Influence of Crystal Fractionation and Fluid Regime Change on the Evolution of Magmas of the Wangtian’e Bimodal Volcanic Association (Northeastern China)

To reconstruct the magmatic evolution of the bimodal association of Wangtian’e Volcano in the Changbaishan area (Northeastern China), we used a comprehensive petrological and geochemical approach. This approach included the study of the chemical composition of volcanic rocks, determination of their isotopic (Sr–Nd) composition, detailed investigation of melt and fluid inclusions in phenocrysts of the entire volcanic rock series, and thermodynamic modeling of melt fractionation and assimilation processes using the alphaMELTS program. The study of melt inclusions in minerals of the tholeiitic basalts and trachytes of the volcano showed that the basaltic melts crystallized at T = 1180–1200°C, whereas the trachytic melts of trachyte extrusions and trachyte lava flows crystallized at 1050–1060°C and 1070–1090°C, respectively. Based on the study of primary CO₂ fluid inclusions in plagioclase of the basalts, the crystallization pressure was estimated as P = 0.1 ± 0.01 GPa, which corresponds to a depth of ~4 km. The analysis of the compositions of rocks and melt inclusions in minerals and their comparison with the results of thermodynamic modeling of assimilation–fractional crystallization allowed us to demonstrate the leading role of fractional crystallization of magmas in the formation of the entire rock series from basalt to trachyte and an insignificant contribution of crustal contamination (Ma/Mc = 15%). Magma fractionation at Wangtian’e Volcano occurred in two stages. The first stage of crystal fractionation of tholeiitic melts followed the Fenner trend, when FeO, TiO₂, and P₂O₅ accumulated in the melts at a minor increase in SiO₂ content. The fractional crystallization of tholeiitic magmas occurred at CO₂ dominance over H₂O in the fluid and was accompanied by extensive degassing of CO₂ (up to 97.5–99%) and partial (up to 50%) H₂O loss. The second stage of crystallization differentiation followed the Bowen trend and involved a change in the fluid regime: the dominant role of CO₂ gave way to the predominance of H₂O in both the fluid phase and the melt. The transition to the Bowen trend of melt differentiation occurred when FeO contents reached 15–20 wt % and titanomagnetite crystallization began. The change in the crystal fractionation trend from the Fenner to Bowen type was associated with a change in the fluid regime of the magmatic system: water accumulation in the melt promoted FeO oxidation to the ferric state and subsequent Fe₃O₄ crystallization. The fluid regime also played an important role in the formation of the bimodal distribution of rock compositions at Wangtian’e Volcano. Highly mobile mafic magmas with a predominance of CO₂ over H₂O in the fluid and trachytic magmas whose viscosity was reduced by melt enrichment in water and alkalis erupted on the surface. Magmas of intermediate composition remained buried in magma chambers owing to their high viscosity.