From Deep Residence to Rapid Exhumation: Constraints on the ~1.81 Ga Collapse of the Trans-North China Orogen from the Zanhuang Complex

The Paleoproterozoic Trans-North China Orogen (TNCO) is central to understanding the assembly of the North China Craton and the Columbia supercontinent. However, whether the metamorphic variations across the TNCO reflect diachronous tectonic events or differential exhumation processes remains a subject of ongoing debate. This study investigates the Daqian garnet-bearing amphibolites from the Zanhuang Complex to constrain the P–T–t evolution of the central TNCO.

Phase equilibria modeling reveals a well-preserved clockwise P–T trajectory characterized by peak high-pressure (HP) amphibolite-facies metamorphism (M2: 1.32–1.63 GPa, 715–750 °C), followed by near-isothermal decompression (M3: 0.61–0.88 GPa, 680–718 °C). Multi-mineral LA-ICP-MS U–Pb geochronology uncovers a significant temporal decoupling between peak and cooling stages: metamorphic zircons record the peak collision at ~1.87 Ga, whereas titanite, rutile, and apatite yield convergent cooling ages at ~1.81 Ga.

Our data indicate that the terrane experienced a prolonged deep-crustal stagnation (~60 Myr) with negligible cooling (<1 °C/Myr), followed by a pulse of extremely rapid exhumation (~22.5 °C/Myr) at ~1.81 Ga. We argue that the Zanhuang Complex underwent synchronous collision at ~1.87 Ga with the rest of the TNCO. Consequently, the observed regional metamorphic variations are not a result of diachronous tectonic arrival but rather reflect differential exhumation and isostatic rebound following lithospheric delamination. This rapid ~1.81 Ga event marks the definitive gravitational collapse of the orogen, providing new insights into the terminal stages of Paleoproterozoic orogenesis.