Phanerozoic lower crustal growth from heterogeneous mantle beneath the North China Craton: Insights from the diverse Hannuoba pyroxenite xenoliths

Abstract

Though many models of continental crustal growth rely on the assumption that juvenile crust is derived from depleted mantle of fairly uniform composition, mafic magmas derived from the mantle – potential candidates for newly formed continental crust – often bear heterogeneous radiogenic isotope compositions. Mafic granulite and some pyroxenite xenoliths from Cenozoic basalts at Hannuoba, in the northern margin of the North China Craton, have younger crystallization ages than the exposed basement rocks, and are generally interpreted to represent juvenile additions to the crust. Here we report mineralogical and geochemical data for the websterite xenoliths from the basalts with the aim of constraining the chemical and isotopic compositions of juvenile continental crust. Two-pyroxene geothermometry suggests that the studied websterite xenoliths originated in the lower crust and last equilibrated at temperatures in the range of 830 to 930 °C. The cumulate texture, together with the geochemical and geochronologic data, indicate that they formed as high-pressure cumulates through crystal fractionation from Phanerozoic mafic magmas. These xenoliths can be subdivided into two groups. Group 1 websterites have low Mg# of 66.1–79.2 and high REE concentrations, and are characterized by extremely evolved radiogenic Sr-Nd-Hf isotopic compositions and high radiogenic Pb ratios. Group 2 websterites have high Mg# of 89.4–89.5, very low REE concentrations, and MORB-like depleted Sr-Nd-Hf-Pb isotopic compositions. We interpret these websterites as additions to the crust formed via magma underplating. The parental magmas for these two websterite groups formed by partial melting of mantle materials with both isotopically enriched (Group 1) and depleted (Group 2) components. The data provide evidence that components from both mantle sources contributed to the underplating magmas and to lower crustal growth beneath the craton.