Using P-T-t estimates to identify and restore out-of-sequence thrusting in the lower part of the Kalak Nappe Complex (Nordkinnhalvøya, Sværholthalvøya), internal Scandinavian Caledonides, Finnmark, N. Norway

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.

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.

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.

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.

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.