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

Two-stage cooling history of pelitic and semi-pelitic mylonite (sensu lato) from the Dongjiu-Milin shear zone, northwest flank of the eastern Himalayan syntaxis

The Dongjiu–Milin shear zone located on the northwest flank of the eastern Himalayan syntaxis, southeast Tibet, separates Indian and Asian plate rocks. It is characterized by a thick sequence of highly strained and ductilely deformed mylonite (sensu stricto and sensu lato) bound between a pair of sub-parallel transtensive brittle normal faults. An integrated geochronological, petrographic, and thermobarometric study of three samples of pelitic and semi-pelitic mylonite (sensu lato) from the shear zone provides new insights into its thermal and structural evolution, and hence the tectonic processes operating in the region since the India–Asia collision. U–Th–Pb in-situ dating of monazite, garnet–ilmenite thermometry, and textural relationships show that mylonitization and peak-thermal staurolite-grade metamorphism occurred at 23.4 ± 0.7 Ma at a temperature of at least ~ 610 ± 30 °C. Cooling of these units through 420–500 °C occurred at c. 11–8 Ma, as constrained by the retrograde breakdown of allanite to form monazite. These data show that the cooling history of mylonite (sensu lato) in the shear zone and the tectonic evolution of the syntaxis region can be divided into two distinct stages. Slow cooling at a rate of 6–12 °C/Myr during c. 23–8 Ma is attributed to exhumation during ductile deformation and mylonitization caused by the ongoing India–Asia collision. This was followed by a period of significantly faster cooling at a minimum rate of 57 °C/Myr since c. 8 Ma, most likely associated with brittle normal faulting that facilitated the final stages of excavation to the surface. These new thermochronological data provide evidence of deep-seated exhumation-related cooling processes occurring in a convergent margin orogenic setting

Monazite geochronology and petrology of kyanite- and sillimanite-grade migmatites from the northwestern flank of the eastern Himalayan syntaxis

A combined geochronological and petrological study of pelitic migmatites from the northwestern flank of the eastern Himalayan syntaxis has constrained the timing and P–T conditions of two high-grade metamorphic events that affected the south Lhasa block (Asian margin) and provides new insight into the tectonothermal evolution of the India–Asia collision. U(–Th)–Pb dating of in situ monazite shows that upper amphibolite-facies sillimanite-grade metamorphism and consequent partial melting occurred between c. 71 and 50 Ma at P–T conditions above 6.3 ± 1.2 kbar and 750 ± 30 °C. Further partial melting at upper amphibolite-facies kyanite-grade conditions occurred between c. 44 and 33 Ma at minimum P–T conditions of 10.4 ± 1.0 kbar and 698 ± 20 °C. These data are interpreted to record a south Lhasa block mid-crustal sillimanite-grade melting event in the Late Cretaceous to Early Eocene related to regional heat advection caused by coeval and prolonged emplacement of Gangdese batholith units. This was followed by a higher pressure and lower temperature kyanite-grade melting event during the Middle Eocene to Early Oligocene associated with deformation and crustal thickening in the south Lhasa block, coeval with kyanite-grade metamorphism along the Himalaya, as a result of the on-going India–Asia collision. These partially-melted crustal lithologies offer potential sources (or otherwise analogs for sources) for the Miocene emplacement of adakitic intrusions previously documented in the eastern Himalayan syntaxis region

Monazite geochronology and petrology of kyanite- and sillimanite-grade migmatites from the northwestern flank of the eastern Himalayan syntaxis

A combined geochronological and petrological study of pelitic migmatites from the northwestern flank of the eastern Himalayan syntaxis has constrained the timing and P–T conditions of two high-grade metamorphic events that affected the south Lhasa block (Asian margin) and provides new insight into the tectonothermal evolution of the India–Asia collision. U(–Th)–Pb dating of in situ monazite shows that upper amphibolite-facies sillimanite-grade metamorphism and consequent partial melting occurred between c. 71 and 50 Ma at P–T conditions above 6.3 ± 1.2 kbar and 750 ± 30 °C. Further partial melting at upper amphibolite-facies kyanite-grade conditions occurred between c. 44 and 33 Ma at minimum P–T conditions of 10.4 ± 1.0 kbar and 698 ± 20 °C. These data are interpreted to record a south Lhasa block mid-crustal sillimanite-grade melting event in the Late Cretaceous to Early Eocene related to regional heat advection caused by coeval and prolonged emplacement of Gangdese batholith units. This was followed by a higher pressure and lower temperature kyanite-grade melting event during the Middle Eocene to Early Oligocene associated with deformation and crustal thickening in the south Lhasa block, coeval with kyanite-grade metamorphism along the Himalaya, as a result of the on-going India–Asia collision. These partially-melted crustal lithologies offer potential sources (or otherwise analogs for sources) for the Miocene emplacement of adakitic intrusions previously documented in the eastern Himalayan syntaxis region