PRESERVED ZR-TEMPERATURES AND U-PB AGES IN HIGH-GRADE METAMORPHIC TITANITE: EVIDENCE FOR A STATIC HOT CHANNEL IN THE HIMALAYAN OROGEN

Titanite grains from ~800 °C gneisses of the c. 5-km thick Greater Himalayan Sequence of central Nepal were analyzed for Zr-in-titanite temperatures and U-Pb ages to investigate the formation and evolution of a former weak mid-crustal channel in Himalaya. Laser-ablation ICP-MS depth profiles and spot analyses of titanite Zr and U-Pb isotopic compositions are inconsistent with diffusional reequilibration on scales greater than 1-2 µm, and instead data appear to faithfully record both temperature and age. Titanite records protracted heating from 700-750 °C at ~37 Ma to 775 °C at ~24 Ma, and possibly slight cooling to 765 °C at ~20 Ma. Such temperatures exceed partial melting reactions in associated metapelites and imply profoundly weak rheologies (i.e. a hot channel), yet predate initiation of the bounding Main Central Thrust and South Tibetan Detachment systems by as much as 15 Myr. Evidently, thick but weak crustal channels may remain stationary with respect to the convergence direction for >10 Myr, even during one of Earth’s biggest continent-continent collisions.