Two episodes of monazite crystallization during metamorphism and crustal melting in the Everest region of the Nepalese Himalaya

New monazite U-Pb geochronological data from the Everest region suggest tha t similar to 20-25 m.y. elapsed between the initial India-Asia collision an d kyanite-sillimanite-grade metamorphism. Our results indicate a two-phase metamorphic history, with peak Barrovian metamorphism at 32.2 +/- 0.4 Ma an d a Later high-temperature, low-pressure event (620 degrees C, 4 kbar) at 2 2.7 +/- 0.2 Ma. Emplacement and crystallization of the Everest granite subs equently occurred at 20.5-21.3 Ma. The monazite crystallization ages that d iffer by 10 m.y. are recorded in two structurally adjacent rocks of differe nt lithology, which have the same postcollisional pressure-temperature hist ory. Scanning electron microscopy reveals that the younger monazite is elab orately shaped and grew in close association with apatite at grain boundari es and triple junctions, suggesting that growth was stimulated by a change in the fluid regime. The older monazite is euhedral, is not associated,vith apatite, and is commonly armored within silicate minerals. During the low- pressure metamorphic event, the armoring protected the older monazites, and a lack of excess apatite in this sample prevented new growth. Textural rel ationships suggest that apatite is one of the necessary monazite-producing reactants, and spots within monazite that are rich in Ca, Fe, Al, and Si su ggest that allanite acted as a preexisting rare earth element host. We prop ose a simplified reaction for monazite crystallization based on this eviden ce.