Stereo nets test Answer all questions; return this paper with answers and tracing paper with your drawings, (the overall mark for the test is 22, each question has a total mark of 2 for each correct answer, in addition 10 marks are given for the correct drawing of planes and lines) Orientation of two limbs of a fold is determined as: 50/70SE and 20/60SE 1. Determine orientation of fold axis (hinge) 2. Determine pitch of the fold axis on both limbs 3. Determine angle between two limbs Two sets of mineral lineations were measured in two locations as: 15→140 and 70→220 4. Determine orientation of the plane containing these lineations 5. Determine apparent dip of this plane on the east to west cross section 6. Determine angle between two sets of lineations Exercise 7.4 answers. Image Descriptions Figure 7.4.1 image description: Approximate temperature range of metamorphic minerals: Chlorite, 50 to 450°C. Muscovite, 175 to 625°C. Biotite, 350 to 725°C. Garnet, 375 to 900°C. Andalusite, 400 to 850°C. Sillimanite, 575 to 1000°C. [Return to Figure 7.4.1] Media Attributions • Figure 7.4.1, 7.4.3ab, 7.4.4: © Steven Earle. CC BY. • Figure 7.4.2: Image edited by Steven Earle, after Keppie, D, and Muecke, G, 1979, Metamorphic map of Nova Scotia, N.S. Dept. of Mines and Energy, Map 1979-006., and from White, C and Barr, S., 2012, Meguma Terrane revisted, Stratigraphy, metamorphism, paleontology and provenance, Geoscience Canada, V. 39, No.1. 7.5 Contact Metamorphism and Hydrothermal Processes Contact metamorphism takes place where a body of magma intrudes into the upper part of the crust. Any type of magma body can lead to contact metamorphism, from a thin dyke to a large stock. The type and intensity of the metamorphism, and width of the metamorphic aureole will depend on a number of factors, including the type of country rock, the temperature of the intruding body and the size of the body (Figure 7.5.1). A large intrusion will contain more thermal energy and will cool much more slowly than a small one, and therefore will provide a longer time and more heat for metamorphism. That will allow the heat to extend farther into the country rock, creating a larger aureole. Contact metamorphic aureoles are typically quite small, from just a few centimetres around small dykes and sills, to several 10s of metres around a large stock. As was shown in Figure 7.3.7, contact metamorphism can take place over a wide range of temperatures—from around 300° to over 800°C—and of course the type of metamorphism, and new minerals formed, will vary accordingly. The nature of the country rock (or parent rock) is also important. A hot body of magma in the upper crust can create a very dynamic situation that may have geologically interesting and economically important implications. In the simplest cases, water does not play a big role, and the main process is transfer of heat from the pluton to the surrounding rock, creating a zone of contact metamorphism (Figure 7.5.2a). In that situation Figure 7.5.1 Schematic cross-section of the middle and mudrock or volcanic rock will likely be upper crust showing two magma bodies. The upper body metamorphosed to hornfels (Figure 7.2.9), has intruded into cool unmetamorphosed rock near to the limestone will be metamorphosed to marble surface and has created a zone of contact metamorphism. (Figure 7.2.6), and sandstone to quartzite (Figure The lower body is surrounded by rock that is already hot 7.2.7). (But don’t forget that marble and quartzite (and probably already metamorphosed), and so it does not have a significant metamorphic aureole. can also form during regional metamorphism!) In many cases, however, water is released from the magma body as crystallization takes place, and this water is dispersed along fractures in the country rock (Figure 7.5.2b). The water released from a magma chamber is typically rich in dissolved minerals. As this water cools, is chemically changed by the surrounding rocks, or boils because of a drop in pressure, minerals are deposited, forming veins within the fractures in the country rock. Quartz veins are common in this situation, and they might also include pyrite, hematite, calcite, and even silver and gold. 249 7.5 Contact Metamorphism and Hydrothermal Processes 250 Figure 7.5.2 Depiction of metamorphism and alteration around a pluton in the upper crust. (a) Thermal metamorphism only (within the purple zone). (b) Thermal metamorphism plus veining (white) related