Relict refractory mantle beneath the eastern North China block: significance for lithosphere evolution

Xenolith-bearing Neogene basalts occur in Hebi county, at the southern end of the Taihangshan–Luliangshan paleo-rift zone in the North China block of the Sino–Korean craton. This locality lies on the North–South Gravity Lineament, which divides the craton into two geophysical zones. The spinel peridotite xenoliths hosted by the basalts can be divided into two groups based on the Mg# values of olivine. The whole-rock compositions of the low-Mg (Fo<91) xenoliths have high Al2O3+CaO (average 3.06 wt.%) and Na2O (average 0.19 wt.%), and low Mg/Si; they are similar to xenoliths from many localities in eastern China and other Phanerozoic volcanic areas. The dominant high-Mg (Fo≥92) group consists of harzburgites (66%) and depleted lherzolites (34%) with coarse-grained (mainly) and porphyroclastic microstructures, and high-Cr spinels (mean Cr#=0.51). The high-Mg xenoliths have low Al2O3+CaO (average 1.36 wt.%) and high Mg/Si, are in general strongly depleted in HREE, Ti, Zr and Y, and are compositionally similar to xenoliths in kimberlites from Archean cratons. The Archean lithospheric root beneath the eastern part of the Sino–Korean craton, which was sampled by Paleozoic kimberlites, was largely replaced by fertile Phanerozoic mantle during Mesozoic extension and subduction events. The high-Mg xenoliths are interpreted as relics of the Archean lithosphere, preserved locally at relatively shallow levels, and re-equilibrated to spinel facies in a regime of high heat flow caused by advective heat transport during extension. Their calculated mean room-temperature density (3.36 g/cm3) and Vp (8.39 km/s) are consistent with this interpretation and with geophysical data for the Hebi area. Regional geophysical data suggest that similar material may be widespread in the uppermost mantle west of the North–South Gravity Lineament, and more locally in the eastern part of the former craton.