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Name: _________________________ SID: ______________________

EPS 50 Lab 4: Metamorphic Rocks Grotzinger and Jordan, Chapter 6

Introduction: Previously we looked at rocks crystallized from magma (igneous rocks) and rocks formed at Earth’s surface through deposition and lithification (sedimentary rocks). In this lab we will classify metamorphic rocks. We will also investigate the relationship between environmental conditions and the effect of temperature and pressure on metamorphic rocks. Remember that the diversity of rock in nature is generally continuous, and we are learning interpretive guidelines. The underlying principle for understanding metamorphic rocks and geological processes in general is that minerals and rocks are stable only under the conditions at which they form. Change the conditions and the rocks will change to adapt to the new conditions. Objectives: Become familiar with the temperature-pressure relationships and tectonic environments associated with the different metamorphic rocks. Answers: Please answer the questions on the separate answer sheet handed out during lab. Only the answer sheet needs to be returned. Explanations should be concise; most questions can be answered with a few sentences or less. All answers should be your own, but we encourage you to discuss and check your answers with 2-3 other students. This lab is graded out of 75 points. Overview of Metamorphic Rocks

Of all the rock types, metamorphic rocks come in the greatest variety. Metamorphic rocks have been subjected to heat or pressure such that the chemistry and structure of the rock is altered from the starting (parent) material: the size, shape, and composition of the component mineral crystals and/or grains change. Another possible change is the rearrangement of atoms to form new minerals. Most of the time no atoms are gained or lost from outside the original rock. One exception is when superheated water from nearby magma reacts with the rocks as they are undergoing changes. Metamorphic changes take place without melting, although the temperature may be high enough to cause the rocks to become quite ductile and plastic. Metamorphic rocks are formed in places in the Earth where pressures and/or temperatures are unusually high, for example at tectonic plate boundaries where rocks in the down-going plate are subjected to high pressures upon subduction. Magma bodies also raise the temperature of surrounding rocks. Minerals that form and break down in response to changes in temperature and pressure can be used to determine the conditions a rock has experienced. The common minerals in metamorphic rocks are the same as those found in igneous

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rocks with the addition of a few high-temperature, high-pressure minerals including kyanite, garnet, sillimanite, andalusite, and corundum. In addition, metamorphic rocks can contain combinations of minerals that would never be found in igneous rocks such as quartz and garnet. Feldspar is also a common mineral in some metamorphic rocks. 1) Before beginning with rock identification, examine the samples in Tray 1. Write down the chemical formula of each mineral. Also note useful diagnostic properties of the minerals (color, cleavage, crystal habit, hardness, etc.) which may help you recognize some characteristic minerals of metamorphic rocks. Some of the minerals will be new. (14 pts)

Metamorphic Textures

One way to examine and classify metamorphic rocks is by their texture. Metamorphic rocks are often subjected to directed pressure. Since pressure depends on depth, the rocks are squeezed vertically, like smashing a piece of clay between a book and a tabletop. This restricts the movement of atoms in the vertical direction but often allows movement horizontally. For this reason many of the common metamorphic minerals are elongated in the direction perpendicular to the directed pressure. These are called foliated rocks. In some cases rocks are subjected to heating without intense pressure or have simple mineralogical makeup and form non-foliated rocks. Foliated Rocks (Tray 2)

The foliation of metamorphic rocks results from the alignment of sheet silicates such as micas and chlorite, or from the alignment of minerals with elongated crystals such as kyanite and amphiboles. Foliated rocks may also contain minerals with equant (equal length along all axes, e.g. cubic) habit. These appear like nuts in a cookie and are called porphyroblasts and are similar to the phenocrysts in igneous rocks.

Foliated metamorphic rocks are often the result of regional metamorphism (conditions common in mountain-building) or shear deformation (conditions common in fault zones). Like sedimentary rocks, they are classified according to the size of the primary mineral grains. Foliated rock classifications are listed and described below in order from low grade to high grade. In general, higher-grade metamorphic rocks have larger crystals, but there are exceptions, such as mylonite.

