7.6: Contact Metamorphism and Hydrothermal Processes
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- 1703
Contact metamorphism takes place where a body of magma intrudes into the upper part of the crust. Any type of magma body can lead to contact metamorphism, from a thin dyke to a large stock. The type and intensity of the metamorphism, and width of the metamorphic aureole will depend on a number of factors, including the type of country rock, the temperature of the intruding body and the size of the body (Figure 7.5.1). A large intrusion will contain more thermal energy and will cool much more slowly than a small one, and therefore will provide a longer time and more heat for metamorphism. That will allow the heat to extend farther into the country rock, creating a larger aureole.
Contact metamorphic aureoles are typically quite small, from just a few centimetres around small dykes and sills, to as much as 100 m around a large stock. As was shown in Figure 7.3.7, contact metamorphism can take place over a wide range of temperatures — from around 300° to over 800°C — and of course the type of metamorphism, and new minerals formed, will vary accordingly. The nature of the country rock is also important. Mudrock or volcanic rock will be converted to hornfels. Limestone will be metamorphosed to marble, and sandstone to quartzite.
A hot body of magma in the upper crust can create a very dynamic situation that may have geologically interesting and economically important implications. In the simplest cases, water does not play a big role, and the main process is transfer of heat from the pluton to the surrounding rock, creating a zone of contact metamorphism (Figure 7.5.2a). In many cases, however, water is released from the magma body as crystallization takes place, and this water is dispersed along fractures in the country rock (Figure 7.5.2b). The water released from a magma chamber is typically rich in dissolved minerals. As this water cools, is chemically changed by the surrounding rocks, or boils because of a drop in pressure, minerals are deposited, forming veins within the fractures in the country rock. Quartz veins are common in this situation, and they might also include pyrite, hematite, calcite, and even silver and gold.
Exercise 7.5 Contact Metamorphism and Metasomatism
This diagram shows a pluton that has intruded into a series of sedimentary rocks.
What type of metamorphic rock would you expect to see at each location: a, b, and c?
a |
b |
c |
[SE]
Contributors
Physical Geology by Steven Earle used under a CC-BY 4.0 international license. Download this book for free at http://open.bccampus.ca.