Identifying the leucogranites in the Ailaoshan-Red River shear zone:Constraints on the timing of the southeastward expansion of the Tibetan Plateau

The uplift of the Tibetan Plateau significantly affected the global climate system. However, the timing of its uplift and the formation of its vast expanse are poorly understood. The occurrence of two types of leucogranites (the two-mica leucogranites and garnet-bearing leucogranites) identified in the Ailaoshan-Red River (ASRR) shear zone suggests an extension event in the southeastern Tibetan Plateau. The age of these leucogranites could be used to constrain the timing of uplift and southeastward expansion of the plateau. Petrography, geochronology and geochemistry investigations, including Sr–Nd isotope analysis, were conducted on the two-mica leucogranites and garnet-bearing leucogranites from the ASRR shear zone. LA-ICP-MS zircon U–Pb dating indicates that these rocks were emplaced at ~27 ?Ma, implying that the Tibetan Plateau had already achieved maximum uplift prior to the late Oligocene. It subsequently started to expand southeastward as a result of crustal flow. Compared to classic metapelite-derived leucogranites from Himalaya, the two-mica leucogranites show high K₂O/Na₂O (1.31–1.92), low Rb/Sr, CaO, lower ⁸⁷Sr/⁸⁶Sr ratios (0.7089–0.7164) and higher ε_Nd(t) (?8.83 to ?3.10). This whole-rock geochemical characteristics likely indicates a mixing source origin, composed predominantly of amphibolite with subordinated metapelite, which is also evidenced by ⁸⁷Sr/⁸⁶Sr vs. ε_Nd(t) diagram. However, The garnet-bearing leucogranites with high SiO₂ contents (72.25–74.12 ?wt.%) have high initial ⁸⁷Sr/⁸⁶Sr ratios (0.7332–0.7535) and low ε_Nd(t) (?16.36 to ?18.98), indicating that they are derived from the source comprised of metapelite and results of fluexed muscovite melting under lower crustal level, which is also evidenced by the Rb–Sr–Ba systematics. These leucogranites formed from partial melting of the thickened lower crust, which resulted in the formation of granitic melt that weakened the crust. The weakened crust aided the left-lateral strike-slip movement of the ASRR shear zone, triggering the escape of the Indochina terrane in the southeastern Tibetan Plateau during the late Oligocene.