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±0.7Ma at a temperature of at least ~610±30°C. Cooling of these units through 420–500°C occurred at c. 11–8Ma, 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. 23–8Ma 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. 8Ma, 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.