| Summary: | U(-Th)-Pb dating of zircon, monazite, and xenotime from metamorphic and igneous rocks at two outcrops along a north-south transect in the Mount Everest region of southern Tibet provide new constraints on the timing and duration of thermal events associated with channel flow and the ductile extrusion of the Greater Himalayan Series (GHS). At the southernmost outcrop in the Kangshung Valley, Th-Pb ages from monazite indicate that prograde metamorphism associated with crustal thickening following the India-Asia collision occurred at least as early as 38.9±0.9 Ma. A subsequent sillimanite-grade metamorphic event at 28.0±1.2 Ma was followed by two phases of leucogranite emplacement at 20.8±0.8 and 16.7±0.4 Ma. At Thongmön, ∼40 km to the northeast of the Kangshung Valley, prograde metamorphism was occurring at ∼25.4 Ma and lasted until 16.1±0.1 Ma, reaching ∼740°C and 5 kbar at 22.4± 0.2 Ma. Immediately following metamorphism, two phases of leucogranite were emplaced at 15.2±0.2 and 12.6±0.2 Ma, with an intervening phase of ductile deformation. These data combined with ages from the Rongbuk glacier and Ama Drime range, north and east of Everest and the North Himalayan Mabja dome 100-140 km to the northeast, suggest that GHS metamorphism lasted ∼20 m.yr. and that migmatization and south-directed channel flow peaked around ∼23-20 Ma and ended by ∼16 Ma. The youngest leucogranites crosscut all ductile fabrics related to the Miocene channel flow.
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