This study presents high-precision Mg and Fe isotopic data for a suite of volcanic rocks ranging from basalt to comendite from Changbaishan region, northeast China. The Millennium Eruption (ME) trachytes and comendites have δ²⁶Mg ranging from −0.37 to as high as 0.14‰. The δ⁵⁶Fe values are 0.17 ± 0.04 and 0.16 ± 0.03 for the two shield-forming basalts, 0.09 to 0.26 for cone-construction and ME trachytes, and 0.30 to 0.37 for the ME comendites. Those rocks with positive δ²⁶Mg values could not have originated by a simple fractional crystallization model. The positive relationship between δ²⁶Mg and ⁸⁷Sr/⁸⁶Sr ratios is indicative of the contribution from weathered basalts instead of the old supracrustal/basement rocks. Combined with an earlier report, this demonstrates that the paleo-weathered products have significantly modified the compositions of intermediate-felsic rocks. By contrast, the well correlated Fe isotopes with indicators of magmatic differentiation (e.g., tFe₂O₃, SiO₂, Mg#) seem unlikely to be controlled by the aforementioned mechanism for Mg isotopes. Most likely, the Fe isotopic variation was produced by crystallization in an open system, which was enhanced by the later compositional effects as suggested by the broadly positive correlation between δ⁵⁶Fe and (Na + K)/(Ca + Mg). In this regard, Fe isotopic data provides evidence for an oxidized environment during the middle Miocene. Our work therefore highlights the potential of MgFe isotopes in tracing the chemical weathering and oxidation state of magma systems.