Pacific slab subduction-induced carbonatite mantle metasomatism in the eastern North China Craton
Abstract:Trace element and Sr–Li isotopic compositions of pyroxene and olivine in mantle peridotite xenoliths entrained by the Cenozoic Changle basalt from the eastern block of the North China Craton (NCC) were analyzed by LA-(MC)-ICPMS. Calcite and carbonatitic melt inclusions occur in clinopyroxene and olivine. Clinopyroxene in these xenoliths generally have a relatively higher Mg# (92.3–93.0) than that of olivine (Mg# = 90.8). Clinopyroxene is characterized by enrichments in LREE and zonations of REE, Li and Sr increasing from the cores to the rims. Furthermore, they have high Ca/Al (8.4–8.7), Zr/Hf (69.9–78.4) and (La/Yb)N (21–36) ratios and low Ti/Eu (618–755) ratios, which are features of mantle peridotite metasomatized by carbonatitic melt. These observations indicate that the peridotitic mantle beneath the Changle area could have suffered a carbonatitic metasomatism.
Clinopyroxene, othopyroxene and olivine in these peridotites exhibit variably low δ7Li (+3.9 to -27.2‰) and slightly high 87Sr/86Sr (0.7030–0.7036), suggesting involvement of recycled crustal material. It has been previously suggested that the Mesozoic destruction of the eastern NCC was closely related to the hydration and mantle upwelling associated with Pacific plate subduction. The Cenozoic Changle basalt locates at the eastern margin of the NCC. Thus, we suggest that the carbonatitic metasomatism of the lithopheric mantle beneath the Changle area could have been induced by the westward subduction of the Pacific plate. The subduction-related carbonatitic metasomatism can transform the depleted harzburgite/dunite mantle to clinopyroxne/orthopyroxene-rich mantle, which could have played an important role in the lithospheric thinning of the NCC.