to dispersal of magmatic fluids into the overlying rock. (c) Thermal metamorphism plus veining from magmatic fluids plus alteration and possible formation of metallic minerals (hatched yellow areas) from convection of groundwater. Heat from the magma body will heat the surrounding groundwater, causing it to expand and then rise toward the surface. In some cases, this may initiate a convection system where groundwater circulates past the pluton. Such a system could operate for many thousands of years, resulting in the circulation of billions of litres of groundwater from the surrounding region past the pluton. Hot water circulating through the rocks can lead to significant changes in the mineralogy of the rock, including alteration of feldspars to clays, and deposition of quartz, calcite, and other minerals in fractures and other open spaces (Figure 7.5.3). As with the magmatic fluids, the nature of this circulating groundwater can also change adjacent to, or above, the pluton, resulting in deposition of other minerals, including ore minerals. Metamorphism in which much of the change is derived from fluids passing through the rock is known as metasomatism. When hot water contributes to changes in rocks, including mineral alteration and formation of veins, it is known as hydrothermal alteration. 251 Chapter 7 Metamorphism and Metamorphic Rocks Figure 7.5.3 Calcite veins in limestone of the Comox Formation, Nanaimo, B.C A special type of metasomatism can take place where a hot pluton intrudes into carbonate rock such as limestone. When magmatic fluids rich in silica, calcium, magnesium, iron, and other elements flow through the carbonate rock, their chemistry can change dramatically, resulting in the deposition of minerals that would not normally exist in either the igneous rock or limestone. These include garnet, epidote (another silicate), magnetite, pyroxene, and a variety of copper and other minerals (Figure 7.5.4). This type of metamorphism is known as skarn, and again, some important types of mineral deposits can form this way. 7.5 Contact Metamorphism and Hydrothermal Processes 252 Figure 7.5.4 A skarn rock from Mount Monzoni, Northern Italy, with recrystallized calcite (blue), garnet (brown), and pyroxene (green). The rock is 6 centimetres across. Exercise 7.5 Contact metamorphism and metasomatism 253 Chapter 7 Metamorphism and Metamorphic Rocks Figure 7.5.5 This diagram shows a pluton that has intruded into a series of sedimentary rocks. What type of metamorphic rock would you expect to see at locations: a) Mudstone? _____________________ b) Limestone? _____________________ c) Sandstone? _____________________ See Appendix 3 for Exercise 7.5 answers. Media Attributions • Figures 7.5.1, 7.5.2, 7.5.3, 7.5.5: © Steven Earle. CC BY. • Figure 7.5.4: 00031 6 cm grossular calcite augite skarn © Siim. CC BY SA. Summary The topics covered in this chapter can be summarized as follows: Section Summary 7.1 Controls Over Metamorphic Processes Metamorphism is controlled by five main factors: the composition of the parent rock, the temperature to which the rock is heated, the amount and type of pressure, the volumes and compositions of aqueous fluids that are present, and the amount of time available for metamorphic reactions to take place. 7.2 Classification of Metamorphic Rocks Metamorphic rocks are classified on the basis of texture and mineral composition. Foliation is a key feature of metamorphic rocks formed under directed pressure. Foliated metamorphic rocks include slate, phyllite, schist, and gneiss. Metamorphic rocks formed in environments without strong directed pressure include hornfels, marble, and quartzite, although the latter two may form in high-pressure situations but not develop obvious foliated textures. Almost all metamorphism can be explained by plate-tectonic processes. Oceanic crustal rock 7.3 Plate can be metamorphosed near the spreading ridge where it was formed, but most other regional Tectonics and metamorphism takes place in areas where mountain ranges have been created, which are most Metamorphism common at convergent boundaries. Contact metamorphism takes place around magma bodies in the upper part of the crust, which are also most common above convergent boundaries. Geologists classify metamorphic rocks based on some key minerals—such as chlorite, garnet, andalusite, and sillimanite—that form at specific temperatures and pressures. Most regional 7.4 Regional metamorphism takes place beneath mountain ranges because the crust becomes thickened and Metamorphism rocks are pushed down to great depths because of the isostatic relationship between the crust and mantle. When mountains erode, those metamorphic rocks are uplifted by crustal rebound. 