Multi-stage metasomatism of lithospheric mantle by asthenosphere-derived melts: evidence from mantle xenoliths in daxizhuang at the eastern North China craton
A detailed study on petrology and mineral chemistry of 12 mantle xenoliths from Late Cretaceous basaltic lava flows at Daxizhuang has been conducted to constrain the nature and secular evolution of the Mesozoic lithospheric mantle beneath the eastern North China Craton (NCC). The Daxizhuang mantle xenoliths are mainly anhydrous spinel lherzolite, with subordinate olivine websterite and rare spinel wehrlite. Based on petrographic and geochemical characteristics, the Daxizhuang spinel lherzolite can be subdivided into two groups. Group 1 lherzolite has olivine with Fo contents from 89.4 to 90.6 and LREE-depleted through spoon-shaped to LREE-enriched REE (rare earth element) patterns in clinopyroxene grains, reflecting variable degrees of partial melt extraction (up to 18%) overprinted by later incipient metasomatism. In contrast, Group 2 lherzolite is strongly metasomatized and enriched in iron, as evidenced by relatively lower Fo contents of olivine (84.9 to 88.2) than that from the Group 1 lherzolite and convex-upward trace element patterns in clinopyroxene grains. The Daxizhuang olivine websterite and spinel wehrlite are rich in Fe, Al and low Mg# in olivine and pyroxenes, and slight Ti-enrichment in spinel. Numerical modeling of the Mg# shows that the Daxizhuang olivine websterite and wehrlite may have resulted from the reaction between host residual lherzolite and evolved, silicate melts at high melt/rock ratios. The result of this study indicates that the lithospheric mantle beneath Daxizhuang underwent multiple metasomatism through asthenosphere-lithosphere interaction, which plays an important role in the transformation of the lithospheric mantle of the NCC at least during Late Cretaceous time.