The presence of paleo-Pacific slab beneath northwest North China Craton hinted by low-δ26Mg basalts at Wulanhada

Low δMg values in basaltic lavas are generally taken to indicate derivation from mantle sources modified by subducted carbonates, making Mg isotopes a potential tool to track subducted slabs. Here, we report Mg isotopic data for Quaternary basalts from the Wulanhada volcanic field, North China Craton, under which stagnant paleo-Pacific slab does not exist according to seismic tomography. These basalts have low δMg ranging from −0.51‰ to −0.32‰ compared with unmodified mantle. Considering their negative Zr, Hf and Ti anomalies, and high Sr/Sm, Sm/Hf, Zr/Hf and Nb/Ta, Mg isotopic variations in these basalts are likely related to subducted carbonates in their mantle sources. This is consistent with earlier studies based on Cenozoic basaltic lavas elsewhere in eastern China, many of which display characteristic low-δMg signature derived from subducted carbonates atop the stagnant paleo-Pacific slab. Our Mg isotopic study hints that the subducted paleo-Pacific slab once existed beneath the Wulanhada volcanic field, but has retreated eastward, or has detached from the stagnant slab and sinked into the lower mantle.