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

摘要

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 peridotites 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 Cenozoic basalts from the North China craton and in the rim (Mg# =76.8-83.9) close to olivine phenocrysts of the host basalts (75.7-79.0). These compositional features as well as rounded crystal shapes and smaller grain sizes (300-800 mum) demonstrate that these zoned olivines are mantle xenocrysts, i.e. disaggregates 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 (low-Mg )and radiogenic isotope-enriched peridotites, leading to the loss of old lithospheric mantle.