Mechanisms and durations of metamorphic garnet crystallization in the lower nappes of the Kalak Nappe Complex (Arctic Norway)

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 c. 630˚C, which is c. 40˚C higher than the temperature obtained from resorbed rims of the largest garnet crystal. The use of a diffusion geospeedometry approach that considers the geometry of the compositional zoning of the entire garnet population, and the higher, more realistic peak temperature, results in an estimated duration of 1 to 4.9 Myr for garnet growth in the Kolvik Nappe. This is approximately one order of magnitude faster than duration estimates calculated when using the apparent, lower temperature obtained from the resorbed garnet rims. Concomitantly to garnet growth in the Kolvik Nappe, garnet overgrowths formed in the Bekkarfjord Nappe for c. 2.5 Myr at metamorphic peak temperatures of c. 560˚C. The garnet growth durations obtained in this work are comparable with the uncertainty on the Lu–Hf garnet–whole-rock isochron ages published previously for the studied rocks (Gaidies et al., 2022). The 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 final stages of the Caledonian Orogeny in Arctic Norway.

Reference: Gaidies, F., Heldwein, O. K. A., Yogi, M. T. A. G., Cutts, J. A., Smit, M. A., Rice, A. H. N. (2022). Testing the equilibrium model: An example from the Caledonian Kalak Nappe Complex (Finnmark, Arctic Norway). Journal of Metamorphic Geology, 40(5):859–886.