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

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<p>The tectonometamorphic evolution of the Kalak Nappe Complex in the northernmost Scandinavian Caledonides is currently uncertain; at least two pre-Caledonian events have been locally recognised within the complex, as well as Caledonian events. To help clarify the evolution of the complex, we document here the P-T-t paths of garnet growth, which represent the peak metamorphic conditions within this relatively unstudied external part of the complex.</p><p>Metamorphic P-T paths for the lower part of the Kalak Nappe Complex were obtained using the THERIA_G model of Gaidies et al. (CMP 2008). In the 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 mass and composition of material used in successive increments of garnet growth is cumulatively subtracted from the matrix bulk-rock composition before calculating the P-T conditions of the next increment of garnet growth.</p><p>There is some latitude with regards to the absolute metamorphic conditions determined using this model, due to the inherent uncertainty of the thermodynamic data and the approximation of the reactive volume composition. However, the slopes of the determined P-T paths, together with lithological, geochemical and Lu-Hf garnet whole-rock isotopic data and garnet crystal size frequency distributions, enabled the identification of three nappes in the study area; from lowest upwards, the Bekkarfjord, Veidnes and Kolvik nappes.</p><p>An early, low-pressure Barrovian-type metamorphic event at ∼464 Ma is preserved in the Veidnes Nappe, where garnet cores (Grt 1V) give a P-T gradient of ∼15 bar/°C, with peak conditions of ∼560 °C and 4.5 kbar. That was followed by moderate-pressure metamorphism in the Bekkarfjord Nappe at ∼423 Ma, resulting in garnet crystallization (Grt 1B, core growth) along a gradient of ∼20 bar/°C, with peak conditions of ∼570 °C and 6.0 kbar. All three nappes then experienced Barrovian-type metamorphism at ∼420 Ma on a steep P-T gradient of ∼40 bar/°C, with peak conditions of ∼560 °C and 6.7 kbar in the Bekkarfjord and Veidnes nappes (Grt 2B, V, rim growth), while the overlying Kolvik Nappe was metamorphosed at peak conditions of ∼590 °C and 7.5 kbar (Grt 1K, core growth). We consider the latter two episodes (423, 420 Ma) to be different stages of the Scandian phase of the Caledonian Orogeny.</p><p>The juxtaposition of the three nappes, with the youngest event having occurred in the structurally highest unit and the oldest event now being sandwiched between the two younger events indicates out-of-sequence thrusting associated with the final continent-continent collision. This has been modeled in “balanced” cross-sections of the ductile thrusting.</p>