Prolonged prograde metamorphism and nonlinear cooling in early plate tectonics: Evidence from garnet diffusion modeling

Abstract

The establishment and evolution of early plate tectonics are crucial for the Earth’s environment to develop toward habitability. However, the rates and timescales of early plate tectonic processes remain poorly constrained due to the limited temporal resolution of in situ radiometric dating. In this study, we apply diffusion chronometry to pelitic granulites from the Jiaobei terrane, North China Craton, integrating phase equilibria modeling, thermobarometry, and U-Pb geochronology. The studied pelitic granulites record peak high-pressure (HP) granulite-facies metamorphism (>1.0 GPa, >800 °C), followed by post-peak decompression with possible heating to 0.5–0.7 GPa and 880–970 °C, and subsequent cooling to ∼ 0.5 GPa and ∼ 600 °C. In situ zircon and monazite U-Pb dating indicates that post-peak decompression and retrograde cooling occurred at 1.86–1.85 Ga. Diffusion simulations of garnet Lu-Hf and Sm-Nd, combined with their isotopic ages, suggest that prograde metamorphism may have persisted for ∼ 80 Myr, implying slow subduction or prolonged continental collision, consistent with lithospheric peeling processes under hotter Paleoproterozoic mantle conditions. The result from the Jiaobei terrane thus shows inefficient surface material transfer to depth during the Paleoproterozoic. Additionally, diffusion modeling of garnet-biotite Fe-Mg exchange reveals a nonlinear cooling history, with an initial rapid cooling phase (800–700 °C) within a few million years, followed by prolonged slower cooling (700–600 °C) over tens of millions of years. The nonlinear cooling behavior resembles that of modern orogenic granulites, but the generally slower rates likely reflect hotter crustal thermal regimes and weaker lithospheric strength compared to their Phanerozoic counterparts. Our findings highlight the power of diffusion chronometry in resolving high-resolution metamorphic timescales beyond the limits of radiometric dating, offering critical insights into the geodynamics of early plate tectonics.