Thermal-chemical conditions of the North China Mesozoic lithospheric mantle and implication for the lithospheric thinning of cratons

Cratons are the most ancient parts of continents that are underlain by thick, cold, old and refractory lithospheric roots. However, how cratonic roots remain stable for billions of years and become remobilized later is still not well understood. The eastern North China Craton (NCC) is the best region to illuminate this issue because of its well-known lithospheric thinning and decratonization during the Mesozoic–Cenozoic. The thinning mechanism is debated because of limited constraints on the thermal-chemical conditions (lithology and P–T–H₂O–fo₂ ) of the Archean lithospheric mantle before and during its removal. Here, we provide constraints on these thermal-chemical conditions for the Archean lithospheric mantle beneath the eastern NCC during its extensive thinning in the form of whole-rock chemical and Sr–Nd–Pb isotopic compositions and mineral (especially olivine) chemistry of the Early Cretaceous primitive basalts (MgO > 10 wt.%) from Yixian and Sihetun in the western Liaoning Province.

Our data support a model in which the Yixian and Sihetun basalts were derived from metasomatized Archean lithospheric mantle under shallow (∼50–60 km), hot (∼1,290–1,350 °C) conditions. This indicates the existence of a relict (∼25 km) of the Archean lithospheric mantle during the Early Cretaceous, supporting gradual or episodic erosion of the eastern NCC lithospheric mantle. Furthermore, the NCC lithospheric mantle was not only widely rehydrated (>1,000 ppm H₂O) but also highly oxidized (Δlogfo_{2 FMQ} =+1.5∼+1.9 at 1.7–2.0 GPa) during its extensive thinning. Such rehydration and oxidization are demonstrated to be closely related to wet upwelling from the Mantle Transition Zone (MTZ) triggered by the deep subduction of the Paleo-Pacific oceanic slab in the period ∼200–125 Ma. We emphasize that the water released from the upwelling MTZ component and associated hydrous melt influx played a key role in the lithospheric thinning of the eastern NCC by oxidizing the lithospheric mantle and lowering its melting point, which led to redox melting, promoting the erosion of cratonic lithosphere. Our study provides key evidence for the role of deep volatile cycling from the MTZ in modifying thermal-chemical conditions and in the lithospheric thinning of cratons.