An Unusually Warm Upper‐Crust in the Late Paleozoic North China Continental Arc: Implications for the Thermal Modification of the Giant Bayan Obo REE Deposit

The genesis and timing of formation of the giant Bayan Obo deposit, the world's largest rare earth element (REE) deposit in the western part of the late Paleozoic northern North China continental arc (NCA), are highly controversial due to complex mineral assemblages and reported ages of mineralization. We conducted new zircon U‐Pb and 40Ar/39Ar dating of metamorphic and igneous Neoarchean to Permian mid‐to upper‐crustal rocks exhumed along a north−south corridor across the western NCA and its retroarc foreland. The results show that the mid‐ to upper‐crust of the western part of the NCA has been strongly affected by thermal modifications during arc construction in the late Paleozoic, while the retroarc foreland remained thermally stable. Our results first reveal an unusually warm upper‐crust around Bayan Obo during the late Paleozoic with high geothermal gradients of 50.0 ± 8.3 to 88.3 ± 8.3°C/km and strong thermal modification of the upper‐crust during arc construction. This unusually warm upper‐crust and high geothermal gradients resulted in intensive thermal perturbations and recrystallization of REE‐bearing minerals in the Bayan Obo deposit, as well as formation of high‐grade REE ores and complete or partial resetting (U‐)Th‐Pb isotopic systems of REE minerals. Our identification of the unusually warm upper‐crust and high geothermal gradients in the western part of the NCA provides important constraints on genesis, timing and thermal modification of the giant Bayan Obo deposit, as well as other REE deposits with complex isotopic ages. Plain Language Summary The Bayan Obo deposit is the largest rare earth element (REE) deposit in the world. Located in North China, its formation and timing of mineralization have been debated for over 60 years. We conducted thermochronometric investigations on metamorphic and igneous minerals (hornblende, muscovite and biotite) of the mid‐to upper‐crusal rocks around Bayan Obo and concluded that anomalously high geothermal gradients led to extensive crustal mobilization and enrichment of the REE minerals to form the ore deposits. We suggest that these thermal perturbation events were also responsible for resetting isotopic clocks (e.g., (U‐)Th‐Pb systems) of REE minerals, ultimately causing uncertainties in the mineralization ages of the Bayan Obo deposit and geological reconstructions of the region. Key Points Archean‐Paleoproterozoic basement rocks in the northern North China continental arc have young 40Ar/39Ar ages of