Table 7.1 Mineralogy of metamorphic rocks related to protolith and grade

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1 Geology 101 Name(s): Lab 7: Metamorphic rocks Metamorphic rocks have been subjected to sufficient heat and/or pressure to melt some of their constituent minerals, but not all of them. As a result of this selective mobilization of chemicals, only certain chemical reactions can occur, and so a whole new set of metamorphic minerals are crystallized. Throw in the presence of fluids such as water and carbon dioxide (yes, at these pressures, even carbon dioxide can be a liquid), and nature has the means to create even more metamorphic minerals and therefore metamorphic rocks. Note that metamorphic rocks must be formed at depth; metamorphism is not a surface process, and so is distinguishable from mere sedimentation. Rocks that have foliation (a sort of wavy layering, though it can resemble horizontal layering) are metamorphic rocks; the foliation indicates that directional pressure was applied to the rock while the mineralogical changes were occurring. On the other hand, some metamorphic rocks are not foliated; they appear crystalline, like coarse-grained igneous rocks. These metamorphic rocks were subjected to isotropic, or nondirected, pressure. Because there are so many metamorphic minerals (of which you have seen but a few), there are all sorts of ways to name metamorphic rocks. We will concentrate on naming rocks by their metamorphic grade (that is, by the maximum degree of heat and pressure they were subjected to, and not their mineral composition), or, in some unusual cases, by their apparent composition (for instance, rocks like marble, quartzite or metaconglomerate, from which you cannot determine the metamorphic grade). The protolith of a metamorphic rock is the original rock that was metamorphosed into what you see today. As you can see from Table 7.1, the protolith s minerals really do determine the resulting metamorphic rock s composition. Note the differences in mineralogy even at the same grade. Table 7.1 Mineralogy of metamorphic rocks related to protolith and grade Metamor- Facies Protolith phic grade Basalt Shale Low Zeolite Calcite, chlorite, zeolite Zeolite, sodium-rich micas Greenschist Chlorite, amphibole, plagioclase, epidote Chlorite, muscovite, plagioclase, quartz Medium Amphibolite Amphibole, garnet, plagioclase, quartz Garnet, biotite, muscovite, quartz High Granulite Pyroxene, plagioclase, garnet Biotite, orthoclase, quartz, andalusite

2 A metamorphic facies is a name of a set of metamorphic minerals which is uniquely created at a particular pressure and temperature. So, in addition to a metamorphic grade, a rock can belong to a particular metamorphic facies as well! Confused? You bet! However, realize that these terms all have their uses. Note that not all minerals in a given cell in the table above will show up in every specimen of that grade/facies/protolith, but all minerals in the specimen will be named in the cell! One other consideration: there are three different types of metamorphism, related to the particular tectonic setting of the metamorphism. As you are aware, the deeper rocks are drawn into the lithosphere, the higher the temperatures and pressures the rocks are subjected to. This is called regional metamorphism. However, there are two other sets of conditions. Dynamic-type metamorphism occurs under high-pressure but low-temperature (high P, low T) conditions. Contact metamorphism occurs under hightemperature but low-pressure (high T, low P) conditions. This means that, depending on the tectonic setting, three different metamorphic rocks could arise from the same protolith. Table 7.2 summarizes these types. Table 7.2 Mineralogy of metamorphic rocks related to protolith and grade Meta. Facies Protolith type Basalt Shale Regional See table 7.1 Dynamic (low grade) Blueschist Blue amphibole, chlorite, Ca-silicates Blue amphibole, chlorite, quartz Dynamic Eclogite Pyroxene, garnet, not observed (high grade) kyanite Contact Hornfels Pyroxene, plagioclase Andalusite, biotite, orthoclase, quartz Needed: Samples M18 and M 19 (Tub 37), R34 through 45 (Tubs 38 49) 1. Some minerals are made under metamorphic conditions. You have seen a few already in Lab 3 (for instance, talc and graphite). Identify these two other metamorphic minerals: Mineral # M-18 Distinguishing features (color, cleavage, hardness, magnetism, density, etc.) Mineral name M-19

