111-32 - GY 111 Lecture Notes D. Haywick (2008-09) 1 GY 111...

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GY 111 Lecture Notes D. Haywick (2008-09) 1 GY 111 Lecture Notes Metamorphism 3: Contact Metamorphism Lecture Goals : A) Metamorphic Aureoles B) Isograds C) Index minerals and metamorphic grade Reference: Press et al., 2004, Chapter 9; Grotzinger et al., 2007, Chapter 6; GY 111 Lab manual Chapter 4 A) Metamorphic Aureoles Contact metamorphism is induced by plutons as they pass upward through the lithosphere and crust toward the surface of the Earth, or by other magma-filled intrustions like dikes and sills. These bodies all contain molten rock (magma) which heats the enclosing parent rock to high temperatures and therefore induces heat only metamorphism. As previously discussed, heat only metamorphism is called contact metamorphism. Unlike regional metamorphism, contact metamorphism is generally localized to the immediate vicinity of the intrusion. In fact, there is a zone of metamorphism that surrounds the intrusion like a halo or an envelop which is officially called a metamorphic aureole (see cartoon at the top of the next page and the image to the left from http://newterra.chemeketa.edu). Within the aureole, metamorphic grade of the parent rock varies from high nearest the intrusion, to low someway from the contact. The width of the metamorphic aureole (or the width of the zone of contact metamorphism) depends on several factors such as the size and
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GY 111 Lecture Notes D. Haywick (2008-09) 2 temperature of the intrusion (dikes and sills are small and therefore induce “thin” aureoles whereas large bodies like plutons and batholiths induce “thick” aureoles), the amount of time the parent rocks were in contact with the intrusion and the type of parent rock. Some rocks conduct heat more readily or are more susceptible to heat than others. Contact metamorphism is relatively easy to
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111-32 - GY 111 Lecture Notes D. Haywick (2008-09) 1 GY 111...

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