Basaltic magma plumbing system for the cone-construction stage of the Changbaishan from mineralogical, geochemical, and SrNd isotopic data

Changbaishan volcano is one of East Asia's most hazardous intraplate volcanoes. The activity of Changbaishan volcano is divided into three stages: the shield-forming stage, the cone construction stage and the caldera-forming stage. During the cone construction stage, the formation of the trachytic volcanic cone was accompanied by eruptions of basaltic monogenetic volcanoes. This study investigates four basaltic monogenetic volcanoes, analyzes the origin of basaltic magma, and constructs a basaltic plumbing system model by merging whole-rock, mineral, and SrNd isotopic data. Based on the differences in SrNd isotopes and TiO2 contents, the considered basalts can be divided into three categories: high-TiO2 basalts (TiO2 > 2.8 wt%, high 87Sr/86Sr values), low-TiO2 basalts (TiO2 < 2.5 wt%, low 87Sr/86Sr values) and transitional basalts (TiO2 > 2.8 wt%, low 87Sr/86Sr values). The geochemical results show that these three types of basalts originated from two mantle source regions containing different proportions of secondary pyroxenite. Based on the differences in TiO2, light rare earth element (LREE)/heavy rare earth element (HREE) ratios, and SiO2, we find that the high-TiO2 basalts and transitional basalts were generated at deeper levels and that the low-TiO2 basalts were generated at shallower levels during the ascent of the same mantle plume containing recycled components. The thermobarometer results indicate that all of the above magmas were trapped in magma chambers in the lower crust before eruption. The zoning pattern and texture of plagioclase reveal that both the low-TiO2 basalts and transitional basalts underwent magma mixing/magma recharge processes before eruption, while the high-TiO2 basalt was formed in a closed system. We constructed a multilevel basaltic plumbing system model, including several lower crustal magma chambers containing high-TiO2, low-TiO2 and transitional basalts. During the Changbaishan cone construction stage, although these discrete basaltic monogenic volcanoes are much less massive and destructive than the Tianchi volcanoes that erupted intermediate to acidic magmas, these small-scale basaltic magmatic activities still need to be evaluated because the recharge of basaltic magmas may be crucial for the long-term viability of the intermediate to the acidic plumbing system of Tianchi volcano. The new model proposed in this paper complements the existing model of the magma plumbing system under Changbaishan volcano and provides a basis for further understanding the eruption mechanism and activity pattern of intermediate to acidic magma since the cone construction stage.