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Chp 7: Metamorphic Rocks

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1 Chp 7: Metamorphic Rocks
Metamorphism – From the Greek “meta” = to change, and “morpho” = shape. Metamorphism – “The altering of rock characteristics and mineral compositions due to heat and/or pressure, or other environmental factors. This changing is a Solid State Reaction, meaning that the rocks subjected to metamorphic processes do not melt (otherwise upon cooling, they would form igneous rocks). It is thought to be a relatively slow geologic process. A great many areas of metamorphism yield abundant mineral reserves of gold, silver, copper, lead, zinc, and other valuable minerals. Metamorphic rocks are formed either by being exposed to heat, pressure, or chemically active fluids, or a combination of these factors to create a rock that has a different texture and mineral content The “parent rock” is the term for the rock prior to metamorphism. It may be igneous, sedimentary, or another metamorphic rock. For example, here are some parent rocks and the rock that they may metamorphose into under certain conditions: Limestone – marble Clay stone – slate Granite – gneiss, etc.

2 Specimen of Chrysotile: fibrous form of serpentine asbestos
Specimen of chrysotile from Thetford, Quebec, Canada. Chrysotile is the fibrous form of serpentine asbestos. p.186

3 Chp 7: Metamorphic Rocks
The effect of metamorphism on rocks is analogous to baking a cake: the resulting cake is dependent upon the ingredients, the amount of fluids, the temperature, and the length of time it was “baked”. A great portion of the continents is metamorphic formed during “continental accretion” during the formation of the Precambrian. Metamorphics form the stable basement rocks called “continental shields” upon which surface sedimentary rocks have been deposited. Metamorphics also comprise a large portion of the crystalline core of many mountain ranges.

4 Occurrence of metamorphic rocks: shields, core of mountain ranges
Figure 7.1 Metamorphic rock occurrences. Shields are the exposed portions of the crystalline basement rocks underlying each continent; these areas have been very stable during the past 600 million years. Metamorphic rocks also constitute the crystalline core of major mountain belts. Fig. 7-1, p.185

5 Chp7: Metamorphic Rocks
Factors Involved in Metamorphism Heat – The source of heat may be from a large intrusive body such as a pluton, or heat from activities associated with s plate tectonics. -At temperatures below 2000 C, only a small amount of fluid is present in most rocks. As the temperature increases many minerals release pore fluid that was trapped in the rock or in crystal lattices of its minerals. This pore fluid may become very chemically reactive, altering the chemistry of the surrounding rocks. -The Geothermal Gradient – On average the temperature of the rocks in the earth increase 25C per kilometer of depth. On the continental cratons, the average is 20C/km. On the continental boundaries it is 40C/km. At subduction zones, it is 10C/km because heat is dissipated into the sea. -At 700C, most rock components become “plastic” where many times the pre-existing crystals rotate, or twist altering the texture of the rock. -Under conditions of high heat, pressure, and chemically active fluids, crystal lattices begin to break down, recreate new types of crystal lattices, rearrange ions, and form new minerals in the process. -Some minerals only form at certain temperature and pressures. If these are found in a metamorphic rock, the temperature of formation can be deduced.

6 Pressure effects on materials: Lithostatic pressure is applied
equally in all directions to rocks buried beneath surface of earth. That pressure increases as depth of burial increases. b. Similar to effects on styrofoam cups lowered to ocean depths of 750m and 1500m. Net effect: retain shape but decrease in volume. Figure 7.2 (a) Lithostatic pressure is applied equally in all directions in Earth’s crust due to the weight of overlying rocks. Thus pressure increases with depth, as indicated by the sloping black line. (b) A similar situation occurs when 200-ml Styrofoam cups are lowered to ocean depths of approximately 750 m and 1500 m. Increased water pressure is exerted equally in all directions on the cups, and they consequently decrease in volume while still maintaining their general shape. Source: (a): From C. Gillen, Metamorphic Geology, Figure 4.4, p. 73. Copyright © Reprinted with the kind permission of Kluwer Academic Publishers and C. Gillen. Fig. 7-2, p.187

