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Metamorphism and Metamorphic Rocks
GEOL: CHAPTER 7 Metamorphism and Metamorphic Rocks
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This slate exposed in the Wasatch Mountains just east of Salt Lake City, Utah, demonstrates “slaty cleavage,” wherein the rock tends to split into flat, plate-like pieces along cleavage planes.
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Learning Outcomes LO1: Identify the agents of metamorphism
LO2: Identify the three types of metamorphism LO3: Explain how metamorphic rocks are classified LO4: Recognize the difference between metamorphic zones and facies LO5: Understand how plate tectonics affects metamorphism LO6: Understand the relationship between metamorphism and natural resources
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Metamorphism Metamorphic rock: any rock that has been changed from its original condition by heat, pressure, and the chemical activity of fluids Metamorphism: the phenomenon of changing rocks subjected to heat, pressure, and fluids so that they are in equilibrium with a new set of environmental conditions
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Figure 7.1 Gneiss This gneiss, found in Canada, is estimated to be about 4.0 billion years old. Gneiss is a foliated metamorphic rock.
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Agents of Metamorphism
Heat Pressure Fluid activity Time for the above to chemically alter rocks is also an important factor
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Heat Increases rates of chemical reactions
Can produce minerals different than those in the original rock Lava Magma Burial: geothermal gradient; subducted rock
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Pressure Pressure increases with depth
Lithostatic pressure: pressure exerted on rocks by the weight of overlying rocks Differential pressure: Pressure not applied equally to all sides of a body Stresses stronger in some directions Common with plate collisions
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Figure 7.2 Lithostatic Pressure
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Figure 7.3 Differential Pressure Differential pressure results from stress 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. In this example from a schist in northeast Sardinia, stress was applied in opposite directions on the left and right side of the garnet (center), causing it to rotate.
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Fluid Activity Water (and carbon dioxide) are present in almost every region of metamorphism Water increases reaction rates and thus metamorphism Water also reacts with some minerals to create new minerals Seawater moving through hot basaltic rock converts olivine to serpentine
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Fluid Activity, cont. Sources of chemically active fluids:
Water trapped in pore spaces of sedimentary rocks Volatile fluid in magma Dehydration of water-bearing minerals
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Three Types of Metamorphism
Contact (thermal) metamorphism Dynamic metamorphism Regional metamorphism
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Contact Metamorphism Metamorphism of country rock adjacent to a pluton
Magma from forming pluton: Raises temperature of country rock Releases hot fluids into country rock Aureole: zone of metamorphism surrounding a pluton
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Contact Metamorphism, cont.
Important factors: Initial temperature Size of the intrusion Fluid content of magma and country rock: hydrothermal alteration creates mineral deposits Degree of metamorphic change decreases with distance from pluton Can also occur with lava flows
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Figure 7.4 Metamorphic Aureole A metamorphic aureole, the area surrounding an intrusion, consists of zones that reflect the degree of metamorphism. The metamorphic aureole associated with this idealized granite pluton contains three zones of mineral assemblages reflecting the decrease in temperature with distance from the intrusion. An inner andalusite–cordierite hornfels zone forms adjacent to the pluton and is reflective of the high temperatures near the intrusion. This is followed by an intermediate zone of extensive recrystallization in which some biotite develops, and farthest from the intrusion is an outer zone characterized by spotted slates.
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Figure 7.5 Contact Metamorphism from a Lava Flow A highly weathered basaltic lava flow near Susanville, California, has altered an underlying rhyolitic volcanic ash by contact metamorphism. The red zone below the lava flow has been baked by the heat of the lava when it flowed over the ash layer. The lava flow displays spheroidal weathering, a type of weathering common in fractured rocks (see Chapter 6).
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Dynamic Metamorphism Occurs in fault zones where rocks are under high degrees of differential pressure Mylonites frequently created Hard, dense, fine-grained Thin laminations
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Figure 7.6 Mylonite An outcrop of mylonite from the Adirondack Highlands, New York. Mylonites result from dynamic metamorphism, where rocks are subjected to high levels of differential pressure. Note the thin laminations (closely spaced layers), which are characteristic of many mylonites.
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Regional Metamorphism
Occurs over a large area Caused by very high temperatures and pressures, occurring simultaneously with deformation Convergent plate boundaries Gradations of metamorphism based on severity of conditions
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Index Minerals and Metamorphic Grade
Index mineral: a mineral that forms within specific temperature and pressure ranges during metamorphism Metamorphic grade: the degree to which a rock has undergone metamorphic change Different rock compositions have different sets of index minerals
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Figure 7.7 Metamorphic Grade 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 colored bars. The progressive appearance of certain minerals, known as index minerals, allows geologists to recognize low-, intermediate-, and high-grade metamorphism.
