The heat introduced by the intrusion controls the metamorphism. The effects of increased temperature are most pronounced where intrusions occur at shallow levels. There, contrasts in temperature between country rock and intrusion are at a maximum. The fluid phase is also an important agent of contact metamorphism. It transports heat and has a profound influence on the chemistry and mineral composition of the rocks with which it comes in contact.
Fluids are particularly important in the metamorphism of carbonate rocks.
Contact metamorphism commonly produces fine-grained rocks termed hornfels . In addition to a variety of common minerals, such as quartz, feldspars, and epidote, hornfels locally contain unique phases. At those shallow levels, the stresses characteristic of orogenic belts are generally absent and contact metamorphic rocks lack foliation. Minerals indicative of these facies include analcite, stilbite, wairakite, pyrophyllite, cordierite, andalusite, sillimanite, K-feldspar, orthopyroxene, sanidine, and mullite. In carbonate rocks, minerals such as talc, tremolite, diopside, forsterite, grossularite, wollastonite, and spurrite may develop.