopic compositions for the Sailipu trachyandesites in the western Lhasa block. Zircon U–Pb dating yields an eruption age of ∼22 Ma. These trachyandesites are high-K calc-alkaline and exhibit intermediate SiO₂ (56.9–59.6 wt.%) and low MgO (2.3–4.2 wt.%) contents, low K₂O/Na₂O (0.8–1.1) ratios, enrichment in light rare earth elements (LREEs), and depletion in heavy REEs (HREEs) with negligible Eu and Sr concentration anomalies. They have high Sr (1080–1593 ppm) and low Y (14.0–26.8 ppm) and Yb (1.08–1.48 ppm) contents, with relatively high Sr/Y (46–95) and La/Yb (46–77) ratios showing adakitic affinities. These Sailipu adakitic rocks display δ¹¹B values of −9.7 to −2.7‰, which are higher than those of mid-ocean ridge basalts (MORBs) but similar to those of arc lavas, indicating contributions from the juvenile crust. However, they have more geochemically enriched Sr–Nd isotopes (⁸⁷Sr/⁸⁶Sr_(i) = 0.7092–0.7095, ε_Nd(t) = −8.09 to −7.25) than the juvenile crust, indicating contributions from ultrapotassic magmas. Thus, the Sailipu adakitic rocks were likely generated by the interaction between the juvenile lower crust and underplated ultrapotassic magmas. Combined with adakitic magmatism in the eastern Lhasa block, we suggest that magma underplating and subsequent crust–mantle mixing could have been a common and important process that induced the reworking of juvenile crust beneath southern Tibet. This process may be related to the foundering of the subducted Indian continental slab.

The history of Indian continental subduction beneath Asian plate remains unclear. Miocene ultrapotassic rocks in southern Tibet, with extremely enriched isotopes, have often been used to trace mantle metasomatism and geodynamic processes associated with Indian continental subduction. These rocks, however, may have been contaminated by Lhasa ancient crust. Uncertainties on primary ultrapotassic magmas obscure their mantle sources. Here we report on first mafic igneous enclaves in Cenozoic lavas of southern Tibet. They consist principally of clinopyroxene, phlogopite, and sanidine and have a zircon U‐Pb age of 21.5 ± 0.3 Ma. Mineral and bulk‐rock geochemical characteristics indicate their crystallization from primary ultrapotassic magmas. Bulk‐rock Sr‐Nd, clinopyroxene Sr, and zircon O isotopes demonstrate their isotopically enriched character. Combined with precollisional basalts, we suggest that the mantle source of the enclaves was enriched by the subducted Indian continent. This study implies that the Indian continent had subducted beneath central Lhasa no later than early Miocene.