Under the agency of heat and magmatic fluids, ascending magmas pick up volatiles, silica, trace elements, and occassionally fragments of wall rock. The heat that the melt gains by leaving behind quick-freezing refractories (an exothermic process) is typically sufficient to compensate for heat lost in the endothermic reactions required for the assimilation (melting) of country rock components. This trade-off ensures that assimilation can proceed without causing the melt to freeze (solidify).
Any wall rock fragments that survive more or less intact, without completely melting or dissolving into the magma, are called xenoliths. Surviving wall rock crystals are called xenocrysts. Together, xenoliths and xenocrysts provide invaluable information about rarely exposed lower crust and mantle levels by carrying these materials up within the ascending magma.
▪ Bowen's Reaction Series