Nature and evolution of lithospheric mantle beneath the western North China Craton: Constraints from peridotite and pyroxenite xenoliths in the Sanyitang basalts

This paper reports petrology, major and trace element abundances in bulk-rocks and minerals, and clinopyroxene Sr-Nd-Hf compositions from a suite of mantle xenoliths entrained in Cenozoic Sanyitang alkali basalts, Jining County in the Inner Mongolia Suture Zone. The aim is to constrain the nature and evolution of the lithospheric mantle beneath the western North China Craton. The Sanyitang mantle xenoliths are mainly anhydrous spinel lherzolites with subordinate websterites. Petrological and geochemical features of the spinel lherzolites suggest that they are refractory mantle residues formed by variable degrees of melt extraction (3–14%), followed by silicate and carbonatite metasomatism. The Sr-Nd-Hf isotopic compositions (87Sr/86Sr = 0.7031–0.7037, εNd = +5.2 − +11.2 and εHf = +8.4 − +29.3) of clinopyroxene separates from the majority of the spinel lherzolites lie on, or close to, the mantle array defined by the composition of ocean-island basalts and mid-ocean ridge basalts. On the other hand, clinopyroxene separates from a minority of spinel lherzolites (SYT09-01, SYT09-09, SYT11-01) have high 87Sr/86Sr (0.7040–0.7049), εNd (+12.4 − +21.9) and εHf (+58.2 − +67.8), above the mantle array, reflecting long-term depletion of the mantle. The websterites are characterized by lower MgO (18–22 wt%) and higher Al2O3 contents (4.5–5.6 wt%), and lower Mg# (86.2–87.3) than the spinel lherzolites (Mg# = 89.3–90.8; Al2O3 = 0.9–3.6 wt%). Their convex upward rare earth element patterns, along with lower Ni contents (334–389 ppm) indicate that they were cumulates fractionated from basaltic magmas circulating through the upper mantle. The trace element patterns of clinopyroxenes from three phlogopite-free websterites show enrichment in large ion lithophile and negative anomalies for the high field strength elements. In addition, the phlogopite-free websterite clinopyroxene Sr-Nd-Hf isotope ratios plot in the enriched lithospheric field (87Sr/86Sr = 0.7058–0.7061, εNd = −9.2 to −8.4 and εHf = −5.2 to −3.7 respectively), which implies the contribution of recycled lower crustal materials to their source. The presence of phlogopite in some Sanyitang websterites, coupled with strong enrichments in both light rare earth elements and large ion lithophile elements, as well as high 87Sr/86Sr (0.7065–0.7068) and negative εNd (−1.5 to −0.3) and εHf (−6.1 to −4.8) values of clinopyroxene, provide strong evidence for the existence of subduction-related hydrous fluids/melts metasomatism. This study demonstrates that the Sanyitang mantle xenoliths and their component minerals record the complex petrological history and local heterogeneity of the subcontinental mantle lithosphere under the western North China Craton.