Garnet megacryst with a multiphase inclusion from intraplate alkali basalts of the Shavaryn Tsaram (Tariat, Mongolia) was the object of the study. This unusual aggregate consists of porous glass, Ti-rich biotite, orthopyroxene, spinel, clinopyroxene, olivine, and ilmenite. WinTWQ 2.32 thermodynamic simulation of this system revealed a few intervals of equilibrium. Pressure and temperature adjustment reflected in the paragenetic minerals of the melt pocket. The capture of already crystallised garnet megacryst was at P = 0.8–1 GPa and T = 1120–1160°C. Mineral crystallisation inside the melt pocket, accompanied by external inputs, occurred at P = 0.75–0.95 GPa; T = 790–1120°C. Symplectite assemblage formed in the garnet megacryst due to decomposition at (P = 0.55–0.7 GPa; T = 850–930°C). The study of the oxygen isotope content in primary garnet and biotite of the melt pocket showed that the δ18OVSMOW values are the same and correspond to that of typical mantle xenoliths. However, the chemical and microcomponent composition of the melt pocket minerals reveals a material that differs from basalts and peridotites. Thus, it has been revealed that the multiphase inclusion in the garnet megacryst formed not only on account of the garnet's substance, but also due to the entrapped material of the Earth's interior.