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05_metamorphic_09_post - 05: Metamorphic Rocks Dalradian...

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Unformatted text preview: 05: Metamorphic Rocks Dalradian Gneiss, Isle of Arran, Scotland. Photo: P.L. Kresan Why this is important • Regionally extensive metamorphic rocks indicate past episodes of plate convergence and mountain-building Metamorphic rock: rock that changed from its original form into a new one, without first becoming a melt or sediment. Fig. 8.21 Lecture 5: 1 Lecture 5: 2 Why this is important Definition of Metamorphism • Assemblage of metamorphic minerals tells us about the pressure and temperature conditions at the time of metamorphism, conditions we can never observe directly. • Changes in mineral assemblage and rock texture that occur in rocks IN THE SOLID STATE as a result of changes in temperature, pressure, and effects of chemically reactive fluids. Fig. 8.2 Approximately circular grains in a limestone (top) and sandstone (bottom) have become elliptical. Photo: P.L. Kresan Lecture 5: 3 Lecture 5: 4 Definition of Metamorphism • Changes in mineral assemblage and rock texture that occur in rocks IN THE SOLID STATE as a result of changes in temperature, pressure, and effects of chemically reactive fluids. Definition of Metamorphism • Changes in mineral assemblage and rock texture that occur in rocks IN THE SOLID STATE as a result of changes in temperature, pressure, and effects of chemically reactive fluids. Recrystallization: rearrange atoms in the Recrystallization: crystalline lattice. Lecture 5: 5 Recrystallization: rearrange atoms in the crystalline lattice (due to Recrystallization: pressure, in this case). Lecture 5: 6 Controlling factors • Heat Controlling factors • Temperature & pressure: mineralogic changes 1. Permits recrystallization 2. Supplied by burial (temperature increases with depth) or igneous intrusions Fig. 8.3 Fig. 8.23 Lecture 5: 7 Graph (phase diagram) shows the range of temperatures and pressures at which the three minerals (which have the same chemical composition, Al2SiO3, but different crystalline structure) would form in metamorphic rocks. Lecture 5: 8 Exercise / iClicker Exercise / iClicker Q1. For a pressure of 8 kbar, what must be the temperature for both kyanite and sillimanite to be present? A. 500°C B. 600°C C. 700°C D. Both minerals will not be present. Q2. For a temperature of 400°C, what must be the pressure for both andalusite and kyanite to be present? A. 2 kbar B. 3 kbar C. 4 kbar D. 5 kbar Lecture 5: 9 Exercise / iClicker Q3. What combination of temperature and pressure will result in all three minerals (kyanite, andalusite, sillimanite) being present? A. 400°C, 3 kbar B. 500°C, 3.5 kbar C. 600°C, 6 kbar D. 700°C, 1 kbar Lecture 5: 10 Controlling factors • Directed pressure, called stress, produces deformation Fig. 8.4 Circle turned into ellipse. Lecture 5: 11 Lecture 5: 12 Rectangle turned into rhombohedron. rhombohedron. Controlling factors Controlling factors • Stress produces preferred orientation (alignment) of grains. Circular grains do not have a long axis or a short axis. • Stress produces preferred orientation (alignment) of mineral grains. Fig. 8.6 Elliptical grains have a long axis and a short axis. In all of these grains, the long axis is oriented updown, and the short axis is oriented left-right. Foliation: alignment of platy minerals (e.g., mica) produced by stress. Fig. 8.6 Lecture 5: 13 Lecture 5: 14 Controlling factors Controlling factors • Non-platy minerals do not form a foliation Photo: Porter & Skinner • Strongly foliated rocks separate along foliation (cleavage) planes Slate shingles used on roofs. Non-platy minerals in quartzite: no foliation Platy minerals in schist: U-shaped foliation Fig. 8.8 Lecture 5: 15 Lecture 5: 16 Types of foliation Types of metamorphic rocks • Foliated rocks are named by texture. Slaty cleavage: well-developed planar foliation in fine-grained rocks. Schistosity: wavy foliation in coarser-grained rocks. Gneissic banding: compositional banding (light and dark bands, commonly folded) in coarse-grained rocks. 1. Slate: fine-grained with well-developed planar foliation (slaty cleavage). 2. Phyllite: somewhat coarser-grained than slate; good foliation. 