In the contact aureole of the Makhavinekh Lake Pluton (MLP), Labrador, garnet resorption caused redistribution of Lu and loss of Hf, creating spuriously young Lu-Hf garnet ages. Garnet grew during granulite facies regional metamorphism at 1860-1850 Ma. At 1322 Ma, garnet rims were replaced by coronas of cordierite and orthopyroxene during contact metamorphism. Garnet-ilmenite Lu-Hf geochronology using bulk-garnet separates yields apparent ages that young from 1876 ± 21 Ma at 4025 m from the contact to 1396 ± 8 Ma at 450 m from the contact. Toward the contact, garnet crystals are progressively more resorbed. Concentrations of Lu measured by LA-ICP-MS along radial traverses on central sections through relict garnet decrease gently away from the cores but rise steeply within 50-200 lm of the edges of the relicts. Enrichments of Lu in rims of relict garnet demonstrates strong partitioning of Lu into garnet during resorption and modest intracrystalline diffusion. Hafnium distributions could not be measured, but considering the strong incompatibility of Hf with garnet, it is likely that nearly all Hf in resorbed portions of the garnet was lost from the crystals. Lu-Hf ages in the aureole are thus controlled predominantly by this retention of Lu and loss of Hf during garnet resorption. This deduction was tested with a simple numerical model in which the partial retention of Lu and loss of Hf is tracked as a population of garnet is resorbed. Assuming a spherical geometry for garnet porphyroblasts, Rayleigh fractionation is used to approximate initial Lu zoning profiles ranging from flat to steeply decreasing toward garnet rims. The model simulates: (i) Lu-Hf decay for a specified period before resorption; (ii) instantaneous resorption with retention of Lu and loss of Hf from the resorbed portion of the crystal and (iii) Lu-Hf decay during a specified period after resorption. Several parameters influence the modelled age, but garnet resorption and Lu retention are the primary factors. When all other parameters are held constant, larger amounts of resorption and higher degrees of Lu retention produce younger apparent ages (false ages). Similarly, flatter initial Lu profiles yield younger apparent ages as a consequence of the larger proportion of Lu and Hf that resides in the outer portions of the porphyroblast. The difference between the apparent and actual ages is greater if the duration of the pre-resorption decay period is large relative to the post-resorption decay period. Larger crystals in a Gaussian crystal-size distribution (CSD) generally dominate the Lu-Hf budget and produce an older apparent age relative to the age of the mean crystal size. Compared to a symmetrical Gaussian CSD, positively skewed CSDs result in reduced resorption of large crystals and produce an older apparent age. Application of the model to the MLP aureole, positing growth at 1850 Ma and resorption at 1320 Ma, yields model ages that young from 1850 to 1374 Ma toward the contact, in good agreement with the apparent ages determined from geochro...

High-resolution, garnet-based pressure-temperature (P-T) paths were obtained for nine rocks across the Himalayan Main Central Thrust (MCT) (Marsyangdi River transect, central Nepal). Paths were created using garnet and whole rock compositions as input parameters into a semiautomated Gibbs free-energy-minimization technique. The conditions recorded by the paths, in general, yield similar T but lower P compared to estimates from mineral equilibria and quartz-in-garnet Raman barometry. The paths are used to modify a model based on a two-dimensional finite difference solution to the diffusion-advection equation. In this model, P-T paths recorded by the footwall garnets result from fault motion at specified times, thermal advection, and alteration of topography. The best fit between the high-resolution P-T paths and model predictions is that from 25 to 18 Ma, samples within the MCT footwall moved at 5 km/Ma, while those in the hanging wall moved at 10 km/Ma. Under these conditions, topography grew to 3.5 km. A pause in activity along the MCT between 18 and 15 Ma allows heat to advect and may be due to a transfer of tectonic activity to the structures closer to the Indian subcontinent. During this time, the topography erodes at a rate of 1.5 km/Ma. Thrusting within the MCT footwall reactivates between 8 and 2 Ma with exhumation rates up to 12 mm/yr since the Pliocene. The results suggest the potential for the highestresolution garnet-based P-T paths to record both the thermobarometric consequences of fault motion and large-scale erosion.Plain Language Summary The Main Central Thrust (MCT) is a major Himalayan fault system largely responsible for the generation of its high topography. Garnets across the MCT record their growth history in the crust through changes in their chemistry. These chemical changes can be extracted and modeled. Here we report detailed pressure-temperature paths recorded by garnets collected across the MCT along the Marsyangdi River in central Nepal. The paths track evolving conditions in the Earth's crust when the MCT was active during the growth of the Himalayas. The results suggest that the MCT formed as individual rock packages moved at distinct times. Further modeling makes predictions about how the Himalayas developed, including that the MCT may have ceased motion 18-15 million years ago, as other faults closer to the Indian subcontinent became active, and that it reactivated 8-2 million years ago, leading to the generation of high topography. The modeling also suggests that very high erosion rates occurred within the range after reactivation. Although garnets have long been used to understand how fault systems evolve, we provide details of an approach that allows higher-resolution data to be extracted from them and show how they could be used to track large-scale erosion.

