Decrypting the polymetamorphic record of the Himalaya

Reconstructions of the tectonometamorphic architectures of orogenic systems rely on petrogenetic indicator minerals, such as garnet, to trace the transport of rocks through space and time. We show the results of new garnet Lu-Hf geochronology and inclusion U-(Th-)Pb geochronology from exhumed midcrustal metamorphic rocks exposed in the Kanchenjunga region of the eastern Nepalese Himalaya. Garnet in three of the five specimens analyzed yielded pre-Himalayan Lu-Hf dates of ca. 292-230 Ma, which contrasts with Himalayan-aged inclusions therein. Garnet in these specimens either grew entirely during the Early Permian opening of the Neo-Tethys Ocean or grew partly during Cambrian-Ordovician orogenesis and partly during the Cenozoic. The remaining two specimens yield Lu-Hf ages of ca. 50 Ma, which are some of the oldest recorded dates for Himalayan metamorphism. The apparent discordance between the geochronological techniques highlights a potential issue with interpretations that rely on single-geochron-method inclusion records. These results further show that some pressure-temperature determinations used in Himalayan models may not be Himalayan in age.

Automated summary

This study used garnet Lu-Hf geochronology and inclusion U-(Th-)Pb geochronology to investigate midcrustal metamorphic rocks in the eastern Nepalese Himalaya. The results showed that the ages of garnet and inclusions from different time periods do not match, which may affect interpretations relying on a single geochronological method. Furthermore, the garnet ages indicate that some pressure-temperature determinations used in Himalayan models may not be of Himalayan age.