Thermal Structure of a Paleoproterozoic Orogen: A Case Study from the Wutai–Fuping Crustal Section

Abstract Exhumed orogenic belts contain the only physical record of the processing of Earth’s crust during tectonic cycles and are, therefore, the critical regions where constraints on geodynamic processes can be developed. Reconstructing metamorphic field gradients and thermal modeling are key approaches for investigating this record. This study examines the thermal structure of the Wutai–Fuping crustal section within the Paleoproterozoic Trans-North China Orogen (TNCO). The crustal section records a decrease in metamorphic grade from the Fuping area in the southeast to the Wutai area in the northwest. The metamorphic pressure–temperature (P–T) evolution of high-grade metamorphism in this crustal section is well established. However, constraints on mid- to low-grade metamorphism recorded in the crustal section are lacking. In this study, we constrain the petrochronology of the mid- to low-grade (epidote-amphibolite facies) metamorphic rocks through the application of phase equilibrium modeling, geothermobarometry, (3-µm) zircon SIMS U–Pb geochronology, and in situ garnet Lu–Hf geochronology. These datasets reveal peak P–T conditions of 0.6–0.8 GPa and 500–530°C at 1888–1863 Ma. To put this new dataset into a broader orogenic context and to understand its relationship to the timing of peak metamorphism at other crustal levels, in situ garnet Lu–Hf geochronology was also conducted on the higher-grade metamorphic rocks from the Wutai–Fuping crustal section. The results yielded peak ages of 1899 ± 31 Ma for lower-amphibolite facies rocks from the Wutai Shizui subgroup, 1872 ± 31 Ma for upper-amphibolite facies rocks from the Wanzi supracrustal rocks, and 1866 ± 29 Ma for granulite facies rocks from the Fuping TTG gneiss. Our data suggest that peak P–T conditions occurred at ca. 1.90–1.86 Ga across all metamorphic grades, defining a contemporaneous metamorphic field gradient varies from 25 to 18°C/km with increasing depth from 20 to 50 km, which reflect the peak geotherm. Combined with metamorphic zircon and monazite U–Pb ages of 1.96 to 1.80 Ga previously reported in the Wutai–Fuping area, our updated dataset constrains the durations of the prograde, peak, and retrograde metamorphic stages, which occurred at 1.96 to 1.90 Ga, 1.90 to 1.86 Ga, and 1.86 to 1.80 Ga, respectively. The thermal structure of the peak stage and the duration of the prograde stage are supported by the results of one-dimensional thermal modeling. The metamorphic field gradient and the durations of the metamorphic stages may characterize the long-lived TNCO across multiple crustal levels.