111-30 - 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 2: Regional Metamorphism Lecture Goals : A) Foliated rocks part 1 B) Folitaed rocks part 2 c) Non-foliated rocks Reference: Press et al., 2004, Chapter 9; Grotzinger et al., 2007, Chapter 6; GY 111 Lab manual Chapter 4 A) Foliated Rocks part 1 If you recall the last lecture, you will realize that regional metamorphism involves the combination of heat and pressure. Pressure is the agent that is responsible for the development of foliation in many of the metamorphic rocks that develop during mountain building. Foliation can be defined as a near parallel alignment of "platy" minerals in metamorphic rocks." Platy minerals are those minerals that have a sheet-like crystallographic structure. Remember this from our discussion of the silicate minerals? The sheet silicates (phyllosilicates) included the common mica group minerals biotite, muscovite and chlorite . It is also necessary to include another mineral group of silicate minerals to the phyllosilicates if we are going to be able to explain the origin of foliation in regionally metamorphosed rocks. The clay 1 group of minerals includes kaolinite , which I know all of you will recall is one of the major chemical weathering products of hydrolysis. Kaolinite and most other clays like biotite, muscovite and chlorite, have a sheet structure; however, unlike the mica minerals, the clay minerals are exceeding tiny (less than 10 microns or 0.01 mm in size), and the only way that you can see their platyness is via an electron microscope (see scanning electron microscope photo of kaolinite to right from: http://www.ktgeo.com). The reason for resurrecting mineralogy at this stage in the class is because it is the clay minerals in parent rocks that ultimately develop foliation in regionally metamorphosed rocks. In order to understand this process, a rather complex diagram is required. Moreover, we need to first consider the composition of a platy-mineral rich parent rock. 1 One of the most confusing things about geology is the double meaning of some geological terms. Clay minerals are a group of very fine-grained phyllosilicates. This is a mineralogical term. Clay-sized sediments are a group of very fine-grained particles. This is a sedimentological term. If it helps, most clays are clay sized sedimentary particles.
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GY 111 Lecture Notes D. Haywick (2008-09) 2 Which platy mineral-rich rock we choose is largely a matter of personal preference. We could choose a mica-rich igneous rock like granite or diorite or andesite, but since we just finished the sedimentary rocks, and considering that your humble instructor is a sedimentologist 2 , let's instead consider a shale. The colour of the shale doesn't matter, but just to be sure that we are as precise as possible, let's make it a red shale. Were you to examine a red shale under a powerful microscope (see cartoon at the top of
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This note was uploaded on 02/04/2012 for the course GLY 111 taught by Professor Haywick during the Fall '11 term at S. Alabama.

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111-30 - GY 111 Lecture Notes D. Haywick (2008-09) 1 GY 111...

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