7 Chp 7- Metamorphic Rocks
Pressure – When rocks are buried, they are subjected to lithostatic pressure - the pressures from all sides by the overburden weight of the country rock. -Differential pressures – may exist whereby the pressures exerted upon the rock are not equal in all directions. This results in a distortion or twisting effect on the rock. -Phenocryst rotation or distortion may occur. This can cause grains in the rock to stretch, rotate, bend, line up in rows, become platy, etc. (i.e. micas forming in mica schists) -Pressure distortion of metamorphic rocks is common around areas of high lithologic stress such as areas around tectonic boundaries. Chemically Active Fluids – Fluids released from igneous intrusions, or other metamorphic processes can cause a constant interaction or exchange of ions altering the rocks. -Metasomatism – the introduction by fluids of ions from an external source not directly associated with the intrusion. Hydrothermal Metamorphism – changes due to migrating superheated water and dissolved ions. Hydrothermal rocks many times appear “bleached” because of the intense chemical reactions.

8 Sharp boundary (red line) between intrusion on left and country rock
on the right. Figure 7.5 A sharp and clearly defined boundary (red line) occurs between the intruding light-colored igneous rock on the left and the dark metamorphosed country rock on the right. The intrusion is part of the Peninsular Ranges Batholith, east of San Diego, California. Fig. 7-5, p.190

9 Differential pressure- Pressure applied un- equally in all directions.
Note garnet in schist Figure 7.3 Differential pressure is pressure that is unequally applied to an object. Rotated garnets are a good example of the effects of differential pressure applied to a rock during metamorphism. This rotated garnet (center) came from a schist in northeast Sardinia. Fig. 7-3, p.188

10 Chp 7- Metamorphic Rocks
Sources of water – 1. Juvenile Water – water given off by cooling magma. 2. Metamorphic Water – water already present the country rock, which is given off during metamorphic processes. 3. Meteoric Water – “groundwater” contained in aquifers encountered in the country rock during metamorphic processes Hydrothermal activities – many times form economically rich mineral deposits of gold, copper, iron, lead, etc. This process is also responsible for the “veining” (“mother loads”) of gold and other valuable minerals. Volcanic events such as calderas usually have associated hydrothermal activities Types of Metamorphism I. Contact – Effects of Heat and Fluids Characteristics: a. “Heat” is the driving force in contact metamorphism. Common where hot magmatic plutons come into contact with the surrounding country rock. b. The degree of metamorphism is related to the temperature of the magma, the size of the intrusion, and the chemically active fluid content of the magma involved. Large intrusions such as batholiths cool for long periods of time -more intense metamorphic change in the country rock.

11 Chp 7- Metamorphic Rocks
c. Temperatures can reach 9000 degrees C next to the intrusion. d. As the heat and associated metamorphic changes alter the country rock, the country rock closest to the intrusion is affected most, furthest from the intrusion is affected least. e.This sets up a “metamorphic halo” or “aureole” in the country rock around the intrusion. The aureole is a gradation of degrees of metamorphism surrounding the intrusion: 1. Shale – unaltered country rock 2. Slate – low grade metamorphism 3. Phyllite –between low and med. Grade 4. Schist – medium grade metamorphism 5. Gneiss – High grade metamorphism 6. Migmatite – Very high grade metamorphism 7. Melting occurs at 900C,above this temperature = formation of igneous rock. f. Two types of contact metamorphic rocks are recognized: 1. those resulting from the “baking” of the country rock 2. those resulting from the actions of chemically active fluids g. Many “baked” types have the texture of porcelain if they contain high amounts of clay such as shale. This effect is seen in the firing of ceramics in a kiln. h. Hydrothermal activity is also common with contact metamorphism resulting in an enrichment of valuable ore deposits.