7.5 Contact Metamorphism and Hydrothermal Processes Contact metamorphism takes place around magma bodies that have intruded into cool rocks at high levels in the crust. Heat from the magma is transferred to the surrounding country rock, resulting in mineralogical and textural changes. Water from a cooling body of magma, or from convection of groundwater produced by the heat of the pluton, can also lead to metasomatism, hydrothermal alteration, and accumulation of valuable minerals in the surrounding rocks. Questions for Review Answers to Review Questions can be found in Appendix 2. 1. What are the two main agents of metamorphism, and what are their respective roles in producing metamorphic rocks? 2. Into what metamorphic rocks will a mudrock be transformed at very low, low, medium, and high metamorphic grades? 3. Why doesn’t granite change very much at lower metamorphic grades? 254 255 Chapter 7 Metamorphism and Metamorphic Rocks 4. Describe the main process of foliation development in a metamorphic rock such as schist. 5. What process contributes to metamorphism of oceanic crust at a spreading ridge? 6. How do variations in the geothermal gradient affect the depth at which different metamorphic rocks form? 7. Blueschist metamorphism takes place within subduction zones. What are the unique temperature and pressure characteristics of this geological setting? 8. Rearrange the following minerals in order of increasing metamorphic grade: biotite, garnet, sillimanite, chlorite. 9. Why does contact metamorphism not normally take place at significant depth in the crust? 10. What is the role of magmatic fluids in metamorphism that takes place adjacent to a pluton? 11. How does metasomatism differ from regional metamorphism? 12. 13. 14. How does the presence of a hot pluton contribute to the circulation of groundwater that facilitates metasomatism and hydrothermal processes? What must be present in the country rock to produce a skarn? Two things that a geologist first considers when looking at a metamorphic rock are what the parent rock might have been, and what type of metamorphism has taken place. This can be difficult to do, even if you have the actual rock in your hand, but give it a try for the following metamorphic rocks: a. Chlorite schist b. Slate c. Mica-garnet schist d. Amphibolite e. Marble Review of Minerals and Rocks Mineral and rock review Steven Earle Mineral and Rock Review Crystals of the mineral native sulphur growing on the rock basalt at an outlet of volcanic gases, Kilauea volcano, Hawaii Now that we’ve covered minerals and all three types of rocks it’s important for you to convince yourself that you’ve got them straight in your mind. As already noted, one of the most common mistakes that geology students make on assignments, tests and exams is to confuse minerals with rocks and then give a wrong answer when asked to name one or the other based on information provided. In this exercise you are given a list of names of minerals and rocks and asked to determine which ones are minerals and which are rocks. For those that you think are minerals you should then indicate which mineral group it belongs to (e.g., oxide, sulphate, silicate, carbonate, halide etc.). For those that you think are rocks, you should describe what type of rock it is (e.g., intrusive igneous, extrusive igneous, clastic sedimentary, chemical sedimentary, foliated metamorphic and non-foliated metamorphic). The answers can be found in Rock and mineral review exercise answers in Appendix 3. 256 257 Chapter 7 Metamorphism and Metamorphic Rocks Mineral or rock name Rock or mineral? Feldspar Calcite Slate Hematite Rhyolite Sandstone Diorite Olivine Pyrite Quartzite Granite Amphibole Conglomerate Chert Halite Gneiss Mica Pyroxene Chlorite Limestone Andesite Media Attributions • Native sulphur: © Steven Earle. CC BY. If it’s a mineral, which group does it belong to? If it’s a rock, what type is it? Homework: Metamorphism | A+ Points 16 Questions 16 Time Limit None Allowed Attempts Unlimited Instructions Please read the Textbook Chapter 7 and Lecture: Metamorphic Rocks before proceeding to answer these questions.
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