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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. [ABSTRACT FROM AUTHOR]

The Thakkhola graben is a large-scale N-S striking, E-W extensional structure located in west-central Nepal that was actively extending ca. 17 Ma. New multi-system geochronological data from the immediate footwall of the Dangardzong fault, the main graben-forming structure in the Thakkhola graben, outline decelerating cooling paths. The cooling rate of the footwall progressively decreases from 55 ± 10°C/Myr in the early Miocene (~22–13 Ma, monazite U-Th/Pb, mica 40Ar/39Ar and zircon U-Th/He), to 23 ± 8°C/Myr in the middle to late Miocene (~13-8 Ma, zircon and apatite U-Th/He), and finally to 10 ± 2°C/Myr from 8 Ma to present day (cooling post apatite U-Th/He closure). The deceleration in cooling rate is interpreted to reflect the widespread development of N-S striking graben structures in the Tibetan plateau in the middle Miocene and the progressive partitioning of strain away from the Thakkhola graben into other, younger, extensional features. The shift in the locus of strain may reflect the progressive under-thrusting of India beneath the Tibetan plateau and/or the eastward flow of mid-to-lower crustal material away from the plateau. [ABSTRACT FROM AUTHOR]