The Distribution of Major and Trace Elements Across a Garnet Population From the Kalak Nappe Complex (Finnmark, Scandinavian Caledonides): Evidence for Size-Dependent Growth and Compositional Equilibration in the Garnet Zone

Abstract

A garnet population from the lower Kalak Nappe Complex in Finnmark (Arctic Norway) was characterized using high-resolution X-ray micro-computed tomography, electron probe micro-analysis and laser ablation inductively coupled plasma mass spectrometry mapping to assess the extent of compositional equilibration and the controlling crystallization mechanisms during garnet growth. The obtained petrological dataset includes the spatial relationships of garnet crystals and the rock matrix fabrics, as well as the two-dimensional distributions of major and trace elements in differently-sized garnet crystals. Our results indicate that the observed elongated shape and clustered distribution of garnet resulted from crystallization in a texturally and chemically differentiated matrix, evidenced by the preferred distribution of biotite porphyroblasts. The major component (Fe, Mn, Mg, Ca) zoning presents systematic variations across differently sized garnet crystals, indicative of progressive nucleation and growth of garnet in equilibrium with an evolving matrix composition at increasing $P$- $T$ conditions. Annular features with the same Ca concentration in the analysed garnet crystals are used as markers of the contemporaneous growth of specific segments in crystals of different sizes. The slopes of compositional gradients correlate with crystal size, with smaller crystals showing steeper gradients for equivalent segments in the largest crystals of the rock. The chemical signature and microstructural properties of garnet suggest that growth rates were anisotropic, interface-controlled and size-dependent. Since similar concentrations and distribution patterns are observed for Sc, Ti, V, Co, Y and rare earth elements (Gd to Lu) across the differently sized crystals at the positions of the markers defined by the low-Ca annuli in the crystals, the quasi-equilibration of these elements at the centimetre scale across the intergranular medium can be inferred. A possible explanation for the observed trace element distribution across the garnet population is a sufficiently slow heating rate during prograde metamorphism, which provided the time required for the efficient transport of trace elements in the intergranular medium during garnet growth. Crystallization simulations using equilibrium thermodynamics indicate garnet growth over an interval of approximately 60°C and 1 kbar until peak conditions of approximately 570°C and 4.5 kbar. Previously published Lu-Hf garnet-whole rock ages coupled with our $P$- $T$ constraints indicate that heating rates could have been as slow as 1.3°C/Myr, suggesting that interface-controlled, size-dependent growth is not restricted to metamorphic garnet that crystallized rapidly and at fast heating rates ( $\ge 100^{\circ }$ C/Myr), as previously observed in a mica schist from the Barrovian garnet zone of Sikkim. The approach developed in this study provides a quantitative means to estimate the minimum length scales of major and trace element equilibration in the intergranular medium. This information is required to critically assess thermodynamic models of metamorphism and the ages used to constrain metamorphic histories. This approach may be helpful to identify crystal growth mechanisms and trace elements' equilibration length scales in natural samples and across the range of metamorphic conditions and durations. The case study presented here emphasizes the importance of the microstructural and chemical characterization of crystal populations for the study of crystallization kinetics and the extent of equilibrium during prograde metamorphism.