3 2. Look at rock sample R34, a regionally-metamorphosed shale. Name two minerals (hint: you did this already in Lab 3). 3. Given that muscovite is present in R34 but hard to see, what grade of metamorphism does this mineralogy imply (use table 7.1)? 4. Still using that table, what metamorphic facies is R34? 5. So what is the name of the rock? To find this, use the diagram below, or table 7-1 (page 145) in the text. One way that metamorphic petrologists try to quantify the conditions of metamorphism for various rocks is to draw a pressure/temperature (P/T) diagram as shown in the figure on the next page. The field of the graph shows the ranges of various metamorphic facies. The vertical axis shows the depth of the

4 metamorphism and the equivalent pressure in kilobars (kb). 1 bar is approximately 1 atmosphere of pressure, and therefore 1 kb is about 1000 atmospheres of pressure. The horizontal axis shows the temperature of the metamorphism in degrees Celsius. 6. Use the facies from question 4 to determine the range of possible maximum pressures and the range of possible maximum temperatures at which R34 formed. Use units of C for temperature and kbar for pressure. 7. Suppose another area where the protolith was found was subjected to less than 1 kbar of pressure but the same temperature range during metamorphism. Name one other mineral (besides the ones you named in the previous question) you would expect to find. As you have seen, some minerals are quite useful in determining the grade or type of metamorphism because they can only form under certain metamorphic conditions. These are called index minerals.

5 8. You are given the following information about a metamorphic rock: Mineral composition: pyroxene, garnet, kyanite Chemical composition: silicon dioxide 50.24%, aluminum oxide 13.32%, calcium oxide 10.84%, iron oxide 9.85%, magnesium oxide 8.39% Which type of composition is more useful in determining the grade and protolith of metamorphism and why? Or do both lists give equivalent information? 9. a. Now look at R35, which is the same metamorphic grade as R34. What are the mineralogical differences? (In other words, what minerals show up in R34 but not R35? In R35 but not R34?) b. But what is the name of this rock, anyway? Hint: kind of a trick question. 10. In fact, for many metamorphic rocks, the most common mineral in the rock is used as an adjective in front of the rock name. Fill in the appropriate mineral name for the samples below, using the suggested test given: Sample # Test Rock name R34 Cleavage schist R35 Obvious mineral schist R36 Color schist R37 Scratch schist

6 Protolith Intensity of metamorphism Low grade grade High shale slate phyllite rhyolite granite basalt limestone sandstone conglom. metaconglomerate schist marble quartzite gneiss amphibolite 11. What changes in foliation thickness and mineral grain size would you expect to see in a shale as it is subjected to greater temperatures and pressures during metamorphism? (Hint: compare, in order, R38, R39, R34, R40) 12. So fill in the following rock names, using your answer to the previous question and the fact that each sample represents a different metamorphic grade: Sample # Metamorphic grade Rock name R38 R39 R R41 and R42 are nonfoliated metamorphic rocks (they are sometimes called granoblastic rocks ); both of these rocks achieved the same grade of regional metamorphism as R34 and R35 did. Identify the rock names using the hints suggested in the characterization column; identify their protoliths from the table above. Sample # Characterization Rock name Rock protolith R41 Glass plate R42 Acid bottle

7 Plate Tectonics and Metamorphic Rocks 14. R43 is blueschist, a unique type of metamorphic rock that forms under conditions of high pressure and low temperature. Label the area on the crosssection below where you might expect blueschist to crystallize. 15. So, if you were to find blueschist as you walked along the Appalachian Trail in North Carolina, what could you infer about the history of the East Coast of the US? 16. R44 is serpentinite, which blueschist often becomes over time. A key mineral in blueschist is forsterite, a form of olivine, with the chemical formula Mg2SiO4. A key mineral in serpentinite is (surprise) serpentine (chemical formula: Mg3Si2O5(OH)4). How does serpentinite form from blueschist? (Hint: consider readily available simple molecules at metamorphic depths and the difference between the two chemical formulae) 17. R45 is hornfels, a unique type of metamorphic rock that forms under conditions of low pressure and high temperature. Label the area on the crosssection below where you might expect hornfels to crystallize. 18. What is hornfels' protolith? Or is there a unique protolith? 19. Why is contact metamorphism such an appropriate term for this type of metamorphism?