Multi-stage mantle accretions and metasomatisms related to peripheral subduction or collision in the northern North China Craton: Evidence from the Nangaoya peridotite xenoliths

Abstract

Peridotite xenoliths from the northern North China Craton (NCC) are characterized by lithological and chemical heterogeneity, related to mantle accretion and metasomatism. However, the accretion mechanism and origin of metasomatic agent are contentious. Here, we report the petrology, whole-rock major elements and ReOs isotopes, and in situ mineral major- and trace-elements as well as Sr isotopic compositions for peridotite xenoliths in the Nangaoya area to evaluate the evolution of the lithospheric mantle beneath the northern NCC. Nangaoya peridotites can be divided into Group A harzburgites and Group B lherzolites. Group A harzburgites are refractory with high Fo contents (91.8–92.9) in olivine and have low whole-rock 187Os/188Os ratios of 0.11418, giving ~2.1 Ga TRD age. Clinopyroxenes in Group A show enrichment of incompatible element with low Ti/Eu and high (La/Yb)N ratios. These characteristics indicate that Group A harzburgites should be the relics of the Archean–Paleoproterozoic “cratonic” mantle and witness carbonatite metasomatism with its agent from asthenosphere. Comparatively, Group B lherzolites are transitional with low Fo contents (90.2–91.9) in olivine and have high whole-rock 187Os/188Os ratios (0.11635–0.112502), giving 1.8–1.2 Ga TRD age. Thus, these lherzolites are considered as accretive mantle components formed by the cooling and re-extracting of the upwelling asthenosphere related to the collision of Eastern and Western blocks. Furthermore, Group B lherzolites can be subdivided into two sub-groups. Group B1 clinopyroxenes without sieve-textured rims are depleted in incompatible element, which was interpreted as weak or without metasomatism, whereas those in Group B2 lherzolites with sieve-textured rims display enriched light rare earth elements (LREE) and various trace element patterns, resulted from different types of strong metasomatism. Clinopyroxenes in Group B2–1 exhibit depletion of high field strength elements (HFSE), with high Ti/Eu but low (La/Yb)N ratios, and the cores of them have higher 87Sr/86Sr ratios (0.70355–0.70443) than the rims (0.70352–0.70406), demonstrating that Group B2–1 lherzolites may have witnessed hydrous CO2-rich silicate metasomatism from subducted plates in cores and a new stage silicate melt metasomatism from asthenosphere in rims. Group B2–2 lherzolites just record later silicate melt metasomatism. These two stage metasomatisms may be triggered by the mantle accretion events in response to the peripheral subduction in the northern NCC during the Phanerozoic. Based on the above results, we suggest that the lithospheric mantle beneath the Nangaoya area underwent multiple modifications through crust–mantle recycling or asthenosphere–lithosphere interaction related to subduction/collision processes, which plays important roles in the evolution of the mantle in the northern NCC.