Unraveling Orosirian geodynamics: high-resolution exhumation and cooling rates in a Paleoproterozoic orogen using Monte Carlo garnet diffusion chronometry

Abstract

Determining exhumation and cooling rates of regional metamorphic rocks is essential for deciphering orogenic dynamics and heat transport in the Earth's crust. Although radiometric dating is commonly used, its temporal resolution becomes coarser for older rocks, limiting its ability to resolve Precambrian metamorphic timescales. Diffusion chronometry, based on mineral zoning, offers age-independent temporal resolution but is affected by uncertainties in pressure–temperature conditions and diffusion coefficients, which have not been fully evaluated in Paleoproterozoic or older orogens. This study integrated Monte Carlo-based garnet Fe–Mg-Ca-Mn diffusion simulations with phase equilibria modeling to quantify exhumation and cooling rates of pelitic granulites from the Paleoproterozoic Jiao–Liao–Ji orogen, explicitly addressing uncertainty propagation. The granulites record peak pressures of 13–14 kbar at 850–870 °C, followed by heating during decompression to ultra-high-temperature conditions (~ 940 °C, ~ 6.5 kbar) within 2–9 Myr at ca. 1.86 Ga. Subsequent cooling to ~ 5 kbar and ~ 600 °C is nonlinear, with rapid cooling (up to 148 °C/Myr) above 800 °C, and slower cooling (~ 5 °C/Myr) below 700–600 °C. These diffusion-based timescales and rates, with uncertainties of 0.3–0.5 orders of magnitude (1σ), outperform current in situ radioisotope geochronology methods, providing refined constraints on Orosirian geodynamics. The heating during decompression and subsequent nonlinear cooling suggest potential parallels with modern mantle upwellings and extensional tectonics; slower cooling and exhumation rates obtained in this study (when compared to modern systems) potentially reflects a weaker Paleoproterozoic lithosphere. This research highlights the power of diffusion chronometry for understanding of early Earth history.