Slate (sample 28): The slaty texture is caused by the parallel orientation of microscopic grains, and the rock is characterized by a tendency to separate along parallel planes. This feature is a property known as slaty cleavage. Don’t confuse slaty cleavage or rock cleavage with mineral cleavage, which is related to the internal atomic structure of the mineral. Slate is formed when cool brittle rocks are subjected to high pressures. They are usually black but may be green, gray, or red. Many gradations exist between shale and slate, and it is not always easy to distinguish them; however, shale tends to fall apart more easily.

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Phyllite (sample 509): A phyllitic texture is formed by the parallel arrangement of platy minerals, usually micas, generally too small to identify. Phyllites have a distinctive surface "sheen" which results from the alignment of mica or other platy grains. Phyllite grain size may be just on the edge of visible with the unaided eye. Schist (sample 629): A schistose texture results from the subparallel to parallel orientation of platy minerals such as chlorite or micas. Schists may also display a surface “sheen” like phyllites. Other common minerals present are quartz and amphiboles. A schistose texture lies between the parallel platy appearance of phyllite and the distinct banding of gneissic texture with an average grain size generally smaller than in a gneiss. Gneiss (sample 434): A coarsely foliated gneissic texture is one in which the minerals have been segregated into discontinuous bands, each of which is dominated by one or two minerals. The individual mineral grains are macroscopic and impart a striped appearance to a hand specimen. Light-colored bands commonly contain quartz and feldspars, and dark bands are commonly composed of hornblende and biotite. Other foliated metamorphic rocks include mylonite (sample 377), a fine-grained rock produced in intense ductile deformation (shearing) zones. A mylonitic texture is one in which pre-existing grains are deformed and recrystallized as finer grains. 2) What grade of metamorphic rocks might you expect to find in a subduction zone? Name one rock that forms under these conditions. What grade might you expect in a continental collision zone such as the Himalayas, and what is a rock you’d expect to find here? (6 pts) Non-foliated Rocks (Tray 3)

Because some minerals like calcite and quartz do not easily form elongated crystals under directed pressure, metamorphic rocks that contain them are usually not foliated. These and others that do not show obvious directional characteristics are called non-foliated rocks. Non-foliated rocks are often the result of contact (thermal) metamorphism on rocks with simple mineral chemistry; quartzite is one example. These rocks often bear characteristics of the original rock, called the protolith, with hornfels being the common exception. A few examples of non-foliated metamorphic rocks are listed and described below. Metaconglomerate (sample 602): Composed of pebbles and gravel that have been flattened due to directed pressure. The parent rock for metaconglomerate is generally a sedimentary deposit which becomes buried, compacted, and cemented. Recrystallization of the pebbles and cobbles and surrounding sediment grains also occurs. Hornfels (sample 549): A fine-grained (i.e. dense-textured), non-foliated, tough rock with a random fabric of interlocking grains. Usually hornfels have nondescript appearance because they

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are some medium to dark shade of gray, lack distinctive structural characteristics, and contain few if any recognizable minerals in hand specimen. Marble (sample 498): Contact metamorphism of limestone forms marble, a coarse-grained rock consisting of interlocking crystals of calcite. The marbling pattern that is common in marble is due to the separation of impurities in the original limestone. Black streaks come from carbon, green from chlorite or other ferromagnesians, gold from pyrite and red from iron oxides. Marble will fizz when HCl contacts its surface unless it is primarily dolomite marble, which fizzes only when in powder form. Quartzite (sample 299): When quartz sandstone is exposed to high temperatures, the quartz grains become plastic and weld together, forming a nearly solid mass of quartz. Calcareous sandstones may also have large amounts of wollastonite (calcium silicate) that forms from the reaction of calcite and quartz at high temperatures.

