Rates of Deep Continental Burial From Lu‐Hf Garnet Chronology and Zr‐in‐Rutile Thermometry on (Ultra)high‐Pressure Rocks

Abstract

Abstract The burial of continental lithosphere in collision zones is a first‐order process in global tectonics. Decades of interdisciplinary research have provided models for continental subduction; however, few empirical constraints exist on the processes and rates of burial, and assessments of the impact of continental collision on the geodynamics of convergent margins are still purely qualitative. A spatially resolved analysis of continental burial in collisional orogens is needed to progress in this field. To this end, we subjected samples collected along a vector parallel to the paleo‐subduction direction of the Western Gneiss Complex—one of the world's largest and best preserved continental ultrahigh‐pressure terranes—to Lu‐Hf garnet chronology and Zr‐in‐rutile thermometry. The Lu‐Hf ages range from 420 to 400 Ma but do not mimic pressure and temperature trends, which decrease up‐slab. Zirconium‐in‐rutile data demonstrate that Lu‐Hf ages of 405–400 Ma represent the cessation of recrystallization at peak conditions during deep burial, whereas ages of 420–410 Ma represent prograde garnet growth, preserved as relics. The differences in P‐T‐t of garnet growth are used to calculate a burial rate of ~5 mm yr −1 , which is much slower than the burial of subducting mature oceanic crust. Comparing our observations with those from other collisional settings, including the India‐Asia Collision Zone, demonstrates that burial rates for continental crust are globally uniform and independent of the precollisional convergence rate and slab angle. These results provide a new quantitative framework for evaluating and predicting changes in the geodynamics of active margins during the collision of (super)continents.