Metasomatism of mesozoic and cenozoic lithospheric mantle beneath the north china craton: Evidence from phlogopite-bearing mantle xenoliths

This paper reports the mineral compositions, clinopyroxene trace element concentrations and Sr-Nd isotopes for the newly-discovered phlogopite-bearing spinel lherzolite and olivine clinopyroxenite xenoliths from three different localities ( Hannuoba, Hebei Province; Jining Sanyitang, Inner Mongolia; Hebi, Henan Province) of the North China Craton. Systematic comparison with those phlogopite-free spinel lherzolite xenoliths from the same localities reveals that the pblogopite-bearing peridotitic xenoliths have relatively higher Al2O3, CaO, Na2O, K2O, TiO2 contents and lower MgO contents than those phlogopite-free counterparts. The former also has higher LREE concentrations, but relatively less radiogenic Sr-Nd isotopic ratios. This demonstrates that mantle metasomatism can not only enrich the basaltic components and trace element concentrations, but also make a decrease in Mg-# of the peridotites and olivines and a relative depletion in Sr-Nd isotopes. Rb-87/Sr-86-Sr-87/(86) Sr isochrons of the phlogopite-bearing xenoliths indicate that mantle metasomatism happened in the Mesozoic and/or Cenozoic time. The metasomatic agent was derived from the asthenosphere. The result also manifests that the widespread similarity of the geochemical features such as major and trace elements and isotopic compositions in the Cenozoic lithospheric mantle beneath the North China Craton to those "oceanic" lithospheric mantle could be as a result of the ubiquitous presence of the interaction between the old refractory peridotites and the infiltrated asthenospheric melt, rather than the actually newly-accreted lithospheric mantle.