Contact metamorphism

Introduction

Contact metamorphism, also known as thermal metamorphism, is a type of metamorphism where rock minerals and texture are changed, mainly by heat, due to contact with magma. This process typically occurs at relatively shallow depths in the Earth's crust, where the temperatures are high but the pressures are relatively low. Contact metamorphism is distinct from regional metamorphism, which involves high pressures and temperatures over large areas and is often associated with mountain building.

Mechanisms of Contact Metamorphism

Contact metamorphism occurs when country rock comes into contact with an igneous intrusion such as a pluton, dike, or sill. The heat from the magma body causes the surrounding rocks to undergo thermal alteration. This process can result in the recrystallization of minerals, the formation of new mineral assemblages, and changes in the rock's texture.

The extent of metamorphism is influenced by several factors:

**Temperature**: The primary driver of contact metamorphism is the high temperature of the intruding magma.
**Duration of Contact**: The length of time the country rock is exposed to the heat source.
**Composition of the Country Rock**: Different rock types respond differently to heat.
**Presence of Fluids**: Fluids can enhance metamorphic reactions by facilitating the movement of ions.

Metamorphic Aureole

A metamorphic aureole, or contact aureole, is the zone surrounding an igneous intrusion where the country rock has been metamorphosed. The size and intensity of the aureole depend on the size of the intrusion, the temperature of the magma, and the thermal conductivity of the surrounding rocks. Aureoles can range from a few centimeters to several kilometers in width.

Within the aureole, the degree of metamorphism decreases with distance from the intrusion. The innermost zone, closest to the magma, experiences the highest temperatures and the most intense metamorphism. This zone is often characterized by the formation of high-temperature minerals such as garnet, pyroxene, and cordierite.

Types of Metamorphic Rocks

Contact metamorphism produces a variety of metamorphic rocks, depending on the original composition of the country rock and the specific conditions of metamorphism. Some common types of contact metamorphic rocks include:

**Hornfels**: A fine-grained, non-foliated rock formed by the contact metamorphism of shale or other clay-rich rocks. Hornfels is characterized by its hardness and dense texture.
**Marble**: Formed from the contact metamorphism of limestone or dolostone. Marble is composed primarily of recrystallized calcite or dolomite.
**Quartzite**: Produced from the contact metamorphism of quartz-rich sandstone. Quartzite is a hard, dense rock composed mainly of recrystallized quartz grains.
**Skarn**: A complex rock type formed by the metasomatic alteration of carbonate rocks in the presence of silica-rich fluids. Skarns often contain a variety of minerals, including garnet, pyroxene, and wollastonite.

Mineral Assemblages

The mineral assemblages found in contact metamorphic rocks depend on the composition of the original rock and the specific conditions of metamorphism. Some common minerals found in contact metamorphic rocks include:

**Andalusite**: A high-temperature aluminum silicate mineral commonly found in contact metamorphosed pelitic rocks.
**Sillimanite**: Another high-temperature aluminum silicate mineral that can form in contact metamorphosed pelitic rocks.
**Cordierite**: A magnesium iron aluminum cyclosilicate mineral that forms in high-temperature, low-pressure conditions.
**Garnet**: A group of silicate minerals that can form in a variety of metamorphic environments, including contact metamorphism.
**Pyroxene**: A group of inosilicate minerals commonly found in high-temperature metamorphic rocks.

Thermal Gradient and Metamorphic Zoning

The thermal gradient, or the rate of temperature change with distance from the heat source, plays a crucial role in determining the extent and intensity of contact metamorphism. In general, the temperature decreases rapidly with distance from the intrusion, leading to the formation of distinct metamorphic zones within the aureole.

These zones are characterized by specific mineral assemblages that reflect the temperature conditions at different distances from the intrusion. For example, the innermost zone, closest to the intrusion, may contain high-temperature minerals such as garnet and pyroxene, while the outermost zone may contain lower-temperature minerals such as biotite and chlorite.

Economic Importance

Contact metamorphism can have significant economic implications, particularly in the formation of mineral deposits. Skarns, for example, are often associated with valuable ore deposits, including tungsten, copper, iron, and zinc. These deposits form when hot, silica-rich fluids from the intrusion interact with carbonate rocks, leading to the precipitation of ore minerals.

Additionally, contact metamorphic rocks such as marble and quartzite are valuable as building materials and decorative stones. Marble, in particular, is prized for its aesthetic qualities and is used extensively in sculpture and architecture.

Case Studies

Several well-known geological sites provide excellent examples of contact metamorphism and its effects on country rocks. Some notable case studies include:

**The Cornubian Batholith in Southwest England**: This large granitic intrusion has produced extensive contact aureoles in the surrounding Devonian and Carboniferous rocks. The aureoles contain a variety of metamorphic minerals, including andalusite, cordierite, and garnet.
**The Skaergaard Intrusion in East Greenland**: This layered mafic intrusion has produced a well-defined contact aureole in the surrounding Precambrian gneisses. The aureole contains high-temperature minerals such as pyroxene and garnet.
**The Bushveld Complex in South Africa**: This large igneous province has produced extensive contact metamorphism in the surrounding rocks, leading to the formation of valuable mineral deposits, including platinum group elements and chromite.

Conclusion

Contact metamorphism is a fascinating geological process that results from the interaction between country rocks and intruding magma bodies. The heat from the magma causes significant changes in the mineralogy and texture of the surrounding rocks, leading to the formation of a variety of metamorphic rock types and mineral assemblages. Understanding contact metamorphism is crucial for geologists, as it provides valuable insights into the thermal and chemical processes that occur in the Earth's crust and can have significant economic implications.