Fertile lithospheric mantle underlying ancient continental crust beneath the northwestern North China craton: Significant effect from the southward subduction of the Paleo–Asian Ocean

The nature of the subcontinental lithospheric mantle beneath the western North China craton is poorly known, which hinders understanding of the cratonic response to the southward subduction of the Paleo-Asian Ocean. This study integrated mineral chemical data from spinel lherzolite xenoliths from newly discovered Late Cretaceous or Cenozoic Langshan basalts in the northwestern part of the craton with data from other localities across the western North China craton to constrain the nature of the subcontinental lithospheric mantle and lithospheric mantle reworking and accretion processes. Compositions of mineral cores (i.e., Mg# in olivine = 88-91) and pressure-temperature estimates (similar to 1.2 GPa, similar to 950 degrees C) suggest that the Langshan xenoliths/xenocrysts represent fragments of the uppermost subcontinental lithospheric mantle and experienced <15% melt extraction, similar to mantle xenoliths from Siziwangqi and Hannuoba along the northern margin of the western North China craton. Disequilibrium characteristics in the Langshan xenoliths/xenocrysts, including spongy coronae around pyroxene and compositionally zoned olivine, can be formed by partial melting and by ionic diffusion with silicate melts in the subcontinental lithospheric mantle, respectively, both occurring shortly before the eruption of host basalts. Metasomatism is recorded in clinopyroxene cores by concomitant enrichments in light rare earth elements and high field strength elements and was likely related to the migration of silicate melts derived from a mantle source modified by slab fluids/melts during Paleozoic time. These lines of evidence suggest that the subcontinental lithospheric mantle along the northern margin of the western North China craton is mainly fertile, similar to the Phanerozoic subcontinental lithospheric mantle in the eastern part of the craton. Considering (1) the coexistence of decoupled fertile lithospheric mantle and Archean continental crust both on the northern margin of the western North China craton and in the eastern North China craton, and (2) the sharp decrease in lithospheric thickness from the inner part to the northern margin of the western North China craton, the subcontinental lithospheric mantle beneath the northwestern North China craton likely has been strongly rejuvenated or replaced by fertile and noncratonic mantle. Combined with other evidence, the mantle replacement and accompanying meta-somatism were likely triggered by the southward subduction of the Paleo-Asian Ocean.