Chapter 8 Metamorphism

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Chapter 8 Metamorphism Metamorphic rocks
Metamorphism - alteration of rocks by heat and
pressure. Low-grade metamorphism Shale to slate
a slight compaction and hardening, slight
deformation of features. Medium-grade
metamorphism significant changes, but parent
rock is apparent. High-grade metamorphism when
changes render the parent rock unclear.
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• Agents of metamorphism
• Heat Nearby magma or lava, deep burial
• Pressure Deep burial or pressure during
  continental collisions
• Chemical activity (fluids) Helps mobilize ions.
• Time
• Heat provides energy to break chemical bonds and
  recrystallize existing minerals or cause minerals
  to combine with other minerals.

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• In regional metamorphic setting continental
  collisions force some of the rock material to
  great depths (geothermal gradient), magmas
  generated by subduction of oceanic plate provide
  heat. Burial within a deep ocean basin may also
  trigger metamorphism. Temp. between 1500 and
  2000 C (8 km depth) can alter clays to micas.
• In a contact metamorphic setting, nearby lava or
  magma provides heat, if intrusion is dry, rock
  may be baked. If fluids are present, more
  changes take place. Host rock type is important
  too. Silicate minerals generally are stable and
  less subject to alteration. Carbonates
  (limestones, marbles) are more susceptible.

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The host rock biotite gneiss was probably
originally a granite under-went regional
metamorphism during the Paleozoic Era. The
diabase dike was intruded during early Mesozoic
Era. Fluids assoc. with the diabase caused
hydro-thermal alteration of micas and feldspars
in gneiss.
Diabase dike intrusion
Biotite Gneiss (regional metamorphism)
3 5 of contact metamorphism
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• Confining pressure (usually with increasing
  depth) causes equal pressure in all directions
  and compaction, recrystallization of existing
  minerals.
• Differential pressure causes deformation along a
  single plain. Near the surface, differential
  pressure may pulverize the minerals into smaller
  grains. At depth, increased temperature causes
  the grains to deform in a ductile fashion.

When under differential pressure, some minerals
rotate while others flow internally, i.e., ions
flow away from direction of high pressure to area
of lower pressure.
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High temperature intense differential pressure
cause flowage (deformation) of the rock.
Differential pressure directions can change over
time.
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• Heat was sufficient to make the rock fold, rather
  than break.

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• Chemically active fluids (primarily water)
• Fluids act as catalysts to enhance ion migration.
• If different minerals are adjacent ionic
  exchange may occur, perhaps changing both
  minerals.
• Higher temperatures may completely expel water
  from rock, changing the composition of clays,
  micas, and amphibolites.

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• Parent rock Many metamorphic rocks are
  chemically similar to their parent rock (shale
  slate, limestone marble, arkose gneiss,
  basalt amphibolite).
• Quartz sandstones stable, little change, except
  recrystallization.
• Limestones Calcite, more soluble. Dirty
  limestones (with clays or sand) are more
  reactive.

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• Calcite Quartz Wollastonite

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• Metamorphic texture size, shape, arrangement
  of grains.
• Deformed metamorphic rocks with platy (mica)
  and/or prismatic/elongate (amphi-bole) minerals
  show a preferred orien-tation (alignment) due to
  the pressure. These are foliated metamorphic
  rocks.
• Foliation forms by 1) Rotation of mineral grains
  2) Recrystallization of minerals to form new
  grains with preferred orien-tation 3) Changing
  the shape of equi-dimensional grains into
  elongated shapes.

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• Text on pp. 227-229 describes various mechanisms
  by which minerals develop a preferred
  orientation.
• Various grades of metamorphism yield various
  foliated textures rock (slaty) cleavage,
  schistosity, and gneissic texture. There are
  gradational textures in some cases.
• Variations in metamorphic grade are due to
  differences in depth of burial and/or proximity
  to igneous plutons.

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• Rock or slaty cleavage causes rocks to split
  into thin tabular slabs. Develops when heat and
  pressure result in the preferred alignment of
  platy minerals into thin zones of parallel flakes
  separated by layers of equigranular (non-platy,
  non-prismatic) minerals. Over time, the platy
  minerals will realign perpendicular to the force
  of pressure, resulting development of rock
  cleavage at an angle to the relict bedding planes.

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• Illustration of slaty cleavage.

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Alignment of clays in shale Initial alteration
of clays in slate Differential pressure causes
development of foliation at angle to original beds
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• Other textures
• Non-foliated Minimal deformation, parent rock
  composed of equidimensional minerals (calcite,
  quartz)
• Porphyroblastic Development of individual,
  large accessory mineral crystals (Staurolite,
  Garnet, Kyanite) with a matrix showing schistose
  or gneissic texture (Fig. 8.11, pg. 230). Parent
  rock mineralogy and heat/pressure conditions may
  also play a role in development of porphyroblasts.

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• Staurolite crystal represents a porphyroblast.

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• Classification of common metamorphic rocks and
  their probable parent rocks Figure 8.12, pg.
  231.
• Slate clay/mica flakes too small to be visible.
  Generally has a dull luster, excellent rock
  cleavage. Metamorphosed shale, mudstone, or
  siltstone, occas. volcanic ash. Black
  organics, Red iron, Green chlorite.
• Phyllite small grains, glossy sheen, sometimes
  wavy.

