GLY4310 88 points 16 took exam Name April 24, 2015 Florida Atlantic University PETROLOGY -- FINAL EXAMINATION KEY Scores to the left of the answer in red are the number of incorrect responses. Instructor comments and answers are in blue. True-False - Print the letter T or F in the blank to indicate if each of the following statements is true or false. Illegible answers are wrong. (1 point each) 0 T 1. Barrow found a much greater change in the pelitic rocks he studied, compared to the sandstones. 0 T 2. Ca-bearing minerals, like laumonite, prehnite, and pumpellyite are stable in waterrich, carbonate-free fluids. 1 F 3. Ordinary prograde metamorphic reactions are common in high-temperature mafic igneous rocks. 0 F 0 F 1 T 6. Hydrous pelitic and quartzo-feldspathic rocks do not reach granulite grade. 0 T 7. Cordierite has a large molar volume, which indicates it is stable at low pressures. 0 T 8. The presence of garnet in a facies indicates it formed at medium to high pressures. 1 F 9. When andalusite is present, the pressure at the time of formation exceeded 0.4 GPa. 2 F 10. Counterclockwise P-T-t paths are found only in granulite facies rocks. 6 F 11. Around catazonic plutons, contact metamorphic aureoles arm narrow and easily distinguished from country rock. 3 T 12. At very low metamorphic grade, the preservation of original igneous textures and minerals is common. 4. For most P-T-t paths, Pmax and Tmax occur at the same time. 5. In the low-grade mafic assemblages, complete alteration of the protolith minerals occurs, and it is easy to define new characteristic mineral assemblages 1 2 T 13. A metamorphic facies may be defined as a set of repeatedly associated mineral assemblages. 1 F 14. All minerals show the transition from brittle to ductile under very similar conditions. 1 T 15. Blueschist preservation, rather than blueschist generation, may be more important in determining whether blueschist rocks are seen in a region. 7 F 16. Cataclasis refers to mechanical crushing and grinding, with no recrystallization. Rocks exhibiting this behavior have undergone ductile flow. 4 T 17. Because there is little difference in mafic rock facies that develop at low, medium, or even high pressure, the biggest argument for the inclusion of the low pressure facies comes from the pelitic rocks. 5 F 18. High pressure favors the formation of low density phases. 2 T 19. The continental geotherm is considerably higher than the oceanic geotherm, due to LIL elements, which concentrate in continental crust and generate significant heat. 8 F 20. Both lithospheric and deviatoric stress can produce deformation in rocks. 1 T 21. Eclogites span a greater temperature range than any other facies. 0 F 22. New minerals will growth during deformation, parallel to σ1. 1 T 23. The effects of contact metamorphism are most evident in shallow, low-pressure environments. 0 T 24. Many orogenic episodes produce repeated episodes of deformation and metamorphism, leaving a polymetamorphic imprint. 1 F 25. Prograde metamorphic reactions are exothermic. 1 T 26. Rock materials formed under high-strain conditions may be either cohesive or non-cohesive. 2 F 27. Ordinary prograde metamorphic reactions are common in high-temperature mafic igneous rocks. 3 T 28. Although garnets exhibit hexoctahedral isometric structures, they rarely appear are cubes or octahedrons. The atomic population of the {100} and {111} family of planes is depleted, resulting in this seemingly strange behavior. 2 7 F 29. Hemimorphite and often amorphous chrysocolla are primary minerals formed in the oxidized portions of zinc and copper deposits, respectively. They are good indicator minerals for potential ores beneath them. As the description indicates, they are secondary, not primary. 