Destruction of the North China Craton: Delamination or thermal/chemical erosion? Mineral chemistry and oxygen isotope insights from websterite xenoliths

Abstract

The destruction (or reactivation) of the North China Craton (NCC) is one of the important issues related to the Phanerozoic evolution of eastern China, although the processes of destruction remain debated. Two main mechanisms – delamination and thermal/chemical erosion – have been proposed based on the geochemistry of Mesozoic–Cenozoic basalts and entrained deep–seated xenoliths. A key criterion in distinguishing between these mechanisms is the nature of the melt, derived from delaminated crust or the asthenosphere, that modified the lithospheric mantle. Here we investigate the mechanism of destruction of the NCC based on mineral compositions and oxygen isotopic data from olivines, and strontium isotopic data for clinopyroxenes within websterite xenoliths from the Early Cretaceous Feixian basalts in the eastern NCC. Olivines in websterite xenoliths have higher Mg# (86–86.4), Ni content (2187–2468ppm), and lower Ca (983–1134ppm), Ti (58.1–76.1ppm), and Mn (1478–1639ppm) contents than olivine phenocrysts (Mg#=71.0–77.3, Ni=233–1038ppm, Ca=1286–2857ppm, Ti=120–300ppm, and Mn=2092–4106ppm) from Late Cretaceous basalts. Additionally, olivines in websterite xenolith have δ18O values of 7.10±0.21‰ to 8.40±0.21‰, evidently higher than those of typical mantle-derived olivines. Similarly, orthopyroxenes (Opx) and clinopyroxenes (Cpx) in the websterite xenoliths have much higher Mg# (86.3–89.2 and 87.5–90.3, respectively), and Ni contents (1097–1491ppm and 581–809ppm, respectively) than orthopyroxene- and clinopyroxene-phenocrysts (Opx: Mg#=82.2–83.9, Ni=730–798ppm; Cpx: Mg#=74.2–84.6, Ni=117–277ppm) from Late Cretaceous basalts. The 87Sr/86Sr ratios of clinopyroxenes in the websterite xenoliths range from 0.70862 to 0.70979, and are much higher than those of clinopyroxene grains from peridotite xenoliths and basalts. These data indicate that olivines are the residue of ancient lithospheric mantle that was modified intensively by a melt derived from recycled continental crust, and that the silicic and calcic metasomatic melt might have been derived from the partial melting of the subducted Yangtze slab and delaminated lower crust of the NCC. The existence of recycled continental crust in the Mesozoic lithospheric mantle implies that delamination was an important mechanism of destruction of the NCC.