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 the giant Bayan Obo deposit, world's largest rare earth element (REE) deposit in western part late Paleozoic northern North China continental arc (NCA), are highly controversial due to complex mineral assemblages reported ages mineralization. We conducted new zircon U-Pb 40Ar/39Ar dating metamorphic igneous Neoarchean Permian mid-to upper-crustal rocks exhumed along a north−south corridor across NCA its retroarc foreland. results show that mid- upper-crust has been strongly affected by thermal modifications during construction Paleozoic, while foreland remained thermally stable. Our first reveal an unusually warm around with high geothermal gradients 50.0 ± 8.3 88.3 8.3°C/km strong modification construction. This resulted intensive perturbations recrystallization REE-bearing minerals as well high-grade REE ores complete or partial resetting (U-)Th-Pb isotopic systems minerals. identification provides important constraints on genesis, other deposits ages.