Apatite-bearing mantle xenoliths in Early Cretaceous basalts: Implications for geochemical modifications and carbon cycling in eastern North China Craton

Abstract

The eastern North China Craton (NCC) has undergone multistage lithospheric modifications through the recycling of subducted crustal materials. However, the mechanisms underlying the formation of carbon-rich domains in the subcontinental lithospheric mantle during the Late Mesozoic evolution remain poorly understood. Here, we report the petrological and in-situ geochemical characteristics of apatite-bearing mantle xenoliths from the Early Cretaceous Fangcheng basalts. The presence of near-equilibrium fluorapatites with olivines and clinopyroxenes along with a high proportion of clinopyroxenes and abundant CO₂ fluid inclusions in the wehrlites and olivine clinopyroxenites indicates the occurrence of carbonatite metasomatism. The geochemical compositions of the apatites show relatively high abundances of F (∼2.78 wt%) and Sr (∼3842 ppm), low Sr/Y (<52) and high (La/Yb)_N (∼215) ratios, resembling apatite crystallizing from carbonatite magmas. Clinopyroxenes exhibit low Al and Ti concentrations and are characterized by low Ti/Eu ratio (517–1867) and high (La/Yb)_N (13–15) and Ca/Al ratios (7–9), consistent with geochemical trends associated with carbonatite metasomatism. Elevated Li concentration (>3 ppm) and low V/Sc ratio (<3) in olivines suggest the contribution of recycled crustal components to the peridotitic lithosphere. Additionally, highly enriched Sr-Nd isotopes reported in the Fangcheng basalts support the inference that Fangcheng wehrlites were metasomatized by carbonatite magmas derived from subducted carbonated pelites before being incorporated into the Early Cretaceous basaltic magmas. This metasomatic process serves as a crucial mechanism for lithospheric modifications and plays a key role in regional carbon cycling in the southeastern NCC, encompassing the subduction of carbonate-bearing continental crusts and subsequent devolatilization through mantle-derived volcanism.