A magma rising through the crust may assimilate country rock material as it passes from its source area to its site of crystallization. The assimilated material will change (contaminate) the chemical signature of the magma, resulting in a 'new' liquid, which when solidified will be distinctly different from the original, uncontaminated magma.

Xenoliths and inclusions within a plutonic or volcanic rock provide evidence for the assimilation process.

Assimilation is a thermodynamic process involving the following principles;

Heat of solution = heat of melting + heat of mixing

where:

1. Heat of solution is the ability of a magma to dissolve an inclusion.

2. Heat of melting is the heat capacity of of the solid phases plus the heat of crystallization of the minerals involved.
3. Heat of mixing is the heat required to mix the phases.

Examples of Assimilation

1. For a basaltic liquid assimilating a granitic solid consisting of quartz, feldspar (plagioclase and alkali feldspar and biotite.

Q, F and B will be melted,

the heat for melting comes from the heat generated by the crystallization of olivine and pyroxene from the liquid,

not from the temperature of the basaltic liquid.

The granite inclusions will not be completely melted ====> partial melting.

The end result is a basaltic andesite with inclusions of chewed up, partially digested granitic material.

2. For the reverse process where granitic liquid incorporates basalt, the anhydrous minerals in the basalt (olivine, pyroxene and plagioclase) become altered to micas, amphiboles and epidote by the addition of H2O.

The heats of crystallization of quartz, feldspar and biotite from a granitic liquid are not large enough to melt the basalt inclusions, resulting in very minor changes in the original liquid composition.

The result is a granite with amphibolite inclusions.