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Classifying Metamorphic Rocks
Foliated texture: Platy and elongate minerals aligned in a parallel fashion Nonfoliated texture: no discernable preferred orientation of minerals
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Foliated Metamorphic Rocks
Foliated texture: Platy and elongate minerals aligned in a parallel fashion From heat and differential pressure Size and shape of mineral grains determine whether foliation is fine or coarse
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Figure 7.8 Foliated Texture
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Slate Low-grade metamorphism Finely foliated
Minerals can only be seen with magnification Slaty cleavage Regional metamorphism of shale
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Figure 7.9 Slate
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Phyllite Coarser grained than slate, but similar composition
Need magnification to see minerals Intermediate grain size between slate and schist
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Figure 7. 10 Phyllite Hand specimen of phyllite
Figure 7.10 Phyllite Hand 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.
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Schist Commonly produced in regional metamorphism
Type depends on intensity of metamorphism and characteristic of original rock Many rock types yield schist Minerals clearly visible Schistosity/schistose foliation Each type has associated minerals
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Figure 7.11 Schist
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Figure 7.11 Schist
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Figure 7.11 Schist
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Gneiss High-grade metamorphism
Segregated bands of light and dark minerals Light bands: quartz and feldspar Dark bands: biotite and hornblende Forms from recrystallization of sedimentary rocks during regional metamorphism, or from igneous or metamorphic rocks
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Figure 7.12 Gneiss Gneiss is characterized by segregated bands of light and dark minerals. This folded gneiss is exposed at Wawa, Ontario, Canada.
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Other Foliated Rocks Amphibolite Migmatites
Dark rock: hornblende and plagioclase Slightly foliated From basalt and ferromagnesian-rich mafic rocks Migmatites Igneous and high-grade metamorphic characteristics
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Figure 7.13 Migmatites A migmatite boulder in the Rocky Mountain National Park, near Estes Park, Colorado. Migmatities consist of high-grade metamorphic rock intermixed with streaks or lenses of granite.
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Nonfoliated Metamorphic Rocks
Nonfoliated texture: no discernable preferred orientation of minerals Mosaic or roughly equidimensional minerals Contact or regional metamorphism of rocks with no platy or elongate minerals
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Figure 7.14 Nonfoliated Texture Nonfoliated textures are characterized by a mosaic of roughly equidimensional minerals, as in this photomicrograph of marble.
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Marble Predominantly calcite or dolomite
Grains range from fine to coarsely granular Contact or regional metamorphism of limestones or dolostones Colors come from impurities in parent rock
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Figure 7.15 Marble Results from the Metamorphism of the Sedimentary Rock Limestone or Dolostone
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The softness of marble, its uniform texture, and its varying colors have made it the favorite rock of builders and sculptors throughout history.
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Quartzite Formed from quartz sandstone
Intermediate to high-grade metamorphism Typically complete recrystallization of quartz grains Impurities add color
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Figure 7.16 Quartzite Results from the Metamorphism of the Sedimentary Rock Quartz Sandstone
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Other Nonfoliated Rocks
Greenstone: dark-green, from mafic igneous rock; low- to high-grade metamorphism Hornfels: many varieties, but mostly from sedimentary rocks; from contact metamorphism Anthracite: hard coal with high % carbon; metamorphism of lower-grade coals
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Metamorphic Zones Metamorphic zone: the region between lines of equal metamorphic intensity known as isograds Metamorphic rocks divided into zones based on presence of distinctive silicate minerals
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Metamorphic Zones, cont.
Successive appearance of index minerals shows progression to higher-grade metamorphism First appearance of a mineral indicates minimum temperature and pressure conditions Isograds: lines of equal metamorphic intensity
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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, approximately 1.5 billion years ago. The lines separating the different metamorphic zones are isograds.
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Metamorphic Facies A group of metamorphic rocks characterized by particular minerals that formed under the same broad temperature and pressure conditions Named after most characteristic rock or mineral Not applicable when original rocks were pure quartz sandstones or pure limestones or dolostones
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Figure 7.18 Metamorphic Facies and Their Associated Temperature–Pressure Conditions A temperature–pressure diagram showing under what conditions various metamorphic facies occur. A metamorphic 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.
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Plate Tectonics and Metamorphism
Associated with all 3 types of boundaries Most common with convergent boundaries Temperature and pressure characteristics Produces typical metamorphic facies Subducting plate heats only slowly at first, so initial metamorphism from high pressure Blueschist facies
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Plate Tectonics and Metamorphism, cont.
As subduction continues, temperatures and pressures increase High-grade metamorphic rocks Plate begins to melt and generate magma: contact metamorphism Divergent boundaries Contact metamorphism from magma Metal-bearing hydrothermal solutions
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Figure 7.19 Relationship of Facies to Major Tectonic Features at an Oceanic–Continental Convergent Plate Boundary
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Metamorphism and Natural Resources
Marble Slate Ore deposits from hot hydrothermal fluids Migrate into surrounding rock Copper: bornite, chalcopyrite Lead: galena Zinc: pyrite and sphalerite Iron: hematite and magnetite Asbestos, talc, graphite, garnets
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