GY 111 Lecture Notes D Haywick 2008 09 1 GY 111 Lecture Notes Metamorphism 4 Metamorphic Facies Lecture Goals A Metamorphic facies Reference Press et al 2004 Chapter 9 Grotzinger et al 2007 Chapter 6 GY 111 Lab manual Chapter 4 A Metamorphic Facies This will be the second last lecture on metamorphic rocks That s the good news The bad news is that the subject for today deals with an important concepts in the study of metamorphism the problem of parent rock composition In GY 111 we usually concentrate on sedimentary rocks when we introduce the idea of parent rocks The reason for this is that we just finished off the sedimentary rocks and shales and sandstones are or should be fresh in your memory But any rock can be metamorphosed which means that the parent rocks can be anything Shale sandstone basalt granite rhyolite gypsum even other metamorphic rocks like slate schist and marble can be re metamorphosed So the problem is that it is hard to keep track of all of the possible metamorphic rocks that could originate from all of the possible parent rocks Matters aren t helped by the fact that many different parent rocks might ultimately be metamorphosed to the same metamorphic rock To avoid this problem and any difficulties that might arise when talking to your colleagues about the metamorphic rocks that you are looking at geologists prefer to talk about groups of rocks that occur under the same conditions of pressure and temperature They refer to these rocks as metamorphic facies GY 111 Lecture Notes D Haywick 2008 09 2 Before we start on the facies it is perhaps best to discuss how the concept works A simple diagram like the one to the left can be drawn to relate temperature to pressure The lowest temperatures and pressure are at the top left hand corner of the diagram Pressure increases downwards along the yaxis of the plot while temperature increases laterally along the x axis Note that the area delineating contact metamorphism heat only is easily identifiable along the x axis of the plot The lowest grade metamorphism occurs in the upper left hand corner of the plot whereas the highest grade occurs in the opposite corner At around 800 C most rocks start to melt which takes us out of metamorphism and back into igneous processes The metamorphic facies are simply added in as fields to this plot The diagram below is my version of the metamorphic facies concept There are 7 distinct facies The lowest grade is the zeolite facies so named because the rocks within this pressure temperature range commonly contain zeolite minerals The zeolites are a large group of silicate minerals with interesting chemical properties Some GY 111 Lecture Notes D Haywick 2008 09 3 are excellent ion exchange materials e g for water purification Others absorb odors out of the air You can buy a back of these zeolites from Lowes or Home Depot however they mistakenly sell them as volcanic rock The rocks that form through contact metamorphism plot within the hornfels facies which is a term that you heard in the previous lecture It should now be clear that hornfels are a group of rocks not just one In the other direction pressure only lies the blueschist facies which as the name implies consists of rocks that are schist like and blue The rocks are blue because the minerals they contain are blue The two major rocks in this facies are kyanite schist and glaucophane schist Now you know that pretty blue mineral kyanite last seen in the mineral section of the lab class is a metamorphic product of high pressure and relatively low temperature The greenschist facies consists of a group of rocks that contain green platy minerals like chlorite and epidote Chlorite schist is an ideal example of the rocks within this facies The amphibolite facies comprises relatively high metamorphic rocks like biotite schist and amphibole schist The granulite facies consists of very high grade metamorphic rocks like gneiss The eclogite facies is an extremely high grade region of the plot and comprises metamorphic rocks that are a bit beyond our level in GY 111 There is however a sample of eclogite in your GY 111L metamorphic rock tray just to keep you on your toes One last facies should be mentioned even though it s not on the plot The Migmatite facies plots right along the melting curve of the facies diagram and comprises rocks that have been partially melted We ll ignore this facies in GY 111 as it has crossed over to the dark side of geology igneous rocks By the way it is possible to add the basic metamorphic rock types to the facies diagram see my attempt to the right All of the rocks follow a straight line commonly called the burial curve which shows how pressure and temperature increase the deeper you go into the Earth GY 111 Lecture Notes D Haywick 2008 09 Important terms concepts from today s lecture Google any terms that you are not familiar with Metamorphic facies Zeolite facies Hornfels facies Blueschist facies Kyanite schist Glaucophane schist Greenschist facies Chlorite schist Amphibolite Facies amphibole schist Granulite facies Gneiss 4
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