Petrogenesis of late Cenozoic basalts from Dalinor, Inner Mongolia: Implications for lateral mantle heterogeneity in eastern China

Abstract

We present major element, trace element and Sr-Nd-Pb-Hf isotopic compositions of the late Cenozoic basaltic rocks from Dalinor, Inner Mongolia, on the west of the Greater Hinggan Mountain Range in Northeast Asia, and discuss the origin of mantle source heterogeneity beneath Northeast Asia. The early-stage volcanoes (EV) from Dalinor produced predominantly quartz tholeiitic and olivine tholeiitic basalts whereas the late-stage volcanoes (LV) produced alkali basalts. Variations in the degree of partial melting and in fractionation crystallization could explain most of the changes in major and trace element compositions within individual groups. Trace element ratios and Sr-Nd-Pb-Hf isotope ratios between the EV and LV groups indicate a heterogeneous mantle source. The EV group is characterized by an ocean island basalt (OIB)-like spider diagram and positive anomalies of Sr, Ba and K, moderately LREE-enriched pattern ((La/Yb)N = 5.7–8.2), high Ba/Nb (10.2–16.5), and negative correlation between Dy/Yb and SiO2. Two thirds of the EV samples have low Nb/U (<37). The source rock for the EV lavas was amphibole-bearing metasomatized lithospheric mantle. The LV lavas have high TiO2 content (2.57–3.65 wt%), high Nb/U (42.4–58.8) and Indian-MORB-like SrNd isotopic compositions, indicating their asthenospheric origin. The Sr-Nd-Pb isotopic compositions of the Dalinor lava overlap those of the Southeast China basalts, and are consistent with source mixing between a depleted mantle component (DM) and an enriched mantle EM2 component. The western part of the trans-Greater Hinggan Mountain Range mantle source profile has a DM + EM2 signature, whereas the eastern part of the profile has a DM + EM1 signature. This lateral mantle source heterogeneity cannot be explained by the stagnant Pacific slab in the mantle transition zone. Instead, we propose that the EM2 component originated from the descending slab during a slab avalanche in the lower mantle near the westernmost leading edge of stagnant slab, before finally residing in the lithospheric mantle.