Influences of the Stagnant Pacific Slab Beyond Its Westernmost Edge: Insights From the Cenozoic Alkaline Basalts in the Dariganga Volcanic Field, SE Mongolia

Abstract

It remains uncertain whether a stagnant slab in the mantle transition zone can affect the asthenospheric mantle beyond its leading edge. To address this question, we investigated Cenozoic alkaline basalts from the Dariganga volcanic field (DVF) in southeastern Mongolia. The DVF is located west of North–South Gravity Lineament (NSGL) in Eastern China, which is spatially coincident with the seismically detected stagnant Pacific slab front. Basalts from the DVF consist of nephelinite, basanite and alkali olivine basalt. These rocks have relatively high Nb/U (average = 58) and Nb/La (>1) ratios and radiogenic Nd–Hf isotopic compositions. They also have high Ca/Al (0.60–1.13), Zn/Fe^T (13.5–16.5), and FeO/MnO (77–112) ratios as well as low δ²⁶Mg (−0.42‰ to −0.26‰) values, reflecting an asthenospheric mantle source modified by carbonated eclogite-derived melts. Pb–Nd–Hf isotope characteristics indicate that the carbonated eclogite-derived melts likely originated from the stagnant Pacific slab. Although Cenozoic basalts from both the east of the NSGL (ENSGL) and DVF domains exhibit light δ²⁶Mg values, basalts from the ENSGL nevertheless have lower δ²⁶Mg values than those in the DVF domain. This suggests a gradual westward decline in the amount of carbonated melts/fluids derived from the stagnant Pacific slab. This variation trend, combined with a more fertile and oxidized asthenospheric mantle toward the ENSGL, indicates that the stagnant slab has affected the mantle and created a compositional aureole beyond its leading edge, which substantially contributed to the formation of the alkaline basalts in the DVF.