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• Pyrite crystals are also porphyroblasts.

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• Schist Dominated by platy minerals (muscovite,
  biotite, chlorite), sometimes with lenses of
  quartz, feldspar. Other less common schists
  include talc and graphite.
• Gneiss Medium- to coarse-grained with bands of
  aligned minerals. Light colored gneisses may be
  derived from arkose or granitic/rhyolitic igneous
  rocks.
• Sometimes called Granitic Gneiss.
• Examples Lithonia Gneiss.

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• Dark gneisses amphibolites from
  basalts/gabbros - Wolf Creek Fm. (Lawrenceville
  area), Pumpkinvine Creek Fm. (I-75 North of
  Marietta). If significant quartz/feldspar is
  present quartz or feldspar amphibolite. If
  quartz/feldspar gt50, amphibole gneiss.
  Pyroxenites gneisses composed of pyroxene are
  fairly rare.
• Layers of platy/elongate minerals within gneiss
  may referred to as schistose layers.

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• Amphibolite outcrop on I-75 North

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• Non-foliated Rocks
• Marble Coarse, crystalline rock derived from
  limestone or dolomite, composed of calcite.
  Impurities produce color varieties and accessory
  minerals (chlorite, muscovite, garnet,
  wollastonite) may produce a banded appearance.
  Popular for carvings, building purposes, but
  susceptible to acid rainfall. Georgia Marble is
  preserved within Murphy Marble Belt (or Murphy
  Syncline). Isolated marble occurrences are
  present in the Brevard Fault Zone, Buford area,
  Gainesville area,

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• Quartzite In moderate- to high-grade
  metamorphism, quartz sandstone is subject to
  fusion of individual grains, i.e., in a
  sandstone, fracturing occurs between grains,
  whereas in quartzite, fracturing occurs across
  grains.
• Marble or Quartzite may be similar in appearance.
  To discern between them, calcite in marble is
  softer than steel effervesces (fizzes) in HCl.
  Quartz in quartzite is harder than steel does
  not fizz.

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Metamorphic Environments 1) Contact 2)
Hydrothermal 3) Regional 4) Burial 5) Impact
6) Fault-associated Contact Metamorphism
adjacent to molten igneous rock. Aureole
zone of alteration. Small aureole diabase dike
in Vulcan quarry (slide 3). Large aureole (near
batholith) may have distinctive zones of
alteration, e.g., garnet adjacent chlorite
margin. Directed pressure is usually not present,
thus foliation usually doesnt form.
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• Hydrothermal Metamorphism caused when hot,
  ion-rich fluids circulate through rock fractures,
  causing mineral alterations. Example in Vulcan
  quarry, diabase dike caused alteration of
  feldspars and other silicates to sericite (a
  fine-grained muscovite). In submarine vulcanism,
  hydrothermal metamorphism can play a role in
  mineral enrichment of fractured rocks near
  seafloor vent (pg. 234). Fractured rocks provide
  more surface area.

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Regional Metamorphism Most widespread type,
regionally, Blue Ridge and Piedmont Provinces
formed during Paleozoic continental collisions
when rocks caught between continents was forcibly
buried (Fig. 8.24, pg. 238). Metamorphic Grade
increases toward the core of the mountain
belts, e.g., Shale-Slate-Phyllite-Gneiss. Burial
Metamorphism Sedimentary rocks at the bottom of
a thick accumulation undergo alteration by heat,
while pressure is uniform, not directional.
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• Fault-zone Associated Metamorphism Rocks caught
  in fault zones are sheared and shattered, taking
  on a foliated or lineated appearance, example
  Mylonites.
• Impact Metamorphism Meteorite impact causes
  melting and passage of shock waves. Lightning
  fused sand stishovite might be included.
  Article pg. 237 Tektites have been found in
  south Georgia.

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Index Minerals and Grade certain minerals are
good indicators of heat and pressure conditions
within metamorphic rocks (see Fig. 8.25, pg.
239). For instance, the presence of garnets in
the mica schists on the GUC campus offers
evidence of Intermediate-grade metamorphism.
One-half mile from the old GPC campus, chlorite
schist was present. Both rock units are probably
in the Wolf Creek Formation.
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• See Fig. 8C for Kyanite P/T conditions

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• Migmatites intense metamorphism causes
  light-colored silicates to melt, while
  dark-colored silicates remain solid.
• Plate Tectonic Settings Subduction Zone - Fig.
  7.24 Low Temp./High Pressure initial portion
  of subduction zone. High Temp./High Pressure
  zone of Partial Melting Rising plutons (though
  pressure is uniform). Near surface High
  Temp./Low Pressure.
• High Temp./Differential Pressure Cont. plate
  collisions.

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High Temp/ Lo Pressure
Low Temp/ High Pressure
Pluton
High Temp/ High Pressure
Different Temperature/Pressure settings in
Subduction Zones
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Continental Shields discussed more fully in
Chapter 20 Core areas of continents. Composed of
ancient igneous and metamorphic rocks, surrounded
by mountain belts built by continental
collisions.
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