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 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 understand because it is heat only driven However the rocks that are produced via this process are not so easy to identify Because there is no pressure involved in this type of metamorphism no foliation can develop even in rocks that initially contained a lot of clays and platy minerals like the micas Instead the rocks just get cooked A slightly cooked shale or a slightly cooked sandstone look pretty much the same as their uncooked equivalents but if the temperature was high enough say equivalent to med high grade metamorphism then metamorphic minerals like chlorite muscovite biotite and even garnet might form in the metamorphic aureole They just won t develop a foliation There are a couple of things that we must address at this point The first is how do we classify or name non foliated cooked rocks Fortunately you don t need to remember a separate name for a cooked sandstone or a cooked shale or a cooked anything Instead we will group them all together and refer to them collectively as hornfels1 In my humble opinion the hornfels are ugly rocks and while you will see one in the lab component of GY 111 you are permitted to not like it The other item that we have to consider is the mineral assemblages that develop within the metamorphic aureole It is obvious that the highest metamorphic grade occurs nearest the intrusion and that the lowest metamorphic grade occurs away from the intrusion Doesn t it stand to reason that the minerals that form under the highest grade conditions will grow nearest the intrusion and that the minerals that form under the lowest grade 1 In an upcoming lecture you will learn that the term hornfels is actually a metamorphic facies Facies are to metamorphic rocks what groups are to minerals They both allow us to cluster similar things rocks and minerals into groups thereby making classification just a bit easier There are potentially hundreds of different types of metamorphic rocks most of which can be grouped into 7 distinct clusters or facies that are related to pressure and temperature The metamorphic facies will comprise an upcoming lecture GY 111 Lecture Notes D Haywick 2008 09 3 conditions will grow well away from the contact If you predict that there will be zones of different minerals within the metamorphic aureole pat yourself on the back There are and they are called isograds see figure at the bottom of the previous page B Isograds Isograds are defined as lines that represent the same pressure temperature conditions think of them as being combinations of isotherm and isobars And with this definition I need to stress that isograds are not restricted to contact metamorphism Isograds as you will see shortly are also associated with regional metamorphism However discussing the isograds for contact metamorphism first is desirable because it is easier to envision a heat only situation than one involving heat and pressure Consider a close up view of a metamorphic aureole i e the cartoon below In this situation the highest grade metamorphism where the heat is the greatest that occurs in direct contact with the pluton is forming biotite The red band in the diagram below is called the biotite isograd because these are the perfect conditions for biotite to grow The next band orange is the interval where muscovite is most likely to form medium grade metamorphism and is called the muscovite isograd The yellow band is the chlorite isograd and it marks the other edge of the metamorphic aureole where the metamorphic grad was the lowest Regional metamorphism is more complex because there are many more minerals that form when pressure and heat are affecting rocks The diagram at the top of the next page which comes from the 2004 version of your text book shows you what I m talking about Here you see isograds for minerals like kyanite staurolite sillimanite a mineral that you do not see in GY 111 and garnet all formed under conditions of high pressure but GY 111 Lecture Notes D Haywick 2008 09 4 variable temperature The region defined by the sillimanite is the highest metamorphic grade of the figure C Index Minerals and Metamorphic Grade Okay we are just about done for today s lecture All that is left is relate minerals to metamorphic grade By now you should understand that there are some minerals like chlorite that only form under minimal metamorphism i e low grade and some minerals like garnet that only form during high grade metamorphism Geologists use this knowledge to map out the extent of metamorphism around intrusions contact metamorphism across faults cataclastic metamorphism and throughout mountain belts regional metamorphism There are many different index minerals that indicate the grade of metamorphism that parent rocks have