Continuous versus episodic metamorphism? Insights from pulsed garnet growth

Mineral reactions occurring during metamorphism govern mass transfer, fluid release, element mobility, and rock strength. Although changes in mineral assemblages with pressure-temperature-composition (P-T-X) conditions are well constrained, the mechanisms and rates of net-transfer reactions involving mineral growth and dissolution remain largely unresolved. Do these reactions proceed continuously or in discrete bursts, and are they primarily controlled by changes in P-T conditions? We address these questions using garnet from two contrasting tectonic settings in China: near-steady P-T conditions in the Wutai area (continental collision), and a prograde P-T path in the South Tianshan suture (oceanic subduction). In both samples, smooth major element zoning appears consistent with continuous growth, e.g., Mn shows bell-shaped profiles predicted by Rayleigh fractionation. In contrast, heavy rare earth elements (HREEs) are systematically more depleted and display sharper gradients, indicating diffusion-limited supply during growth. Diffusion-limited uptake modeling reveals four and three growth pulses in Wutai and Tianshan garnets, respectively, with progressively accelerating growth rate. The pulse boundaries coincide with HREE fluctuations, implying possible transient changes in fluid availability. These results show that garnet growth can be episodic and kinetically controlled, rather than continuously tracking P-T evolution. Although chemical potential gradients induced by changes in P-T conditions provide thermodynamic driving force, transient changes in element transport properties may act as a kinetic switch that triggers mineral reaction. More broadly, this implies that some crustal processes may operate in short-lived pulses rather than continuous, steady-state regimes.