Geology 101

Name(s): Lab 4: Sedimentary and metamorphic rocks

More sedimentary rocks 1. Now consider the actual mineral grains in rock samples R18, R19, R24 and R25. Because sand-sized dark minerals are very hard to identify, sedimentary petrologists (much to the horror of igneous petrologists) use the term “dark lithic fragments” to categorize lots of little dark minerals. Sample R18

Most common mineral

R19 R24 R25 Sedimentary rocks can be classified in a number of ways. For our purposes, the first division to be made is between clastic sedimentary rocks (those that are made of weathered and eroded grains) and non-clastic or other sedimentary rocks (these include sedimentary rocks of biological and chemical origin). You will use the Sedimentary Rock ID flow chart on the next page. One other thing: fossils (Latin for “dug up”) are the remains of living organisms. If the fossil is literally the body of the organism (or parts such as skeleton or shell), it is called a hard parts fossil; if the fossil merely records the shape of an organism (like a leaf impression in silt) or the passage of an organism (like preserved footprints), then it is called a trace fossil. For any sedimentary rock, if it contains any fossils, use the adjective fossiliferous in front of the rock name. Flow chart for identifying sedimentary rocks — If the rock is made of grains or other materials which have been deposited by wind, water or ice, or else was generated by biological or surface chemical activity, it's a sedimentary rock. First step. If the rock is made of broken up bits of rock (including extremely fine grains) → GO TO Second step alternative A.

Else → GO TO Second step alternative B.

Second step alternative A. Consider the most common grain size in the rock from the following list. cobble or pebble sand silt clay

> 2 mm

easily visible to naked eye; "grains" may contain identifiable minerals 0.062 — 2 mm visible to naked eye 0.005 — 0.062 mm not visible but can be felt between fingers or across teeth < 0.005 mm not visible; cannot be felt between fingers or across teeth

If the most common grain size is cobble or pebble → conglomerate If the most common grain size is sand → sandstone If the most common mineral is quartz → arenitic sandstone If the many dark minerals are present → arkosic sandstone If the most common grain size is silt → siltstone If the most common grain size is clay → claystone (shale) Second step alternative B. Identify the most common mineral in the specimen (use mineral ID chart if necessary). If the most common mineral is quartz → chert If the most common mineral is halite → rock salt If the most common mineral is gypsum → rock gypsum If it is black-colored, not very dense and flaky → coal (also look for plant fibers) If it fizzes, the most common substance is calcium carbonate, usually in the form of the mineral calcite (be careful you are not fizzing the cement) If the rock is not very dense and pure white → chalk If the rock is made of broken-up shells → coquina If the rock is dense and white, gray or black → limestone 2. Fill in the following table for clastic sedimentary rocks. Begin by determining the average grain size of the clasts in the rock (use the grain size terms in the flow chart), then the grain sorting (the choices are: well-sorted, moderately sorted, poorly sorted and unsorted) and the grain roundness (the choices are; well-rounded, sub-rounded, sub-angular and angular). See the diagrams to determine which type of rounding and sorting the grains have. Under fossils,

your choices are none, hard parts or trace fossils. Finally, identify the rock, using the flow chart. Clastic sedimentary rocks Sample #

Grain size

Grain sorting

Grain roundness

Fossils

Rock name

R18 R19 R20 R21 R22 R23

3. Fill in the following table for “other” sedimentary rocks. Begin by determining the rock’s mineral composition. Then add any other details that help identify it. Under fossils, your choices are none, hard parts or trace fossils. Finally, identify the rock, using the flow chart.

“Other” (chemical and biological origin) sedimentary rocks Sample #

Mineral composition

Other defining details

Fossils

Rock name

R24 R25 R26 R27 R28 4. Return to R18 and R19 and circle the correct answers: a. Which rock contains the most stable mineral clasts? (at the Earth's surface) b. Which rock is composed of rounder grains?

R18

R19

R18

R19

c. Which rock is more well-sorted?

R18

R19

d. Based on a-c, which sample was deposited furthest from its source (and thus is called mature)?

R18

R19

5. The energy of the system (how much force is behind the medium of transport (air or water)) can be characterized by the size of the particles the system can carry. For instance, high-energy systems can carry large grains; low-energy systems can carry small grains. Examine and rank rocks R18, R21 and R20 in order from highest energy to lowest energy depositional system.

6. a. Some limestones (R33) are dense, fine-grained and black. So is basalt (R5). What test can you perform to tell them apart, and how does each behave in the test?

b. By the way, in general, why didn’t you worry about fossils in igneous rocks?