10 T 30. In the system Forsterite-Nepheline-Silica as pressure increases the eutectic shifts away from silica apex. Multiple-Choice - Choose the best response to each statement or question. Print the letter corresponding to your choice in the blank. (1 point each) 0 D 1. Fluid may refer to: A. Liquid B. Gas C. A single phase above the critical point D. All of the above 1 C 2. Xenoliths which fall into magma chambers may be rapidly heated, and show substantial alteration, or wall rocks in or near volcanic necks may show a type of contact metamorphism known as: A. Burial metamorphism B. Hydrothermal metamorphism C. Pyrometamorphism D. None of the above 13 B 3. In the English Lake District, Ordovician Skiddaw Slates are intruded by granite and granodiorite bodies. The metamorphic aureole is divided into the inner, middle, and outer zones. The basis for this classification within the zones is: A. Mineralogy B. Structure C. Temperature D. All of the above 10 A 4. Rocks that contain cordierite and andalusite are likely to have protoliths rich in which element? A. Al B. Ca C. Fe D. Mg 3 2 C 5. Who proposed the addition of the albite-epidote and hornblende hornfels facies to Eskola's original set of five facies? A. P. Eskola B. D. Coombs C. W. Fyfe D. V. Goldschmidt 1 C 6. As pressure increases, amphibolite or granulite facies rocks transform to eclogites. It is the initial breakdown of which mineral that marks this transition? A. Enstatite B. Hornblende C. Plagioclase D. Garnet 7 C 7. Which of the following sets of metamorphic facies is typical of rocks seen in areas where the protolith reacts quickly, there is a high geothermal gradient, and fluids are abundant? A. Albite-epidote hornfels, hornblende hornfels, pyroxene hornfels, and sanidinite B. Blueschist and eclogite C. Zeolite and prehnite-pumpellyite D. None of the above 2 D 8. Which of the following terms is used in place of fault-zone metamorphism? A. Dislocation metamorphism B. Shear-zone metamorphism C. High-stress metamorphism D. All of the above 6 B 9. Who first formulated the idea of metamorphic facies? A. C.E. Tilley B. Pentii Eskola C. George Barrow D. Alfred Harker 1 B 10. Who proposed the addition the zeolite and prehnite-pumpellyite facies to Eskola’s original set of five facies? A. Eskola B. Coombs C. Fyfe D. All of the above 4 1 B 11. The Barrovian type of metamorphic trajectory is characteristic of which baric series? A. High P/T B. Medium P/T C. Low P/T D. May be any of the above 2 A 12. At the high temperature end of the prehnite-pumpellyite facies, prehnite may break down to yield what mineral? A. Actinolite B. Hornblende C. Enstatite D. Forsterite 11 B 13. Eclogites associated with migmatic gneisses are usually associated with which temperature group? A. Low B. Medium C. High D. Any of the above 3 D 14. Where might a low P/T baric series be found? A. Rift zone B. High heat flow orogenic zone C. Contact metamorphic zone D. All of the above 3 A 15. Who proposed the addition of the granulite, epidote amphibolite, and glaucophane schist facies to Eskola’s original set of five facies? A. Eskola B. Coombs C. Fyfe D. Goldschmidt 1 C 16. During ocean floor metamorphism, which pair of elements is removed from the protolith? A. Fe, Mg B. Na, Mg C. Ca, Si D. K, Ti 5 1 C 17. A cohesive high-strain rock that has undergone significant recrystallization is known as what? A. Phyllonite B. Microbreccia C. Blastomylonite D. Cataclasite 1 C 18. The starting material from which a reaction or recrystallization begins is called what? A. Granofels B. Isograd C. Protolith D. Spillite 1 C 19. Which of the following describes tension? A. σ1 > σ2 > σ3 B. σ1 > σ2 . σ3 C. σ1 > σ2, both + ; σ3 is negative D. Either A or B 0 B 20. Which of the following is the most effective mineralizer? A, Water B. Fluoride ion C. Carbon dioxide D. Sulfur dioxide 0 B 21. Where might the geothermal gradient be lower than average? A. Near zones of crustal thickening or crustal extension B. In a subducting plate C. Near an igneous intrusion D. Above a hot spot 2 A 22. Albite-epidote hornfels, hornblende hornfels, pyroxene hornfels, and sanidinite all belong to which group of metamorphic facies, as classified by Yardley? A. Low pressure facies B. Medium pressure facies C. High pressure facies D. Low grade facies 3 A 23. The perisiterite "solvus" occurs in which plagioclase composition range? A. An7-17 B. An27-37 C. An47-57 D. An77-87 6 4 C 24. Most exposed granulite facies rocks are found: A. Associated with basalts and gabbros near a former ocean ridge, where the heat flow was very high B. Associated with shield volcanism, in areas of greatly elevated geothermal gradient C. In deeply eroded, Precambrian rocks associated with continental shields D. All of the above 4 D 25. Elements described as "Iron-loving" and composed of elements, typically iron and alloying elements, which form dense phases, which are often metallic, although