Miocene magmatism in the Western Nyainqentanglha mountains of southern Tibet: An exhumed bright spot?

• A Tibetan tectonic window (the Western Nyainqentanglha) reveals three events:
• (1) 213–201 Ma tectonism associated with north–south Lhasa terrane accretion
• (2) 140–52 Ma ‘Gangdese’ magmatism from subduction of Neotethys
• (3) 25–8 Ma magmatism from partial melting of the thickened Tibetan crust
• The latter is argued to represent the exhumed equivalent of a geophysical bright spot.

The Western Nyainqentanglha (WNT) mountain range of south-central Tibet predominantly comprises granitoids that intrude into metasedimentary strata, and was exhumed from ~ 15 to 20 km depth in the footwall of the Yangbajain graben during the late Neogene. The range provides a rare exposure of deeper crustal levels of Tibet, which can be used to gain insight into processes that may be occurring beneath the plateau. Field, petrological, thermobarometric and U–Pb geochronological analyses are applied to determine the magmatic and metamorphic history of the WNT, revealing three tectonothermal events: (1) 213–201 Ma magmatism and amphibolite-facies metamorphism associated with north–south Lhasa terrane accretion, (2) 140–52 Ma magmatism resulting from subduction of Neotethys preceding India–Asia collision, and (3) 25–8 Ma magmatism that we suggest to have resulted from partial melting of the thickened Tibetan plateau crust. The latter is correlated with seismic bright spots imaged in the region at ~ 15–18 km depth, indicating that restricted partial melting at mid-crustal levels may have been ongoing since 25 Ma, in accordance with observations from xenolith data and predictions made by thermal modelling of thickened crust.