The place in which the sediment is deposited or the organisms lived is called the depositional environment. Examples of depositional environments include terrestrial environments (like lakes, deserts and rivers), transitional environments (like beaches and tidal flats) and marine environments (like continental shelves and the abyss). Note that, over time, a beach area may be uplifted by plate tectonics so that you will find a transitional depositional environment quartz-rich sandstone deep in a mountain range!

7. In what depositional environment did rock R25 form? Hint: these kinds of rocks are called evaporites. Please explain how they form.

8. Look at sedimentary structure S1, which is an example of ripple marks. Are they symmetrical or asymmetrical? Based on that answer and on the wavelength of the ripples, is it more likely that these ripples were originally deposited in a desert, a river, or a tidal flat? How are they preserved so that you can see them today?

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 4.1, the protolith’s minerals really do determine the resulting metamorphic rock’s composition. Note the differences in mineralogy even at the same grade. 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. Table 4.1— Mineralogy of metamorphic rocks related to protolith and grade MetamorFacies Protolith phic grade Basalt Shale Low Zeolite Calcite, chlorite, zeolite Zeolite, sodium-rich micas Medium Greenschist Chlorite, amphibole, Chlorite, muscovite, plagioclase, epidote plagioclase, quartz Amphibolite Amphibole, garnet, Garnet, biotite, plagioclase, quartz muscovite, quartz High Granulite Pyroxene, plagioclase, Biotite, orthoclase, garnet quartz, andalusite

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. Convergence-type metamorphism occurs under high-pressure but lowtemperature (high P, low T) conditions. Contact metamorphism occurs under high-temperature 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 4.2 summarizes these types. Table 4.2 — Mineralogy of metamorphic rocks related to metamorphic type Meta. Facies Protolith type Basalt Shale Regional See table 4.1 Convergence Blueschist Blue amphibole, Blue amphibole, (low grade) chlorite, Ca-silicates chlorite, quartz Convergence Eclogite Pyroxene, garnet, not observed (high grade) kyanite Contact Hornfels Pyroxene, plagioclase Andalusite, biotite, orthoclase, quartz Needed: rock samples R34 through R44. 9. Look at rock sample R34, a regionally-metamorphosed shale. a. Name three minerals in R34.

b. What grade of metamorphism do those ranges imply (use table 4.1)?

c. In what metamorphic facies is R34?

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 metamorphism and the equivalent pressure in kilobars (kb). 1 bar is 100000 Pa or 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.

10 a. Use the named minerals from question 9b 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.

b. Suppose another area where this rock is found was subjected to less than 1 kbar of pressure during metamorphism. Name two minerals (besides the ones you named in part a) 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. The next figure shows some ranges under which certain minerals will form under regional metamorphic conditions, and some of the associated rocks from a shale protolith.

11. 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?

12. a. Now look at R35. List some minerals in this regionally-metamorphosed rock.

b. What is its metamorphic grade? What is its protolith?

c. How does R35’s grade compare to R34’s grade?

Protolith

Intensity of metamorphism Low grade

shale rhyolite granite basalt limestone sandstone

High grade

slate schist gneiss amphibolite marble quartzite

13. Now find R40 and R41; both of these rocks achieved the same grade of regional metamorphism as R34 and R35 did. Determine the protoliths of these rocks (hint: the standard mineral tests work well here). R40’s protolith: R41’s protolith: 14. In what two ways would you be able to distinguish rock R38 from its protolith (note that rock R38's protolith is also in the same drawer)? (Hint: ignore color. Look at the shape and “brokenness” grains within the rocks).

15. Why doesn't rock R40 have a foliation? (Hint: look at its mineralogy)

16. 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, R37, R34, R36)

17. Fill in the following table (you have most of the answers by now). For composition, your choices are: a mineral name, "clay minerals" and "rock fragments". For foliation, write "F" for rocks with observed foliation and "NF" for non-foliated rocks. For texture, your choices are slaty cleavage, schistosity, gneissic banding or crystalline (meaning “unfoliated”); you decide the criteria. Finally, identify the rock. Rock

Composition

Foliation

Texture

Rock name

R34 R35 R36 R37 R38 R39 R40 R41 Plate Tectonics and Metamorphic Rocks 18. a. Rock R42 is blueschist, a unique type of metamorphic rock that forms under conditions of high pressure and low temperature. Label the area on the cross-section on the next page where you might expect blueschist to crystallize. b. 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?

19. Rock R43 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)

20. a. Rock R44 is hornfels, a unique type of metamorphic rock that forms under conditions of low pressure and high temperature. Label the area on the crosssection where you might expect hornfels to crystallize. b. What is hornfels' protolith? Or is there a unique protolith?

c. Why is contact metamorphism such an appropriate term for this type of metamorphism?