bonding to sulfur or carbon is possible, belong to which Goldschmidt classification. A. Atomphile B. Chalcophile C. Lithophile D. Siderophile 4 D 26. If the weight percent alumina concentration approximately equals the weight percent (Na2O) + (K20) concentrations, the rocks are classified as? A. Metaluminous B. Peralkaline C. Peraluminous D. Subaluminous 4 B 27. On the continuous side of Bowen’s Reaction Series, what solid solution series is crystallizing? A. Olivine B. Plagioclase feldspar C. Potassium feldspar D. Amphiboles Fill-Ins - Write in the word or words which best completes each statement or answers each question. (1 point per blank) 0 1. In metamorphic assemblages, the symbol ± is short for what phrase? WITH OR WITHOUT 3 2. Granulites are thought to form in regions of crustal thickening. Why? AREAS OF CRUSTAL THICKENING HAVE VERY HIGH GEOTHERMS, WHICH ALLOWS GRANULITES TO REACH THE 700-1000EC RANGE THEY REQUIRE TO FORM. 0 3. As grade increases, the color of hornblende changes from green to brown. Why? DUE TO INCREASING TI CONTENT IN THE HORNBLENDE 7 8 4. Non-foliated cohesive rocks include microbreccias and cataclasites. How do they differ? MICROBRECCIAS HAVE LESS THAN 70% CLASTS. CATACLASITES HAVE MORE THAN 70% CLASTS 5.5 5. The term psammitic refers to what? A MIXTURE OF SAND AND SHALE, WHICH MAY BECOME A PROTOLITH. 7 6. Chlorite-quartz rocks formed by ocean-floor metamorphism may be the protolith of cordierite-anthophyllite metamorphic rocks. They have a distinct chemical signature, which no igneous or sedimentary rock has. What is this signature? THEY ARE HIGH IN MG, LOW IN CALCIUM. 2 7. Parallel belts like the Sanbagawa and Ryoke belts, usually separated by a fault, are seen in a number of regions around the Pacific Ocean. What name did Miyashiro give to such belts? PAIRED METAMORPHIC BELTS 2 8. Most museum meteorite collections are dominated by irons. When meteorites are collected at fall sites, stones comprise about 94% of collected samples. Explain what causes this discrepancy. IRONS ARE MUCH EASIER TO IDENTIFY, BECAUSE THEY ARE USUALLY MAGNETIC, VERY DENSE, AND DON’T RESEMBLE MOST TERRESTRIAL SAMPLES. STONES OFTEN LOOK LIKE TERRESTRIAL ROCKS, ESPECIALLY WHEN THEY WEATHER. 9-10. The geothermal gradient varies from place to place on the earth. What are the sources of heat, and why does it vary? 4.5 9. THE SOURCES ARE HEAT FROM THE ORIGINAL ACCRETION AND GRAVITATIONAL DIFFERENTIATION OF THE EARTH, AS WELL AS ONGOING CRYSTALLIZATION OF Fe-Ni ALLOY IN THE CRUST, AND DECAY OF LONG-LIVED RADIOACTIVE ISOTOPES, WITH HALF-LIVES ON THE ORDER OF A BILLION YEARS OR MORE, SUCH AS U, Th, AND K. 8 9 10. RADIOACTIVE ISOTOPES CONCENTRATE IN FELSIC ROCKS, SO HEAT ESCAPE WILL BE MUCH HIGHER ABOVE FELSIC ROCKS THAN MAFIC ROCKS. 9 11. In order to justify using the IUGS gabbroic chart, what criteria must be met? P + M > 90% 7 12-13. In an ophitic texture, 12) PLAGIOCLASE LATHS are completed surrounded by 8 13) 7 14. The type of rock associated with the magma series OIA is ALKALINE BASALT 4 15. MAARS CLINOPYROXENE crystals are negative topographic features, with craters excavated by hydromagmatic (phreatic) explosions. Such explosions occur when magma interacts with meteoritic water (either surface or groundwater). The water flashes into steam, with a huge increase in volume. 5.5 16. What is the primary reason some gabbros may melt at considerably lower temperatures than basalt, despite similar magmatic chemistries? BASALT MAGMAS ARE USUALLY ANHYDROUS, WHEREAS SOME GABBROIC MAGMAS HAVE A HYDROUS COMPONENT. WATER DEPRESSES THE MELTING POINT. 9 Diagram - Each of the following questions refers to Figure 1(1 point each) Figure 1 2 1 A 1. Which path is associated with orogenic belt crustal thickening? A. a B. b C. c D. All of the above 2. What does path “b” represent? PATH "B" REPRESENTS A SITUATION IN WHICH AT ROCK IS HEATED AND COOLED UNDER NEARLY ISOBARIC CONDITIONS. 3 3. For path “a”, do pressure and temperature increase together? PRESSURE AND TEMPERATURE DO NOT INCREASE TOGETHER. THE RELATIVE CHANGES IN BOTH QUANTITIES WITH TIME VARY CONSIDERABLY ALONG THE PATH. THIS IS AN IMPORTANT DIFFERENCE WITH THE TRADITIONAL VIEW OF REGIONAL METAMORPHISM. 