Older orogens cooled slower: new constraints on Orosirian tectonics from garnet diffusion modeling of metamorphic timescales, Jiaobei terrain, North China Craton

Metamorphic timescales harbor crucial information for quantifying lithospheric processes and identifying tectonic regimes. The Orosirian Period is known for global orogenesis during 2.1‒1.8 Ga that possibly signifies the emergence of global-scale plate tectonics. While the metamorphic pressure–temperature (P–T) paths and T/P ratios have been extensively used to characterize 2.1–1.8 Ga orogens, their detailed metamorphic timescales remain underutilized. In this study, we use garnet diffusion modeling to explicitly explore the metamorphic timescales of dominant pelitic granulites in the Jiaobei terrane, the southwest part of the Jiao-Liao-Ji orogenic belt (JLJB), North China Craton. The pelitic granulites record high-pressure granulite-facies metamorphism of 13–14 kbar and 860–870 °C, followed by fast decompression (4‒21 mm yr−1) with some degree of heating to medium-pressure granulite-facies retrogression of 7‒8 kbar and 865‒950 °C at c. 1.85 Ga. The subsequent cooling is nearly isobaric, but likely nonlinear with initially faster and later slower cooling (17‒32 °C Myr−1, 950/865–700 °C; 5–7 °C Myr−1, 700–600 °C, respectively). The determined clockwise P–T–t path provides evidence for convergent plate boundary processes and the fast exhumation is likely related to an extensional setting. The relatively fast exhumation and cooling rates at high temperatures, as obtained via diffusion chronometry, represent the fastest rates yet reported from 2.1 to 1.8 Ga orogens and are several times to one order of magnitude slower than those observed in modern orogens. In the light of recent numerical modeling, we suggest that these slower metamorphic rates of the c. 2.1‒1.8 Ga JLJB orogenic rocks are consistent with the features of orogenesis occurring at higher mantle temperatures, such as hotter orogenic thermal structures, less over-thickened orogenic crusts, and distributed strain patterns. Therefore, the timescale information from the JLJB indicates that Paleoproterozoic plate tectonics under higher temperatures might have caused subduction and collisional orogenesis to be distinct from their modern style. Notably, the detailed ancient metamorphic timescales achieved by diffusion modeling cannot be resolved geochronologically, highlighting the necessity of applying diffusion chronometry to quantify ancient tectonic processes.