Oct 15 2014
Metamorphism is a process of change in the arrangement of minerals within pre-existing rocks due to pressure, heat and the penetration of chemically active fluids.
Contact metamorphism is a type of metamorphism which occurs adjacent to intrusive igneous rocks due to an increase in temperature resulting from an intrusion of hot magma into the rock. The metamorphosed zone around an igneous rock is known as the metamorphic aureole.
Contact metamorphic rocks, also known as hornfels, are often fine-grained and do not indicate the signs of strong deformation. The size of the aureole is dependent on the difference in temperature between the wall rocks and the heat of intrusion.
In general, dikes have small aureoles with minimal metamorphism while large ultramafic intrusions have thick and well-developed contact metamorphism.
The rate of metamorphism gradually increases in all directions with respect to the level of intrusion. The temperature difference between the intruded magma and the surrounding rock is large at the low-pressure shallow regions of the crust. As a result, contact metamorphism is also known as high temperature, low pressure metamorphism.
The metamorphic rate of aureole is measured by a peak metamorphic mineral which is formed in the aureole. This is associated with the metamorphic temperatures of alumonisilicate or pelitic rocks and their related minerals. The metamorphic grades of aureoles include pyroxene hornfels, sillimanite hornfels and andalusite hornfels.
References and Further Reading