Petrology Contact Metamorphism
Prof. Stephen A. Nelson
This document last updated on 08-Apr-2002
As discussed previously, contact metamorphism occurs as a result of a high geothermal gradient produced locally around intruding magma. Contact metamorphism is usually restricted to relatively shallow depths (low pressure) in the Earth because it is onlyat shallow depths where there will be a large contrast in temperature between the intruding magma and the surrounding country rock. Also, since intrusion of magma does not usually involve high differential stress, contact metamorphic rocks do not often show foliation. Instead, the common rocks types produced are fine grained idioblastic or hypidioblastic rocks called hornfels. The areasurrounding an igneous intrusion that has been metamorphosed as a result of the heat released by the magma is called a contact aureole. We will here first discuss contact aureoles, then look at the facies produced by contact metamorphism. Contact Aureoles Within a contact metamorphic aureole the grade of metamorphism increases toward the contact with the igneous intrusion. An example of a contact aureolesurrounding 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 0.5 to 2.5 km around the intrusion. Two zones representing different contact metamorphic facies are seen within the aureole. The outer zone contains metapelites in the Hornblende Hornfels Facies, andthe 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 andtemperature of the intrusion. This will control how much heat is available to heat the surrounding country rocks.
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The thermal conductivity of the surrounding rocks. This will control the rate at which heat can be transferred by conduction into the surrounding rocks. In general, the rate of heat flow Q, depends on the thermal conductivity, K,and the temperature gradient, (∂T/∂x)
Q = K(∂T/∂x) 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 surroundingcountry 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 fora 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 complicatedbecause 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 1100oC, into dry country rock at a temperature of 0oC. The width of dike is assumed to be 100 m, and the latent heat of crystallization is assumed to be released between 1100o and 800o....
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