Records of Himalayan Metamorphism and Contractional Tectonics in the Central Himalayas (Darondi Khola, Nepal)

Catlos, Elizabeth J.

doi:10.1002/9781119773856.ch6

Cited by 2 publications

(1 citation statement)

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“…Such an approach has been extensively applied in studies of rocks that experienced Barrovian-type metamorphism utilizing thermodynamic equilibrium models and the assumption of sequential fractionation of the effective bulk rock composition during garnet growth (e.g., Catlos, 2023;Faryad et al, 2022;McCarron et al, 2014). Although largely employed, this approach does not account for the kinetics of garnet crystallization, which is related to the energy barriers of interface processes and diffusion across the system (e.g., Carlson, 2011;Gaidies et al, 2017).…”

Section: Introductionmentioning

confidence: 99%

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

Yogi,

Gaidies,

Heldwein

et al. 2024

Journal Metamorphic Geology

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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.

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