Contact Metamorphism

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Contact metamorphism occurs when the country rock is affected by heat from a large igneous intrusion. Because temperature differences between the surrounding rock and the intruded magma are greater at shallow levels in the Earth's crust where pressure is low, contact metamorphism is described as high temperature, low pressure metamorphism. High temperature, not pressure leads to the formation of altered, recrystallised, unfoliated rocks in a zone surrounding the intrusion. This zone is the metamorphic aureole. Around a large igneous intrusion, such as a batholith, the metamorphic aureole may be up to 10km wide. Temperature decreases with distance from the contact with the intrusion and for this reason the effects of contact are greater closer to the intrusion.

Metamorphic grade increases in all directions towards the intrusion.

Contact Metamorphism of Shale

Chemical composition of minerals in shale is varied and so a range of different metamorphic rocks are formed, depending on the temperature and therefore the distance away from the intrusion.

close to the contact with the intrusio, temperatures are high and so a high grade metamorphism occurs. Shale is completely recrystallised to form a fine grained, hard, splintery, granoblastic metamorphic rock called hornfels

further away from the contact, where the heat is less intense, medium grade metamorphism occurs. Clusters of a new metamorphic mineral andalusite for porphyroblasts. This partly recrystallised rock is andalusite slate or rock.

in the outer part of the metamorophic aureole, temperatures are lower. Some recrystallisation occurs, causing clusters of dark minerals to grow in separate spots, iron, carbon or biotite mica will form the spots. The rock in this outer part of the metamorphic aureole is called spotted rock and is formed by low grade metamorphism

Factors controlling the width of metamorphic aureoles

Volume of the magma

The size of intrusions ranges from batholiths down to minor instrusions. Dykes and sills are not large enough and do not produce enough heat to develop a metamorphic aureole. Because the volume of magma is small it cools quickly and there is only sufficient heat to change the rock for a few centimetres on either side. This narrow zone of bleached and hardened rock is known as a baked margin.

Larger intrusions cool slowly and heat the surrounding rocks over long periods of time, allowing a wide metamorphic aureole to develop.

Temperature of the Magma

The volumes of magma in an intrusion affects the maximum the maximum temperature reached at any point and also the time it takes for temperatures to rise in the country rocks.

Metamorphism will not occur unless the temperature rises above 200 degrees for an extended period of time. A small intrusion produces little metamorphic change because the rock has little time to warm up and there is not enough time for metamorphic reactions to occur before the rock cools down. With larger intrusions there is time for metamorphic reactions to take place and for new minerals and recrystallisation to occur, because temperatures remain high for much longer periods of time.

Composition of Magma

Mafic magma may be intruded at a temperature of 1200 degrees, whilst silicic magma may be intruded at 850 degrees. Silicic magmas contain more volatiles. When they enter the country rock they speed up metamorphic reactions. This compensates for the lower temperature of the magma, because metamorphic aureoles surrounding silic intrusions are of similar size to those around mafic ones.

Composition of the Country Rock

Rocks largely composed of one mineral, such as limestone and orthoquartzite, show much less variation than clay-rich rocks such as shale. Quartzite and marble have larger crystals the nearer they are to the igneous intrusion and are uniform. Metamorphic aureoles formed in sandstone country rocks are typically narrower than those formed in clay-rich rocks. If the country rock is permeable and contains groundwater, heat will be able to move by convection, allowing a wider aureole to develop.

Dip of the Contact

The dip of the sides of the intrusion has a major effect on the width of the metamorphic aureole. A shallow angle of dip gives a wide aureole and a steep angle of dip gives a narrow aureole. If the sides of the intrusion dip at different angles, then the metamorphic aureole will be asymmetric.

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