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The 3D microstructure and compositional zoning of garnet populations in micaschists from the Kolvik and Bekkarfjord nappes indicate the quasi‐equilibration of their major components across the rock matrices during interface‐controlled, size‐independent garnet growth. There is microstructural evidence for foliation‐parallel, small‐scale resorption of garnet rims in the Kolvik Nappe, influencing the metamorphic peak conditions obtained from thermodynamic modelling. The local chemical compositions of rims less affected by resorption indicate a peak temperature of ~630°C, which is ~40°C higher than the temperature obtained from resorbed rims of the largest garnet crystal. Using a diffusion geospeedometry approach that considers the geometry of the compositional zoning of the garnet population, as well as the higher, more realistic peak temperature, a duration of 1 to 4.9 Myr is obtained for garnet growth in the Kolvik Nappe. This is approximately 1 order of magnitude faster than duration estimates obtained when using the apparent, lower temperature estimated from the resorbed garnet rims. Concomitantly to garnet growth in the Kolvik Nappe, garnet overgrowths formed in the Bekkarfjord Nappe for circa 2.5 Myr at metamorphic peak temperatures of ~560°C. The garnet growth durations obtained here are comparable with the uncertainty on the Lu–Hf garnet–whole rock isochron ages of 419.9 ± 2.4 Ma and 423.0 ± 1.9 Ma, previously obtained for these rocks. These results provide new insight into the timescales of repeated Barrovian‐type metamorphic events experienced by the lower nappes of the Kalak Nappe Complex during the Caledonian Orogeny in Arctic Norway. This study emphasizes the importance of microstructural and chemical characterization of garnet populations in assessing metamorphic crystallization mechanisms and the extent of equilibration of garnet‐forming components during prograde metamorphism. Moreover, our results provide means for reducing the uncertainty on metamorphic durations obtained via diffusion geospeedometry and, so, contributing to our understanding of geological timescales and processes. [ABSTRACT FROM AUTHOR]

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 stages of the Scandian phase during the Caledonian Orogeny. An additional orogenic event at c. 464 Ma is preserved in the Veidnes Nappe, which was juxtaposed with the Scandian Nappes by repeated out‐of‐sequence thrusting associated with the final continent–continent collision. This nappe provides a unique insight into an earlier stage of the Caledonian Orogeny, indicating a low‐pressure Barrovian‐type metamorphic history with a gradient of ∼15 bar/°C and peak conditions of ∼560°C and 4.5 kbar. [ABSTRACT FROM AUTHOR]