12 Metmorphic aureole commonly surrounds many intrusions: this
Model has 3 zones of mineral assemblages around the intrusion, reflect decreased pressure and temperature effects away from the intrusion Figure 7.4 A metamorphic aureole typically surrounds many igneous intrusions. The metamorphic aureole associated with this idealized granite batholith contains three zones of mineral assemblages reflecting the decreases in temperature with distance from the intrusion. An andalusite–cordierite hornfels forms adjacent to the batholith. This is followed by an intermediate zone of extensive recrystallization in which some biotite develops, and farthest from the intrusion is the outer zone, which is characterized by spotted slates. Fig. 7-4, p.189

13 Table 7-2, p.202

14 Chp 7- Metamorphic Rocks
II. Regional Burial – Effects of Lithostatic Pressure Characteristics: a. Occurs over a very broad area b. Rocks are altered due to tremendous pressures (and the resulting high temperatures), resulting in deformation within deeper portions of the crust. c. Very common along convergent and divergent plate boundaries. d. Index minerals are minerals that are known to form only under certain temperatures and pressures. The following is a sequence of known minerals that form from low grade metamorphism to high grade: chlorite – (forms around 200 C), muscovite, biotite, garnet, staurolite, kyanite (forms around 500C) e. Quartz and feldspars can be present in both igneous and metamorphic rocks, but some minerals such as andalusite, sillimanite, and kyanite (all 3 minerals are forms of Al2SiO5) form only from these metamorphic conditions. f. The presence or absence of these minerals is an indication of the degree of pressure (and resulting heat) in the formation of the rock in question. g. Examples of regional burial rocks are: marble from limestone, quartzite from quartz sandstone, and argillite from clay.

15 Mineralization due to metamorphism of shale: new minerals form
at different Temperature levels. Progression of minerals indicates temperature level Figure 7.7 Change in mineral assemblage and rock type with increasing metamorphism of shale. When a clay-rich rock such as shale is subjected to increasing metamorphism, new minerals form, as shown by the various colored bars. The progressive appearance of particular minerals allows geologists to recognize low-, intermediate-, and high-grade metamorphic zones. Fig. 7-7, p.191

16 metamorphic aureole in sedimentary rocks of Michigan
Figure 7.17 Metamorphic zones in the Upper Peninsula of Michigan. The zones in this region are based on the presence of distinctive silicate mineral assemblages resulting from the metamorphism of sedimentary rocks during an interval of mountain building and minor granitic intrusion during the Proterozoic Eon, about 1.5 billion years ago. The lines separating the different metamorphic zones are isograds. Source: From H. L. James, G. S. A. Bulletin, vol. 66, plate 1, page 1454, with permission of the publisher, the Geological Society of America, Boulder, Colorado. USA. Copyright © 1955 Geological Society of America. Fig. 7-17, p.200

17 Chp7- Metamorphic Rocks
III. Dynamic Metamorphism (“Dynamo-thermal”)- Characteristics: a. Usually associated with the pressures around fault zones. b. “Mylonites” is the term used to describe rocks formed in this way. c. Typically, the extent of metamorphism is restricted to narrow margins adjacent to faults. d. Myolinites are hard, dense, fine-grained rocks, many of which have laminations or layerings. e. These also can be associated with tectonic settings. Textures of Metamorphic Rocks I. Foliated Textures - a. Typically associated with contact metamorphism. b. Minerals are arranged in a platy, parallel fashion. c. The size and shape of the mineral grains determines if the foliation is fine or coarse. d. A coarse foliation usually indicates a higher degree of heat such as in gneiss. e. A fine foliation usually indicates a lower degree of heat such as in schist. Slate has a very fine foliation exhibiting the lowest grade of contact metamorphism.

18 Mylonite from Adirondack Mtns…note thin laminations
Figure 7.6 Mylonite from the Adirondack Highlands, New York. Note the thin laminations. Fig. 7-6, p.190

19 When rocks are subjected to differential pressure, mineral grains
Typically align in parallel fashion-producing a ‘foliated’ structure. b. Foliated metamorphic rock… Figure 7.8 (a) When rocks are subjected to differential pressure, the mineral grains are typically arranged in a parallel fashion, producing a foliated texture. (b) Photomicrograph of a metamorphic rock with a foliated texture showing the parallel arrangement of mineral grains. Fig. 7-8, p.192

20 Figure 7.8 (b) Photomicrograph of a metamorphic rock with a foliated texture showing the parallel arrangement of mineral grains. Fig. 7-8b, p.192

