Abstract Recent studies in the Yardoi gneiss dome of the Tethyan Himalaya, southern Tibet demonstrated that the Himalayan orogenic belt experienced a major episode of crustal melting in the Mid-Eocene ( c. 44–40 Ma), represented by the genesis of a suite of high Sr/Y two-mica granites (TMGs) accompanied by leucogranites. Geochemical and U–Pb age data indicate that (1) both the leucogranite and the porphyritic leucogranite formed at c. 44–41 Ma are characterized by similar Sr–Nd–Hf isotope systematics to those in the less evolved TMGs; (2) these three suites of rocks might represent a Mid-Eocene igneous complex derived from partial melting of amphibolite under crustal thickened conditions; and (3) in contrast with the TMGs, leucogranites experienced hydrothermal-magma interactions as well as plagioclase fractional crystallization as revealed from pronounced negative Eu anomalies, much lower Sr concentrations, and elevated common Pb and U concentrations in the latest magmatic zircon overgrowth. Our data suggest that magmatic differentiation of more primitive magmas could have played a critical role in the generation of leucogranites. Documentation of partial melting of deep crustal rocks during the earliest phase of the tectonic evolution of the Himalayan orogen implies that the large-scale collisional orogenic belt could experience intensive partial melting under contraction and thickening conditions, which could profoundly affect the geophysical and geochemical properties as well as rheological properties and facilitate the initiation of movement or flow of deep crustal rocks.

Four structural-metamorphic zones separated by ductile shear zones comprise the Sulu high- to ultrahigh-pressure metamorphic terrane. From south to north, they are the southern high-pressure zone (0.2–0.4 GPa, 350–400 °C), the central very high-pressure zone (1.5–2.5 GPa, 500–600 °C), the northern ultrahigh-pressure (UHP) zone (>2.8 GPa, 650–800 °C) dominated by paragneiss, and the northern UHP zone dominated by granitic gneiss. Exhumation was associated with retrograde amphibolite- to greenschist-facies metamorphism and oblique southeastward ductile extrusion within the subduction channel. Geologic observations and seismic reflection profiles suggest an eroded asymmetric antiformal slab intruded by numerous Mesozoic granitic plutons. Zircon U/Pb ages indicate that peak metamorphism in the UHP zone occurred at 240–220 Ma, and 40 Ar/ 39 Ar thermochronology documents later exhumation at 220–200 Ma. In contrast, in the high-pressure zone, peak metamorphism and exhumation ages are older than 253 Ma and 253–240 Ma, respectively, by the same mineral. Together with previous results, these new data suggest that subduction and exhumation of different parts of the Yangtze slab occurred at different times, depending upon vertical variations in the physical properties of the subducted continental crust.