Within a contact metamorphic aureole the grade of metamorphism increases toward the contact with the igneous intrusion. An example of a contact aureole surrounding the Onawa Pluton in Maine is shown here.
The granodiorite pluton was intruded into slates produced by a prior regional metamorphic event. The aureole is a zone ranging in width from about 0. Two zones representing different contact metamorphic facies are seen within the aureole. The outer zone contains metapelites in the Hornblende Hornfels Facies, and the zone adjacent to the pluton contains metapelites in the Pyroxene Hornfels Facies. The zones are marked by an isograd , which represents a surface along which the grade of metamorphism is equal.
The size of a contact aureole depends on a number of factors that control the rate at which heat can move out of the pluton and into the surrounding country rock.
Among these factors are:. The size and temperature of the intrusion. This will control how much heat is available to heat the surrounding country rocks.
The thermal conductivity of the surrounding rocks. This will control the rate at which heat can be transferred by conduction into the surrounding rocks. Thus, the rate at which heat moves by conduction increases if the thermal conductivity and temperature gradient are higher.
The initial temperature within the country rock. This, in combination with the temperature of the intrusion, will determine the initial temperature gradient, and thus the rate at which heat can flow into the surrounding country rocks.
The latent heat of crystallization of the magma. As you recall, the total amount of heat available in a liquid is not only dependent on the temperature, but also involves the heat released due to crystallization. Thus, if the latent heat of crystallization is large, their will be more heat available to heat the surrounding country rocks.
The heat of metamorphic reactions. In order for a metamorphic reaction to take place some heat is necessary and this heat will be absorbed by the reactions without increasing the temperature in the intrusion.
The amount of water in and the permeability of the surrounding country rock. If the country rock is permeable and contains groundwater, heat will be able to move by convection. Solutions to the heat equation given above are complicated because most the terms in the equation are functions of temperature, time, and position. Solutions for a simple case are shown below. In this simple case a basaltic dike is assumed to have intruded at temperature of o C, into dry country rock at a temperature of 0 o C.
The width of dike is assumed to be m, and the latent heat of crystallization is assumed to be released between o and o. Solidification of the intrusion is thus complete at o , after 10, years. Note how the temperature of the country rock near the contact reaches a maximum of about o after about years, and how the temperature in the country rock at distances greater than about m from the center of the dike continues to rise, while temperatures near the contact drops.
The model above assumes that all heat moves by conduction. Limestone will be transformed into marble and quartzite into sandstone. Friday, November 12, Sign in. Forgot your password? Get help. Password recovery. Geology Page. Home Latest News Video. Debris Flow Dynamics. Sampling Hot Molten Lava. Incredible moment Anak Krakatau erupts, Oct Download Google Earth For Free. 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. Share to:.
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