Tectonic Evolution of Metasediments from the Gangdise Terrane, Asian Plate, Eastern Himalayan Syntaxis, Tibet

Abstract

The Eastern Himalayan Syntaxis of Namche Barwa carries critical information for understanding the geodynamics of the Indian-Asian collision. In this syntaxis, the Nyingchi Group is a sequence of medium- to high-grade metasediments, located on the north side of the Indus-Yarlung suture zone. Results from zircon U-Pb ages coupled with their Hf isotope systematics for the Nyingchi Group and a two-mica granite intruding the paragneisses reveal strong contrasts in provenance and depositional age within the sequence. Detrital zircons from a garnet paragneiss of the lower Nyingchi Group display 206Pb/238U ages from 493 Ma to 2466 Ma, with peaks at 506 Ma, 821 Ma, and 1032 Ma. The minimum detrital zircon age of 493 Ma constrains the maximum depositional age of the lower Nyingchi Group. Detrital zircons from a paragneiss of the middle Nyingchi Group yield 206Pb/238U ages ranging from 55 Ma to 1362 Ma, with peaks at 60 Ma, 95 Ma, 138 Ma, and 171 Ma. This indicates that the maximum depositional age is less than 55 Ma. Zircons from a two-mica granite, intruding the upper Nyingchi Group, yield a magma crystallization age of 22 ± 1 Ma and inherited zircon ages of 52 Ma to 911 Ma. Because the Hf isotopic ratios together with the ages of detrital zircons suggest an origin from anatexis of the middle Nyingchi Group sediments, these data reduce the maximum depositional age of the middle Nyingchi Group to less than 52 Ma. Results preclude the Nyingchi Group from representing Precambrian basement of the Gangdise terrane. The detrital zircon and inherited zircon age distributions, combined with their Hf isotopic compositions, reveal that whereas the lower Nyingchi Group may be correlated with crystalline units from the Himalayan terrane, the middle-upper Nyingchi Group is a much younger deposit derived from the Gangdise magmatic arc. In order to explain the medium- to high-grade metamorphism of the middle-upper Nyingchi Group, we suggest that following the Indian-Asian collision, a thrust slice from the leading edge of the Asian continental slab (the middle-upper Nyingchi Group) became detached and subducted, together with Himalayan lithologies from the Indian continental slab. These results provide new insight into the tectonic evolution of the Eastern Himalayan Syntaxis.