Rongbuk Re-Visited: Geochronology of Leucogranites in the Footwall of the South Tibetan Detachment System, Everest Region, Southern Tibet

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The Hermit's Gorge area adjacent to the Rongbuk Glacier on the north side of Mount Everest is a critical location to establish the timing and duration of movement along the ductile strand of the South Tibetan Detachment system (STDS), a low-angle, north-dipping normal fault that bounds the upper part of the Greater Himalayan Sequence. Monazite from four leucogranite samples in the Hermit's Gorge that bracket the timing of ductile fabric development has been dated using both U/Th–Pb ID-TIMS and LA-MC-ICPMS. Results suggest that the earliest, ~ 16.4 Ma, leucogranite sills have been folded and deformed along with the host sillimanite gneisses and calc-silicates whereas structurally higher sills and dykes that post-date fabric development are slightly younger, all within uncertainty of one another at 15.6 to 15.4 Ma. Field relations combined with age data constrain ductile fabric formation associated with movement along this strand of the STDS as being on-going at 16.4 Ma but had ceased prior to 15.6 Ma, while brittle faulting along the STDS is younger than 15.4 Ma. Combined with data from the Everest massif and surrounding region, ages of granite crystallization and ductile shearing propagated up-structural section and northward with time.