Multi-stage metamorphism of high-pressure granulites in the northern Trans-North China Orogen records continental subduction/collision to exhumation: Implications for the Paleoproterozoic tectonic regime

The Paleoproterozoic era is an important tectonic transition period through Earth's history, and understanding its tectonic style may provide critical information on the early plate tectonics and orogenic processes. High-pressure (HP) granulites from the root of the early Precambrian orogens are natural laboratories for addressing this issue. Here, we present an integrated study of a suite of newly discovered Paleoproterozoic granulite-facies rocks in the Xuanhua complex within the northern Trans-North China Orogen (TNCO). Geothermobarometry and pseudosection modeling suggest that the Xuanhua HP granulites record clockwise P−T paths, including prograde stage M1 (8.5−10 kbar/690−730 °C) to peak stage M2 (13.0−14.8 kbar/850−870 °C), followed by heating with decompression to M3a (8.5−9.0 kbar/915−940 °C), and further cooling to M3b (6.1−7.3 kbar/c. 720 °C). Peak metamorphic P−T conditions lie in the medium T/P Barrovian facies series, suggesting that the thermal gradient (574-669 °C/GPa) of the Paleoproterozoic orogenic belt (TNCO) is ob­viously higher than those of the Phanerozoic HP/ultrahigh-pressure (UHP) subduction zones. Geochemical features and zircon U−Pb−Hf−O results indicate that the enriched mid-ocean ridge basalt (E-MORB) type protoliths of granulites were emplaced at c. 2.1-2.0 Ga with low-temperature hydrothermal alteration. Metamorphic zircons/titanites of all samples yield c. 1.92−1.90 Ga HP granulite-facies metamorphism and c. 1.84−1.81 Ga retrograde metamorphism. Thus, we proposed a new tectonic scenario involving the continental subduction/collision occurred prior to c. 1.92 Ga, and was followed by rapid exhumation from the base of a thicken lower crust (< 50 km; c. 1.92−1.90 Ga) to middle crust (c. 28−30 km; c. 1.84−1.81 Ga) accompanied with final cooling (< 1.81 Ga). The tectonothermal cycle was a long-lived process (> 110 Myr), and the exhumation occurred within c. 70−80 Myr at a slow rate of c. 0.5−1 mm/yr, which is much lower than the rapid exhumation rate in the Phanerozoic orogens. Combining the distribution of the similar retrograded eclogites/HP granulites on Earth, we further reveal a Paleoproterozoic (2.0−1.8 Ga) global continental subduction/collision pattern during the assembly of the Nuna supercontinent. Based on the global metamorphic dataset, the higher T/P and the slower exhumation rates of our data support that the warm and shallow subduction/collision is an important part of the Paleoproterozoic plate tectonic style, which is distinct from the modern plate tectonics.