21 Chp 7- Metamorphic Rocks
Examples of Foliated Textured Metamorphic Rocks: Slate –has a very fine foliation due to it having formed at the lowest grade of contact metamorphism. It possesses a slaty cleavage, easily cleaving or parting along the axis of layering. It is used for pool tables, chalkboards, and building tiles for this reason. The different colors of slates are due to the presence of minerals such as chlorite (green), graphite (black), or iron oxide (red). Phyllite – similar to slate but coarser grained. It is more lustrous or glossy due to tiny mica minerals. Grains are too small to be identified with the unaided eye. Schist – is most commonly produced by regional burial metamorphism. It can also be produced by medium grade contact metamorphism. Metamorphosed clay rich sedimentary rocks typically produce schists (although other rocks may also produce them). All schists contain more than 50% platy and elongated minerals all of which large enough to identify. The degree of schistosity reflects the temperature of formation: the greater the temperature, the greater the degree of schistosity. Schists are common in low to medium grade metamorphic environments. Schists are named as to the most abundant mineral: mica schist, talk schist, biotite schist, chlorite schist, etc.

22 Hand specimen of slate Fig. 7-9a, p.194
Figure 7.9 (a) Hand specimen of slate. Fig. 7-9a, p.194

23 Large sheet of slate-note the original bedding: up right to lower left
Figure 7.9 (b) This panel of Arvonia Slate from Albemarne Slate Quarry, Virginia, shows bedding (upper right to lower left) at an angle to the slaty cleavage. Fig. 7-9b, p.194

24 Phyllite: note lustrous sheen and bedding at angle to cleavage
Figure 7.10 Specimen of phyllite. Note the lustrous sheen as well as the bedding (upper left to lower right) at an angle to the cleavage of the specimen. Fig. 7-10, p.194

25 Garnet-mica schist Fig. 7-11a, p.195
Figure 7.11 Schist. (a) Garnet–mica schist. Fig. 7-11a, p.195

26 Hornblende-mica-garnet schist
Figure 7.11 Schist. (b)Hornblende–mica–garnet schist. Fig. 7-11b, p.195

27 Chp 7 Metamorphic Rocks Texture continued….
Gneiss – is a streaked or has segregated bands of alternating light and dark minerals. Quartz and feldspar are the major light colored minerals and biotite and hornblende are the principle dark colored minerals. Gneiss typically forms from regional metamorphism of clay-rich sedimentary rocks, from contact metamorphism of granites, or from meta- morphism of older metamorphic rocks. Amphibolite – a dark-colored, slightly foliated rock consisting primarily of hornblende and plagioclase. The metamorphism of mafic rocks such as basalt produce amphibolites. Migmatites – “mixed metamorphics” – These have characteristics of both igneous and metamorphic rocks indicating very high heat and pressure. Examples include the rocks touching an intrusion: the very highest grade contact metamorphism. Most contain granite components, or lenses (small pieces of other rocks), and appear to have been twisted or wavy. This may be due to partial melting of the country rock.

28 Gneiss-recognized by segregated bands of light and dark minerals
Gneiss-recognized by segregated bands of light and dark minerals. This gneiss has been folded… Figure 7.12 Gneiss is characterized by segregated bands of light and dark minerals. This folded gneiss is exposed at Wawa, Ontario, Canada. Fig. 7-12, p.195

29 Migmatites-high grade metamorphic rock, with streaks or lenses of
granite. Picture from Lake Huron, Ontario, Canada Figure 7.13 Migmatites consist of high-grade metamorphic rock intermixed with streaks or lenses of granite. This migmatite is exposed at Thirty Thousand Islands of Georgian Bay, Lake Huron, Ontario, Canada. Fig. 7-13, p.198

30 Chp 7 Metamorphic Rocks II. Nonfoliated Textures - Characteristics:
These textures result from the metamorphosing of rocks whose minerals do not show a preferred orientation, and therefore are not foliated. Most non-foliated rocks result from contact or regional burial of rocks that are devoid of platy or elongated crystals. Two Types of Nonfoliated rocks: those composed of mainly one mineral (marble or quartzite) those composed of mineral grains that are too small to be seen- hornfels or greenstones. Examples of Non-foliated Textured Metamorphic Rocks: Marble – the parent rock is a limestone (mostly calcite) or dolostone (mostly dolomite) that was subjected to contact or regional burial. It may be fine-grained to coarse-grained. Color variation is due to impurities in the parent rock. Because of its texture and softness, marble has been used extensively for sculpturing. Quartzite – the parent rock is a quartz sandstone subjected to medium to high grade contact or regional burial resulting in a hard, coarse-grained compact rock. Pure quartzite is white but impurities may alter the color. Since it is so hard from the re-crystallization of the quartz, it is commonly used for the bases of roads and buildings.