3. Schist: still coarser grained; schistosity (wavy foliation) 4. Gneiss: coarse-grained rock characterized by compositional banding (light and dark bands) Plate, p. 226 Fig. 8.18 Lecture 5: 17 Lecture 5: 18 Types of foliated rocks Types of metamorphic rocks • Non-foliated rocks are named by composition. Gneiss Schist Slate Manhattan Schist Schist http://3dparks.wr.usgs.gov/nyc/ima ges/fig14.jpg Lecture 5: 19 Photo: B.P. Kent Phyllite Lecture 5: 20 Types of metamorphic rocks Progressive metamorphism • Non-foliated rocks are named by composition. Fig. 8.18 Photo: B.P. Kent Marble (limestone) Photo: B.P. Kent Quartzite (sandstone) Metamorphic rock (parent rock) Increasing metamorphism Metaconglomerate Shale--> Slate --> Phyllite --> Schist --> Gneiss (conglomerate) Photo: W.K. Hamblin Lecture 5: 21 Lecture 5: 22 Types of metamorphism Types of metamorphism Contact metamorphism: result of baking by igneous intrusions Regional metamorphism: acts over broad areas, and is a result of compression produced at convergent plate boundaries. 1. Occurs in mountain belts. • Intensity decreases away from intrusion • No foliation (absence of stress) • Hornfels: contactmetamorphic rock. 2. Foliation is well developed. Fig. 8.23 At point A, temperature=400°C, pressure=3.5 bar Lecture 5: 23 Degree of contact metamorphism is analogous to changes in pottery clay when it is baked at various temperatures in a kiln. Lecture 5: 24 Fig. 8.26 Types of metamorphism Regional metamorphism: acts over broad areas, and occurs at convergent plate boundaries. Types of metamorphism shale Regional metamorphism: acts over broad areas, and is a result of compression produced at convergent plate boundaries. Regional metamorphism in ancient mountain belt (Appalachians) Fig. 8.21 hornfels Lecture 5: 25 gneiss - schist - slate Rock cycle Succession of events that results in the transformation of earth materials from one rock type to another, then another, and so on. Lecture 5: 26 Rock cycle Succession of events that results in the transformation of earth materials from one rock type to another, then another, and so on. Example of rock cycle 1. Lava cools to form igneous rock 1 2 2. Igneous rock weathers to become sediment 3. Sediment transported to and deposited in the ocean 4. Sediment buried and lithified to become sedimentary rock 5. Sedimentary rock transformed to metamorphic rock when subjected to heat and pressure as a result of burial 6. Metamorphic rocks melt to form magma, which makes its way to the surface to form lava… Lecture 5: 27 Lecture 5: 28 3 5 6 www.bbc.co.uk 4 Review Questions / iClicker 5-1. A. True / B. False: Metamorphism takes places while rocks are solid. 5-2. A. True / B. False: Pressure and heat produce mineralogic changes. 5-3. Which type of foliation is usually associated with the smallest grain size? A. slaty cleavage B. schistosity C. gneissic banding D. stratification 5-4. Which of the following metamorphic rocks is not foliated? A. gneiss B. marble C. schist D. slate 5-5. What is the proper sequence for the progressive metamorphism of shale? A. gneiss-->phyllite-->schist B. gneiss-->schist-->phyllite C. phyllite-->schist-->gneiss D. schist-->gneiss-->phyllite 5-6. Differential stress will cause mineral grains to align in a preferred orientation unless the mineral grains are ____________. A. platy B. non-platy 5-7. Two common metamorphic rocks that typically lack foliation are ____________. A. slate and phyllite B. gneiss and migmatite C. quartzite and marble D. schist and metaconglomerate 5-8. ____________ commonly serves as a parent rock (protolith) in the formation of marble. A. Limestone B. Sandstone C. Shale D. Slate Lecture 5: 29 Review Questions / iClicker 5-9. ____________ commonly serves as a parent rock (protolith) in the formation of quartzite. A. Limestone B. Sandstone C. Shale D. Slate 5-10. Thermal (contact) metamorphism occurs ____________. A. in areas surrounding igneous intrusions B. only where gneiss is in contact with schist C. as consequence of the sinking of a broad region to great depth D. only at the Earth’s surface, where rock is in contact with the atmosphere 5-11. Which of the following processes cannot occur in the formation of metamorphic rock? A. realignment of minerals so that they develop a preferred orientation B. segregation of minerals into layers of different compositions C. solid-state rearrangement of atoms or ions to create a new assemblage of minerals D. complete remelting of the rock, followed by solidification to form a new rock 5-12. Thermal (contact) metamorphism produces ____________. A. foliated rocks only B. nonfoliated rocks only C. both foliated and nonfoliated rocks 5-13. A body of gneiss is subjected to heat and forms a melt. Later the melt cools and crystallizes to form a(n) ____________. A. metamorphic rock B. igneous rock C. sedimentary rock 5-14. The parent rock (protolith) subjected to metamorphism ____________. A. is always metamorphic rock B. is always igneous rock C. is always sedimentary rock D. may be any of the primary rock types Lecture 5: 30 ...
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This note was uploaded on 09/25/2011 for the course GEOLOGY 100 taught by Professor Lepre during the Fall '11 term at Rutgers.

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