Olivine chemistry of the Quaternary Datong basalts of the Trans-North China Orogen: insights into mantle source lithology and redox–hydration state

Abstract

AbstractCenozoic intraplate basalts are widespread above the Big Mantle Wedge (BMW) and its front in East Asia. While the mantle source lithology and redox-hydration state have been demonstrated to be crucial in the generation of basalts above the BMW, their nature and role in the basalts above the front of the BMW is poorly constrained. To address this, we report olivine compositions of the Quaternary Datong basalts. Datong basalts exhibit ocean island basalt-like trace-element compositions and depleted Sr–Nd isotopes with slightly enriched signatures (enriched mantle I, EMI) for tholeiitic basalts. Olivines of the Datong basalts show high Ni and Fe/Mn, and low Ca, Mn and Mn/Zn values, pointing to a pyroxenite source. Applying V and Ca partition coefficients between olivine and whole rock, respectively, the Datong basalts lie −0.44 to 0.64 log units above the fayalite–magnetite–quartz buffer for fO2, and contain 2.1–3.4 wt% H2O but highly variable H2O/Ce values (265–1498). Both fO2 and H2O/Ce in the basalts vary with whole-rock and olivine compositions, indicating that the source was the main control. Thus, there is a heterogeneous redox–hydration state in the source, the EMI component being relatively reduced but extremely wet and recycled oceanic crust being relatively oxidized but dry. The extremely wet EMI component was probably derived from the mantle transition zone. In the light of our findings, we propose a model in which mantle upwelling carried the recycled oceanic crust and EMI component from the mantle transitional zone to shallow mantle, owing to the Pacific slab stagnating in the mantle transitional zone, to form pyroxenite, which subsequently melted to generate Datong basalts.