contactmeta - Contact Metamorphism EENS 2120 Prof Stephen A...

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This document last updated on 30-Mar-2011 EENS 2120 Petrology Prof. Stephen A. Nelson Tulane University Contact Metamorphism 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 only at 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 area surrounding 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 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 0.5 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, 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: Contact Metamorphism 3/30/2011 Page 1 of 7
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z The size and temperature of the intrusion. This will control how much heat is available to heat the surrounding country rocks. z 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, ±w T/ w x) Q = K ±w T/ w x) Thus, the rate at which heat moves by conduction increases if the thermal conductivity and temperature gradient are higher. z 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. z
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contactmeta - Contact Metamorphism EENS 2120 Prof Stephen A...

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