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

Abstract

The Western Nyainqentanglha (WNT) mountain range of south-central Tibet predominantly comprises granitoids that intrude into metasedimentary strata, and was exhumed from ~15 to 20km 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) 213–201Ma magmatism and amphibolite-facies metamorphism associated with north–south Lhasa terrane accretion, (2) 140–52Ma magmatism resulting from subduction of Neotethys preceding India–Asia collision, and (3) 25–8Ma 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 ~15–18km depth, indicating that restricted partial melting at mid-crustal levels may have been ongoing since 25Ma, in accordance with observations from xenolith data and predictions made by thermal modelling of thickened crust.