Question

Difference between contact metamorphism and contact metasomatism

Answer 1

The term metamorphism refers to changes in a rock’s mineral compositions where the geochemical composition stays mostly the same with the exception of volatile losses (usually water is expelled or at least necessary as an intermediary to enable any sort of reaction to take place).

Metasomatism or (hydrothermal) alteration refers to changing a rock’s composition in an open system, i.e. a system where mass can enter and leave, not just volatiles but also refractory elements. Usually one would be looking at some form of water with dissolved components passing through a rock, dissolving some elements and precipitating others. The extent to which elements have been lost or gained can be estimated by using a mass balance approach if the original, unaltered rock composition can be estimated.