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

ABSTRACT

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.

KEYWORDS:

• Humbu Himalaya
• U(-Th)/Pb geochronology
• ⁴⁰Ar/³⁹Ar thermochronology
• U(-Th)/He thermochronology

Acknowledgments

1. Lihter, A. Brubacher and F. Gervais are thanked for their companionship and discussions in the field. Field logistics were organized by Teke Tamang and carried out by Pradap Tamang and his excellent crew. Dr. Ananta Gajurel from the Department of Geology at Tri-Chandra College is thanked for his support in obtaining the various permits required for this work. This work benefited from the comments of three anonymous reviewers and the editorial handling of R. Stern. Supplementary information and data can be accessed online: https://osf.io/6xp8q/.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Highlights

• New U(-Th)-Pb geochronological data documents two main pulses of anatexis in the Khumbu

• ⁴⁰Ar/³⁹Ar geochronological data from the same specimens show rapid cooling after emplacement

• Rapid cooling coincides with movement along the South Tibetan detachment system

• U(-Th)/He geochronological data indicate slower cooling post-Ar closure in mica

Supplementary material

Supplemental data for this article can be accessed here

Additional information

Funding

This work was supported by the Canada Foundation for Innovation [31301]; Natural Sciences and Engineering Research Council of Canada [RGPIN-2016-06736,RGPIN-2018-06023,RGPIN-2020-05658].