Contact Metamorphism

Contact metamorphism is the name given to the changes that take place when magma is injected in the surrounding solid rock (country rock).

The changes that occur are greatest wherever the magma comes into contact with the rock because the temperatures are highest at this boundary and decrease with distance from it. Around the igneous rock that forms from the cooling magma is a metamorphosed zone called a contact metamorphism aureole.

Depending on whether the alternation occurs during metamorphism, contact metamorphism can be divided into heat contact metamorphism and contact alternation metamorphism. Heat contact metamorphism doesn’t experience alternation, and the periphery experiences metamorphism due to the heat roast from the lava; contact alternation metamorphism experiences obvious alternation besides the influence of lava heat. Under these two kinds of contact metamorphism, the original rock of different compositions can form different mineral combinations, and the contact metamorphic rock lacks bedding.

The product of the contact metamorphism depends mainly on the protolith composition and temperature, secondly on pressure, and the stress effect is not obvious. The original rock with thermal contact metamorphism is mainly mudstone, carbonate rock, clastic rock, and igneous rock that can form different thermal contact metamorphic rock types. Thermal contact metamorphic rock type of mudstone is spotted slate, knotty slate, and hornstone; carbonate rock will develop into marble after thermal contact metamorphism; clastic rock is not easily changed—under thermal contact metamorphism, if the metamorphism is shallow, the sandy texture still maintains and forms metamorphic sandstone; and igneous rock is generally the combination of high-temperature minerals, thus changing a little under thermal contact metamorphism.

Contact alternation happens in the contact zone between the lava intrusion and peripheral rocks, and the evaporate components and hydrothermal solution precipitated in the later lava crystallization with the heat from the lava alternate the peripheral rocks.

When a rock is contact altered by an igneous intrusion it very frequently becomes more indurated, and more coarsely crystalline. Many altered rocks of this type were formerly called hornstones, and the term hornfels is often used by geologists to signify those fine grained, compact, non-foliated products of contact metamorphism.

A shale may become a dark argillaceous hornfels, full of tiny plates of brownish biotite; a marl or impure limestone may change to a grey, yellow or greenish lime-silicate-hornfels or siliceous marble, tough and splintery, with abundant augite, garnet, wollastonite and other minerals in which calcite is an important component.

A diabase or andesite may become a diabase hornfels or andesite hornfels with development of new hornblende and biotite and a partial recrystallization of the original feldspar. Chert or flint may become a finely crystalline quartz rock; sandstones lose their clastic structure and are converted into a mosaic of small close-fitting grains of quartz in a metamorphic rock called quartzite.

If the rock was originally banded or foliated (as, for example, a laminated sandstone or a foliated calc-schist) this character may not be obliterated, and a banded hornfels is the product; fossils even may have their shapes preserved, though entirely recrystallized, and in many contact-altered lavas the vesicles are still visible, though their contents have usually entered into new combinations to form minerals that were not originally present.

The minute structures, however, disappear, often completely, if the thermal alteration is very profound. Thus small grains of quartz in a shale are lost or blend with the surrounding particles of clay, and the fine ground-mass of lavas is entirely reconstructed.

Magmatic fluids coming from the intrusive rock may also take part in the metamorphic reactions. An extensive addition of magmatic fluids can significantly modify the chemistry of the affected rocks. In this case the metamorphism grades into metasomatism.

Skarns

Sometimes when a siliceous magma intrudes carbonate rocks like limestone and dolostone, significant chemical exchange (metasomatism) takes place between the magma and the carbonate rock. Such a metasomatized rock is refered to as skarn.

If the intruded rock is rich in carbonate the result is a skarn. Fluorine-rich magmatic waters which leave a cooling granite may often form greisens within and adjacent to the contact of the granite. Metasomatic altered aureoles can localize the deposition of metallic ore minerals and thus are of economic interest.

A special type of contact metamorphism, associated with fossil fuel fires, is known as pyrometamorphism.

The formation of Foliated Metamorphic Rock