Timescales of melt generation and the thermal evolution of the Himalayan metamorphic core, Everest region, eastern Nepal

In the Everest region of the Nepalese Himalaya, ⁴⁰Ar/³⁹Ar and U-Pb geochronology provide evidence for a complex thermal history marked by multiple episodes of granite intrusion. The oldest mobilized melt formed syn-deformational granitic sills that have U-Pb crystallization ages of 21.33±0.03 and 21.80±0.05 Ma. Preserved in these same granites is a record of earlier magmatic crystallization of xenotime, zircon and monazite between ca. 26 Ma and ca. 23 Ma. This pattern of accessory phase crystallization is interpreted to reflect incremental melting and crystallization in the source region of the sills before ultimate melt migration, and provides the earliest evidence for anatexis in the Everest region. The beginning of crustal melting in the Everest region predates the earliest known movement on both the Main Central Thrust and the South Tibetan fault systems, but is temporally associated with the implied pressure decrease between "Eohimalayan" and "Neohimalayan" metamorphism.