31 Chp 7- Metamorphic Rocks
Greenstone – this is the name given to any compact, dark green, altered, mafic igneous rock that formed under low to high grade metamorphic conditions. The green color is due to the minerals chlorite, epidote, and hornblende. These are commonly the rocks found in “greenstone belts” along the transitional zones of sialic continental plates to mafic oceanic plates. Hornfels – fine-grained, nonfoliated rock formed from contact metamorphism. The grains are equidimensional with its composition dependent upon the composition of the parent rock. Most are formed from contact metamorphism of clay-rich sedimentary rocks or impure dolomites. Anthracite – is a black, lustrous, hard coal that is high in carbon and low in volatiles. Its parent rock is bituminous coal that was subjected to regional burial.

32 Marble results from the metamorphism of sedimentary rocks known
Limestone or dolostone Figure 7.15 Marble results from the metamorphism of the sedimentary rock limestone or dolostone. Fig. 7-15, p.199

33 Photomicrograph of marble-note the mosaic of roughly equidimensional minerals-indicates non-foliated texture Figure 7.14 Nonfoliated textures are characterized by a mosaic of roughly equidimensional minerals, as in this photomicrograph of marble. Fig. 7-14, p.198

34 Quartzite results from the metamorphism of quartz sandstone
Figure 7.16 Quartzite results from the metamorphism of quartz sandstone. Fig. 7-16, p.199

35 Chp 7- Metamorphic Rocks
Metamorphic Zones or Facies – a. A “metamorphic facies” is a group of metamorphic rocks characterized by particular mineral assemblages (more than one mineral is present) under the same broad temperature/pressure conditions. b. Each facies is named after its most characteristic rock or mineral. c. Metamorphic facies are usually are applied to areas whose parent rocks were originally clay-rich. Metamorphic facies cannot be applied to areas where the parent was pure limestone or pure quartz sandstones because they would produce only marbles and quartzites respectively. Examples of Metamorphic Facies: a. Greenschist Facies – forms whenever the rock is rich in the mineral chlorite and is subjected to relatively low temperatures and pressures. b. Granulite Facies and Amphibolite Facies – form under similar chemistries but the pressures are significantly greater. c. Blueschist Facies – form at subduction zones where, due to the presence of seawater, the temperature is low, but because of the tectonic activity, the pressure is high. This results in an abundance of a blue-colored amphibole mineral named glaucophane. The presence of a blueschist facies indicates to the geologist the presence of ancient subduction zones.

36 Metamorphic facies produced along oceanic-continental boundary
Figure 7.19 Metamorphic facies resulting from various temperature–pressure conditions produced along an oceanic–continental convergent plate boundary. Metamorphic facies produced along oceanic-continental boundary Fig. 7-19, p.201

37 Chp 7- Metamorphic Rocks-Summary
Metamorphic Rocks form as a result of ‘metamorphism’…an alteration of rock charac- teristics and chemical composition due to application of heat and/or pressure, or chem- ically active fluids. “Parent rock” is term applied to the rock being metamorphosed-it may be igneous, sedi- mentary or even another metacmorphic rock. Metamorphic rocks commonly occur in- -core of mountain ranges -continental shields (sedimentary rocks commonly deposited on top of them…) -original continental accretion in PreCambrian Factors applied during metamorphism: -Heat -Pressure -Chemically active fluids

38 Table 7-1, p.192

39 Chp 7- Metamorphic Rocks-Summary
Types of Metamorphism: A. Contact metamorphism: results from heat and fluids -metamorphic ‘halo’ known as aureole is generated (shale) -baked zones common -hydrothermal effects occur.. B. Regional burial: occurs over large area -gradation of minerals common as a result of high pressure -specific minerals indicate different levels of pressure/temperature C. Dynamic metamorphism: usually associated with fault zones - mylonites Economic uses- mining slate, hydrothermal minerals suggest proximity to gold or silver??

