Miocene anatexis, cooling and exhumation in the Khumbu Himal, Nepal

The Khumbu region of Nepal is host to numerous large leucogranite bodies that comprise some of the most iconic peaks in the Himalaya. These leucogranites occur throughout the high-metamorphic grade rocks exposed there in the immediate footwall of the orogen-scale South Tibetan detachment system. Previous work on leucogranites in the area has been limited spatially but has yielded U-Th/Pb ages between ~23 and 16 Ma. The current study expands upon that work with 522 new individual zircon, monazite and xenotime U-Th/Pb analyses across 10 leucogranite specimens collected from across the Khumbu. The dates returned outline the episodic crystallization of leucogranites at ~19 and 17 Ma, both of which contain significant inherited age components presumably from their source rocks. ⁴⁰Ar/³⁹Ar dating of muscovite from the same leucogranite bodies indicate rapid cooling through Ar closure (~510 °C) on the scale of ~500 kyr. The weighted mean of all ⁴⁰Ar/³⁹Ar dates, 16.48 ± 0.05 Ma, coincides with the timing of motion along the normal-sense Qomolangma detachment indicating a potential genetic relationship between the leucogranite cooling and extension. Finally, U(-Th)/He dating of apatite and zircon show that cooling slowed significantly post argon closure and that the rapid cooling may have been ephemeral.