Numerical simulations of diffusion‐controlled nucleation and growth of garnet porphyroblasts in regionally metamorphosed rocks constrain interfacial energy and rates of nucleation and Al intergranular diffusion. The 13 rocks analysed in this study were collected from seven localities exhibiting a diverse range of crystallization conditions. Kinetic parameters governing nucleation and intergranular diffusion were adjusted iteratively to achieve fits between simulated and natural porphyroblastic textures. Model fits were assessed primarily from textural characteristics precisely measured by high‐resolution X‐ray computed tomography. Interfacial energy for heterogeneous nucleation ranges from 0.007 to 0.255 J m−2 for the sample suite, assuming shape factors in the range 0.01–1.0. Nucleation rates change through space and time due to growth and impingement of Al depletion zones surrounding porphyroblasts. In some models, the overall rock‐wide nucleation rate rises steeply, achieves a steady state, and then falls rapidly as reactants are consumed; in others, the steady state is not achieved, but instead the rate simply peaks before falling. Maximum rock‐wide nucleation rates range from 10−14.7 to 10−10.7 nuclei cm−3 s−1, and maximum local rates range from 10−13.7 to 10−9.7 nuclei∙cm−3 s−1 depending on Al supersaturation. Diffusive fluxes of Al are well constrained by the simulated textures, but rates of intergranular diffusion are subject to uncertainties in Al solubility and interconnected porosity. Best estimates of Al diffusivities at 600 °C span 10−12.3 to 10−10.5 m2 s−1 for the sample suite, a narrow range considering natural variability and the uncertainties in solubility and porosity. Eliminating some models suspected of higher uncertainty for these quantities yields diffusivities at 600 °C near 10−11.0 m2 s−1, with dispersion of less than half an order of magnitude. These simulations, which are among the first attempted for regionally metamorphosed rocks, emphasize that: (i) nucleation rates vary markedly in time and space during crystallization; (ii) nucleation extends well beyond equilibrium conditions; (iii) Al diffusivity likely varies over only a narrow range across common metamorphic circumstances; and (iv) better determinations of both Al solubility and interconnected porosity are needed to constrain rates of Al intergranular diffusion more precisely.

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Numerical models of diffusion‐controlled nucleation and growth of garnet crystals, which successfully replicate diverse textures in 13 porphyroblastic rocks, yield quantitative estimates of the magnitudes of departures from equilibrium during crystallization. These estimates are derived from differences in chemical potential between subvolumes containing stable product assemblages and those containing persistent but metastable reactant assemblages. The magnitude of disequilibrium is evaluated in terms of the thermal overstepping, which is commonly referenced to the garnet‐in isograd; the reaction affinity in the intergranular fluid at the site and time of each nucleation event, and on average throughout the rock, and the ‘latent energy of reaction’ per unit volume, a measure of the average unreacted capacity of the bulk rock, which describes its overall metastability. Across all of the models, the first crystals nucleate after 5–67 °C of thermal overstepping (correspondingly, 0.7–5.8 kJ mol−1 of 12‐oxygen garnet); the maximum reaction affinity averaged across the intergranular fluid is between 4.7 and 16.0 kJ mol−1 of 12‐oxygen garnet; and the maximum latent energy of reaction ranges from 7.3 to 51.7 J cm−3. These results demonstrate that impediments to crystallization significantly delay nucleation and retard reaction, with the consequence that nucleation of new crystals extends throughout nearly the entire crystallization interval. This potential for protracted reaction during prograde metamorphism, with reactions continuing to temperatures and pressures well beyond equilibrium conditions, suggests the likelihood of overstepping of multiple – possibly competing – reactions that can progress simultaneously. Isograds and ranges of stability for metamorphic assemblages along a metamorphic field gradient may therefore be significantly offset from the positions predicted from calculations based on equilibrium assumptions, which poses a substantial challenge to accurate interpretations of metamorphic conditions and processes.