Bilevel Magnetic Lower Crust Preserved in the North China Craton

Abstract:

Bilevel magnetic lower crust is preserved in the northern North China Craton revealed by the magnetic properties of the lower crustal xenoliths in Cenozoic basalts, and terrain granulite facies rocks. The upper layer (~24−33 km) is representative of strongly magnetic Archean terrain granulites, of which the mafic granulites have the highest susceptibility (κ, mean 65.33 × 10⁻³ SI) and natural remanent magnetization (NRM, mean 0.57 Am⁻¹). The magnetic minerals in the terrain granulites are dominantly coarse granular multi−domain magnetite and ilmenite−hematite solid solution with lamellar texture, which dominate the magnetic properties of the rocks and have > 90% contribution (average ~ 95%) to low-field susceptibility (χ_ferri%, here as the low-field susceptibility subtracting paramagnetic susceptibility) for most of the samples. By contrast, the lowermost layer (~33−42 km) is mainly composed of mafic granulite and pyroxenite as xenoliths, which are weakly magnetic with the κ and NRM of 0.9−1.5 × 10⁻³ SI and 0.04−0.1 Am⁻¹, respectively. The magnetic properties of xenoliths are controlled by paramagnetic silicates (olivine, pyroxene and feldspar) and small amounts of interstitial fine granular Fe−Ti oxides (e.g. magnetite) and sulfides (e.g. pyrrhotite), of which the χ_ferri% varies between 5% and 90% (average ~ 40%). As continuous underplating of mantle−derived basaltic magma at the prior crust−mantle boundary occurred from the late Mesozoic to Cenozoic (peak at ~120 Ma), a 8−10 km lowermost crust accreted (represented mainly by the weakly pyroxenite and mafic granulite xenoliths), associated with thinning of the old Archean crust and shallow of the original Moho to ~33 km, which is about 10 km away from the new Moho today (~41−42 km). Therefore, we suggest that this model of bilevel magnetic lower crust with varying depth and thickness may exist widespreadly beneath the North China Craton.