3) Give two examples of how contact metamorphism might occur and two examples of how regional metamorphism might occur (think of what conditions prevail at different types of tectonic plate boundaries). Provide a brief description of each process. (8 pts) Naming Metamorphic Rocks (Tray 4)

Metamorphic rock names are commonly derived utilizing one or more of the following criteria (Yardley, 1989):

a) nature of the parent material (bulk composition) b) metamorphic mineralogy c) rock texture (grain size and fabric development) d) any appropriate special name To name a metamorphic rock, the texture and characteristic mineralogy must be identified. For example, a schist with numerous garnets and abundant biotite would be referred to as a garnet-biotite schist. 4) Examine the five samples in tray 4. Describe the texture (e.g. gneiss, schist, phyllite), the dominant minerals (if grains are too small to identify, write N/A), and assign a name for each sample. Also note the probable protolith, the rock type before metamorphism occurred (e.g. igneous, sedimentary, or even metamorphic. Refer to Table 6.1 (pg. 159) of Grotzinger and Jordan. Note that without chemical analyses, it is difficult to determine the protolith for some high-grade metamorphic rocks. (20 pts)

5) This handout describes contact metamorphism and regional metamorphism. Describe two

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other processes or ways by which rocks can undergo metamorphism. (4 pt) Metamorphic Facies (Tray 5)

One of the primary goals of metamorphic petrology is to interpret the pressure (P) and temperature (T) history of a rock or set of rocks, because it provides information about the tectonic history of a region. Finnish geologist Pentii Eskola devised a facies scheme in 1915 (that was later revised by Berkeley faculty member Francis Turner) to identify assemblages of minerals formed under particular P-T conditions.

Metamorphic minerals such as garnet and andalusite that help determine which facies a rock belongs to are called index minerals. The assemblage of minerals that defines a facies only forms at certain temperature and pressure ranges and provides a crude geothermometer and geobarometer just by looking at the rocks. Rock names in the facies classification (e.g. schist, gneiss) are unique metamorphic rock names that may also describe the texture or protolith of the rock. The metamorphic facies are shown in the chart below:

6) Tray 5 contains examples of rocks from most of the metamorphic facies. Examine each of these rocks and list the rock ID#, mineralogy, and texture (e.g. foliation, grain alignment) for each sample. Based on mineralogy and texture, find which facies the rock belongs to and list the temperature and pressure ranges during formation using the P-T diagram above. Refer to your book for a description of the two without samples and include those as well. Tables and graphs shown on pages 161-162 of Grotzinger and Jordan will help you decide which facies the samples

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belong to. (10 pts)

7) What does the term retrograde metamorphism mean? How might you identify it in a rock sample? (4 pts)

A tiny crystal of garnet can tell us about the history of the place where it was found. Geologists use variations in the chemical composition of garnet porphyroblasts to deduce the relative rates at which the rocks containing them were buried and then exhumed, and these rates reflect particular plate tectonic settings. The chemical composition of a garnet porphyroblast generally varies progressively from its center to its edge. This progression gives us a sense of change in pressure or temperature as a function of time (the center of the crystal records earlier conditions, and the edge of the crystal records later conditions). In particular, changes in the calcium content of garnet track changes in pressure, whereas its iron content is more sensitive to temperature changes. Specifically, an increase in relative calcium content indicates an increase in pressure, and an increase in relative iron content indicates an increase in temperature. The following data were obtained by measuring the number of atoms of four elements at the center and at the edge of a garnet porphyroblast:

Element Abundance at center Abundance at edge Ca 0.3 0.05 Fe 2.25 2.23 Mg 0.2 0.52 Mn 0.25 0.2

8) Based on this table, calculate the relative abundances (in ratio) of calcium and iron at the center and the edge of the crystal, respectively. (show your work, 4 pts) 9) Based on your calculation above, make a time plot of pressure and temperature (each with a separate curve). Do either pressure or temperature change significantly from center to edge? What kind of condition (igneous intrusion or subduction) do you think best represents the geologic history in this crystal? Briefly explain. (5 pts)

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