Contact (thermal) metamorphism
Hot magma rises from depths and intrudes cooler country rock at a shallow depth.
Heat flows - magma cools and solidifies, and country rock heats up.
Hydrothermal fluids circulate through intrusion and country rock.
Highest-grade rocks form immediately adjacent to pluton, where temperatures are highest.
Progressively lower-grade rocks form farther from the heat.
Band of metamorphic rock around igneous intrusion = metamorphic aureole.
Takes place without stress, so aureoles contain hornfels, a non-foliated metamorphic rock.