Mantle olivine xenocrysts entrained in Mesozoic basalts from the North China craton：Implication for replacement process of lithospheric mantle

Mesozoic (125 Ma) Fangcheng basalts fromShandong Province contain clearly zoned olivines that arerare in terrestrial samples and provide first evidence for thereplacement of lithospheric mantle from high-Mg peridotitesto Iow-Mg peridotites through peridotite-melt reaction.Zoned olivines have compositions in the core (Mg# = 87.2--90.7) similar to those olivines from the mantle peridotiticxenoliths entrained in Cenozoic basalts from the NorthChina craton and in the rim (Mg# = 76.8--83.9) close to oli-vine phenocrysts of the host basalts (75.7--79.0). Thesecompositional features as well as rounded crystal shapes andsmaller grain sizes (300—800 μm) demonstrate that thesezoned olivines are mantle xenocrysts, i.e. disaggregates ofmantle peridotites. Their core compositions can representthose of olivines of mantle peridotites. The zoned texture ofolivines was formed through rapid reaction between the oli-vine xenocryst and the host basalt. This olivine-basaltic meltreaction could have been ubiquitous in the Mesozoic litho-spheric mantle beneath the North China craton, i.e. an im-portant type of the replacement of lithospheric mantle. Thereaction resulted in the transformation of the Paleozoic re-fractory (high-Mg) peridotites to the late Mesozoic fertile(Iow-Mg) and radiogenic isotope-enriched peridotites, lead-ing to the loss of old lithospheric mantle.

Mantle olivine xenocrysts entrained in Mesozoic basalts from the North China craton: Implication for replacement process of lithospheric mantle

Mesozoic (125 Ma) Fangcheng basalts from Shandong Province contain clearly zoned olivines that are rare in terrestrial samples and provide first evidence for the replacement of lithospheric mantle from high-Mg peridotites to low-Mg per-idotites through peridotite-melt reaction.Zoned olivines have compositions in the core (Mg#=87.2—90.7) similar to those olivines from the mantle peridotitic xenoliths entrained in Ceno-zoic basalts from the North China craton and in the rim (Mg#=76.8—83.9) close to olivine henocrysts of the host basalts (75.7—79.0). These compositional features as well as rounded crystal shapes and smaller grain sizes (30—800 μm) demon-strate that these zoned olivines are mantle xenocrysts, i.e. disag-gregates of mantle peridotites. Their core compositions can represent those of olivines of mantle peridotites. The zoned texture of olivines was formed through rapid reaction between the olivine xenocryst and the host basalt. This olivine-basaltic melt reaction could have been ubiquitous in the Mesozoic lithospheric mantle beneath the North China craton, i.e. an important type of the replacement of lithospheric mantle. The reaction resulted in the transformation of the Paleozoic refractory (high-Mg) peridotites to the late Mesozoic fertile (Iow-Mg) and radiogenic isotope-enriched peridotites, leading to the loss of old lithospheric mantle.