Fe–Ti oxide–orthopyroxene symplectites in the eastern Tianshan layered intrusions, NW China: Insights into late-stage reactive melt flow and re-equilibration

• Fe-Ti oxide-orthopyroxene symplectites result from melt-primocryst reactions. • Fe-Ti oxides in both primocryst and symplectite phases underwent extensive re-equilibration. • Niumaoquan deposit formed under more reduced conditions with faster melt migration than Weiya deposit. Layered intrusions are primary reservoirs for Fe–Ti oxide mineralization, preserving crucial records of the crystallization and solidification of mafic magmas. Notably, symplectites, a widespread microstructure in these intrusions, serve as key evidence for melt migration and post-cumulus disequilibrium. However, the formation of symplectites remains unclear. This study investigates symplectites from two representative layered intrusions associated with Fe–Ti oxide ores (Niumaoquan and Weiya) in the eastern Tianshan, comparing primocryst and symplectite Fe–Ti oxide textures. We infer that symplectite formation preceded Fe–Ti oxide re-equilibration and was driven by melt percolation and reaction. This interpretation is supported by Fe–Ti oxide-based thermo-oxybarometric estimates, which indicate that both types of Fe–Ti oxides underwent re-equilibration at temperatures of approximately 500–700°C and corresponding f O 2 (ΔFMQ −4 to 0). Mass-balance calculations indicate that symplectite formation required an external input of Fe and Si. Based on these constraints, we propose a model in which a late-stage residual melt, relatively poor in Fe but rich in Si, reacted with primocryst Fe–Ti oxides, progressively dissolving them while becoming enriched in Fe and retaining Si. Subsequently, the Fe-Si-bearing melt reacted with primocryst olivine: Si facilitated its replacement by orthopyroxene, while Fe precipitated as Fe–Ti oxides within the symplectites. Our results demonstrate that the formation of Fe–Ti oxide-orthopyroxene symplectites reflects both the behavior of immiscible Fe- and Si-rich melt components and late-stage re-equilibration, and emphasizes melt migration and reaction during post-magmatic evolution.