10 2 D 4. Which path is common in high-grade gneisses and granulite facies terranes. It is believed to result from the intrusion of large quantities of mafic magma into the lower to middle crust. A. a1 B. a2 C. b D. c Problem - SHOW ALL WORK - there will be no credit for answers not supported by sufficient work to justify your answer. Show the formula used in your calculation, and identify all parameters, including units, used in the formula. Clearly label your answer, including units if any. Be sure to express your answer to the correct number of significant figures. 22.5 Suppose a reaction takes place with a constant energy barrier of 3850 cal/mol. Calculate K at 645 C. (4 points) The energy barrier figure is good to four significant figures. where K = equilibrium constant (dimensionless) E = energy barrier (calories/mole) R = gas constant = 1.987 cal/E mole T = temperature (Kelvin) - EC + 273.15 = Kelvin e = natural logarithm base Once again, for a class that did very well on the homework, the results here were terrible. 11 Discussion Questions - Answer all of the following questions completely but concisely. Points as indicated. Diagrams (labeled) may be used to supplement your written answers, where appropriate. 14 1. In Scotland, Comrie schists were intruded by diorite. The inner aureole is granofels, containing a mineral associations of orthopyroxene + potassium feldspar. In the English Lake District, Ordovician Skiddaw Slates are intruded by granite and granodiorite bodies. In the innermost zone, the rocks are hornfelsic. What is the difference between hornfels and granfels? What is responsible for the formation of granofels rather than hornfels? (2 points) BOTH GRANOFELS AND HORNFELS ARE NON-FOLIATED ROCKS TYPICALLY PRODUCED BY CONTACT METAMORPHISM. THE TEXTURE OF GRANOFELS IS CONSIDERABLY LARGER THAN HORNFELS. THE CAUSE OF THE INCREASED GRAIN SIZE WAS HEAT, SINCE A DIORITIC MAGMA IS MUCH HOTTER THAN FELSIC MAGMAS LIKE GRANITE OR GRANODIORITE. IN ADDITION, SLATE IS A MUCH FINER GRAINED PROTOLITH, SO IT REACTS FASTER, BUT IT PRODUCES SMALL GRAINS AS A PRODUCT 13 2. Mafic rocks may remain unaltered while nearby sedimentary rocks are completely changed by metamorphism. Why? (2 points) HIGH-TEMPERATURE MAFIC IGNEOUS ROCKS CONTAIN VERY LITTLE WATER. ORDINARY PROGRADE METAMORPHIC REACTIONS ARE NOT POSSIBLE IN THESE ROCKS, SINCE THEY CANNOT BE DEHYDRATED. SEDIMENTARY ROCKS ARE QUITE WET, AND PROGRADE REACTIONS OCCUR READILY. 9 3. The Sanbagawa belt lies nearer the subduction zone than the Ryoke belt. Which belt is richer in alkaline elements? Why? (2 points) SANBAGAWA. THE SANBAGAWA BELT IS FORMED FROM THE OCEANWARD ACCRETIONARY WEDGE, WITH A MIXTURE OF ARC-DERIVED SEDIMENTS AND VOLCANICS MIXED TOGETHER WITH OCEANIC CRUST AND MARINE SEDIMENTS. IT IS THE MARINE SEDIMENTS THAT CONTRIBUTE THE HIGH SODIUM CONTENT NECESSARY TO FORM GLAUCOPHANE. 12 8 4. Did Barrow find a greater change in the sandstones or the pelitic rocks he studied? Why? (2 points) THE PELITIC ROCKS, WHOSE PROTOLITH WAS SHALE, SHOWED A MUCH GREATER CHANGE. FINE-GRAINED ROCKS, WHICH HAVE A MUCH HIGHER SURFACE AREA TO VOLUME RATIO, USUALLY REACT MUCH FASTER. 2 5. What two minerals are considered characteristic of impact metamorphism? What other material is often found in impact zones? Describe two other features that are commonly associated with impacts. (5 points) COESITE AND/OR STISHOVITE, HIGH-PRESSURE POLYMORPHS OF SILICA, ARE THE CHARACTERISTIC MINERALS. AMORPHOUS GLASS, PRODUCED BY IMPACT MELTING, IS OFTEN PRESENT. SHOCKED QUARTZ, CHARACTERIZED BY THE PRESENCE OF SHOCK LAMALLAE, AND SHATTER CONES ARE OFTEN SEEN. 5 6. A student studies the fluid inclusions in a rock. They form a planar array. Will careful analysis of the fluid allow the student to determine the composition of the original fluid associated with metamorphism? Why or why not? ( 2 points) PROBABLY NOT. PLANAR ARRAYS ARE USUALLY FORMED BY POST-METAMORPHIC FLUIDS SEEPING INTO THE ROCK ALONG CRACKS 13 23 7. What type of stress is produced by tectonic forces? What are three sub-categories of this type of stress? (4 points) Deviatoric stress Tension σ1 > σ2, both + σ3 is negative Compression σ1 > σ2 > σ3 (Folding) σ1 > σ2 . σ3 (Flattening) Shear Pure shear = flattening σ1 > σ2 . σ3 Simple shear = Movement at an angle to σ1, like pushing at the top of a deck of cards AVE A GREAT SUMMER AND ENJOY SFC IF YOU ARE GOING ON IT. IT'S BEEN FUN! 14
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