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.

The equilibrium model has been tested using Barrovian garnet-zone micaschists of the Kalak Nappe Complex. In our model, equilibrium in the MnNCKFMASHT system was established across the entire rock volume during prograde metamorphism except for garnet, which developed growth zoning preserved at levels controlled by the kinetics of intracrystalline diffusion. The preservation potential of the disequilibrium fluctuations required for nucleation of garnet has been considered in our simulations, using a moving boundary multicomponent diffusion and growth model. Results indicate that the core of garnet that crystallizes during regional metamorphism does not retain the major element composition of the nucleus but reflects the compositional signature of the immediate overgrowth. The differences between the metamorphic conditions of successive garnet growth steps of the samples indicate characteristic trends in their pressure-temperature evolutions that can be predicted with the equilibrium model. There is some latitude with regard to the absolute metamorphic conditions due to the inherent uncertainty of the thermodynamic data and the approximation of the reactive volume composition. However, the slopes of the pressure-temperature paths together with systematic trends in the lithological, geochemical, and Lu-Hf garnet whole-rock isotopic properties of the rocks, as well as their garnet crystal size frequency distributions, enable the identification of the Veidnes, Bekkarfjord, and Kolvik Nappes involved in the Caledonian Orogeny and provide new insight into their metamorphic evolutions. According to our findings, the base of the Kalak Nappe Complex experienced wide-spread Barrovian-type metamorphism at c. 420 Ma with a gradient of $40 bar/ C and peak conditions of $560 C and 6.7 kbar in the Bekkarfjord and Veidnes Nappes, whereas the hinterland-placed Kolvik Nappe was metamorphosed at peak conditions of $590 C and 7.5 kbar. This event was preceded by moderate-pressure metamorphism at c. 423 Ma resulting in garnet crystallization exclusively in the Bekkarfjord Nappe, along a gradient of $20 bar/ C and peak conditions of $570 C and 6.0 kbar. We consider both of these metamorphic and deformational episodes to be different

The freshwater reservoir effect (FRE) in the Canadian Subarctic complicates development of high-resolution age-depth models based on radiocarbon dates from lake sediments. Volcanic ashfall layers (tephras) provide chronostratigraphic markers that can be used to estimate age offsets. We describe the first recorded occurrence of a visible tephra in a lacustrine sequence in the central Northwest Territories. The tephra, observed in Pocket Lake, near Yellowknife, is geochemically and stratigraphically attributed to the White River Ash east lobe (WRAe; A.D.833-850; 1,117-1,100 cal BP), which originated from an eruption of Mount Churchill, Alaska.We also observed the WRAe as a cryptotephra in Bridge Lake, 130 km to the NE, suggesting that records of this tephra are potentially widespread in CNT lakes. The identification of this tephra presents opportunities for use of the WRAe as a dating tool in the region and to quantify the magnitude of the FRE in order to correct radiocarbon age-depth models. Two well-dated sediment cores from Pocket Lake, containing a visible WRAe record, indicate a FRE of ~200 years at the time of the ash deposition, which matches closely with the estimated FRE of ~245 years at the lake sediment-water interface. Although additional results from other lakes in the region are required, this finding implies that FRE estimates for the late Holocene in the region, may be based either on down-core WRAe/radiocarbon age model offsets, or on radiocarbon dates obtained from the sediment-water interface.

Uranium-lead (U-Pb) accessory mineral petrochronology has been increasingly used to constrain the timing of tectonometamorphic events. However, because mafic rocks commonly lack minerals with a high U/Pb ratio, they may be underrepresented in the chronologic record. This study on polymetamorphic mafic granulites from the Archean Rae craton (northern Canada) provides a striking example of a metamorphic cycle that has been entirely overlooked. We utilized Lu-Hf garnet geochronology and equilibrium phase diagram modeling to characterize two high-pressure granulite-facies mineral assemblages that affected the 2.6 Ga protolith. Zircon and garnet recrystallization occurred at 1.87 Ga within a gneissic foliation, while a coarse-grained garnet precursor nucleated 230 m.y. earlier during a stage of high heat flow within thickened lower crust, the latter of which is nearly absent in the zircon and monazite age record except for rare igneous occurrences. Combined garnet geochronology and petrological modeling reinforce a ca. 1.9 Ga age for high-grade overprinting in the southern Rae craton and clearly show within the same sample that U-Pb accessory minerals did not grow during a newly identified 2.11 Ga granulite-facies event.

The burial and exhumation of continental crust during collisional orogeny exert a strong control on the dynamics of mountain belts and plateaus. Constraining the rates and style of exhumation of deeply buried crust has proven difficult due to complexities in the local geology and thermochronometric methods typically used. To advance this field, we applied trace-element and U-Pb laser ablation inductively coupled plasma mass spectrometry analyses to rutile from eclogite and amphibolite samples from the Western Gneiss Complex of Norway-an archetypal continental (ultra)high-pressure (UHP) terrane. Peak temperature and timing of midcrustal cooling were constrained for samples collected along a subduction-and exhumation-parallel transect, using Zr-in-rutile thermometry and U-Pb rutile geochronology, respectively. Peak temperatures decrease from 830°C in the UHP domain to 730°C at the UHP-HP transition, remain constant at 730°C across most of the terrane, and decrease to 620°C at the eclogite-out boundary. U-Pb results show that most of the terrane cooled through 500°C at 380-375 Ma except for the lowest grade region, where cooling occurred approximately 20 million years earlier. The results indicate that exhumation was a two stage process, involving (1) flexural rebound and slab flattening at depth combined with foreland-directed extrusion, followed by (2) synchronous cooling below 500°C across the, by then, largely flat-lying Western Gneiss Complex. The latter implies and requires relatively homogeneous mass removal across a large area, consistent with erosion of an overlying orogenic plateau. The Caledonides were at near-equatorial latitudes at the time. A Caledonian paleo-plateau thus may represent a so far unrecognized factor in Devonian and Carboniferous atmospheric circulation and climate forcing.