40 Slate mine in Wales, England
Slate mine in Wales, England. Formed by mountain building process dated at 400 to 440 million years ago (MYA) Figure 7.21 Slate quarry in Wales. Slate, which has a variety of uses, is the result of low-grade metamorphism of shale. These high-quality slates were formed by a mountain-building episode that occurred approximately 400 to 440 million years ago in the present-day countries of Iceland, Scotland, Wales, and Norway. Fig. 7-21, p.202

41 Chp 7- Metamorphic Rocks-Summary
Metamorphic textures: A. Foliated-results from contact metamorphism -varies from coarse to fine slate, phyllite, schist, gneiss, amphibolite, migmatite B. Non-Foliated- no preferred orientation to minerals -2 types: single mineral, grains too small to be seen with naked eye marble, quartzite, greenstone, hornfels, anthracite Metamorphic Zones/Facies: metamorphic rocks characterized by specific mineral assemblages that reflect pressure-temperature regime rock experienced: 1. Greenschist: contain chlorite, low temperature, lo pressure 2.Granulite/Amphibolite: similar but higher pressure 3. Blueschist: fairly low temperature, high pressure. Indicative of subduction zones. Glaucophane mineral….

42 Pressure-temperature diagram showing where metamorphic facies occur.
Each facies is named after its most characteristic mineral. Figure 7.18 A pressure–temperature diagram showing where various metamorphic facies occur. A facies is characterized by a particular mineral assemblage that formed under the same broad temperature–pressure conditions. Each facies is named after its most characteristic rock or mineral. Source: From AGI Data Sheet 35.4, AGI Data Sheets, 3rd edition (1989) with the kind permission of the American Geological Institute. Fig. 7-18, p.200

43 Chp 7- Metamorphic Rocks

44 This Greek kouros, which stands 206 cm tall, has been the object of an intensive authentication study by the Getty Museum. Using a variety of geologic tests, scientists have determined that the kouros was carved from dolomitic marble that probably came from the Cape Vathy quarries on the island of Thasos. Garry Hobart/GeoImagery Fig. 7-CO, p.182

45 Figure 7.8 (a) When rocks are subjected to differential pressure, the mineral grains are typically arranged in a parallel fashion, producing a foliated texture. Fig. 7-8a, p.192

46 Different colored slates used for roof in Michigan
Different colored slates make up the roof of this elementary school in Mount Pleasant, Michigan. p.193

47 Slate roof in Switzerland
Figure 7.9 (c) Slate roof of Chalet Enzian, Switzerland. Fig. 7-9c, p.194

48 Franciscan Complex in CA: low temperature, high pressure subduction
Figure 7.20 Index map of California showing the location of the Franciscan Complex and a diagrammatic reconstruction of the environment in which it was regionally metamorphosed under low-temperature, high-pressure subduction conditions approximately 150 million years ago. The red line on the index map shows the orientation of the reconstruction to the current geography. Source: From “Effects of Late Jurassic-Early Tertiary Subduction in California,” San Joaquin Geological Society Short Course, 1977, 66, Figure 5-9. Fig. 7-20, p.201

49 Figure 7.20 Index map of California showing the location of the Franciscan Complex and a diagrammatic reconstruction of the environment in which it was regionally metamorphosed under low-temperature, high-pressure subduction conditions approximately 150 million years ago. The red line on the index map shows the orientation of the reconstruction to the current geography. Source: From “Effects of Late Jurassic-Early Tertiary Subduction in California,” San Joaquin Geological Society Short Course, 1977, 66, Figure 5-9. Fig. 7-20a, p.201

50 Figure 7.20 Index map of California showing the location of the Franciscan Complex and a diagrammatic reconstruction of the environment in which it was regionally metamorphosed under low-temperature, high-pressure subduction conditions approximately 150 million years ago. The red line on the index map shows the orientation of the reconstruction to the current geography. Source: From “Effects of Late Jurassic-Early Tertiary Subduction in California,” San Joaquin Geological Society Short Course, 1977, 66, Figure 5-9. Fig. 7-20b, p.201

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