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					Geology for Engineers




Minerals and Rocks (II)

                     Gneiss
        Metamorphic Rocks
Rocks may be altered through
• Temperature
• Pressure
• Deformation
• Fluid influx
        Metamorphic Rocks
• The original rock is termed a protolith

• The composition of the protolith and the
  conditions of metamorphism and
  deformation will largely determine the end
  structure and composition of the
  metamorphic rock
        Metamorphic Rocks
• Protoliths may be sedimentary, igneous or
  even metamorphic
• Metamorphism may result from a general
  increase (prograde), or decrease
  (retrograde) of temperature and pressure
Pressure-Temperature-Time
        Metamorphic Rocks
Metamorphism may be:
• Dynamic (i.e. some deformation involved),
  e.g. gneiss

• Static (i.e. the end rock does not have a
  preferred shape fabric) e.g. hornfels
Gneiss
         Metamorphic Grades
•   Anchi-metamorphic   • Increasing
•   Greenschist           temperature and
•   Amphibolite           pressure
•   Granulite
•   Eclogite

• Blueschist
P-T fields of metamorphic grades
Index of Metamorphic Minerals




                       csmres.jmu.edu/
Index of Metamorphic Minerals




                       csmres.jmu.edu/
Isograd Map
    Different Styles of Metamorphism
•   Barrovian (regional) metamorphism
•   Buchan (regional) metamorphism
•   Contact metamorphism
•   Shock metamorphism
•   Dynamic metamorphism (e.g. shearing)
      Contact Metamorphism
• This shows a
  basalt (igneous
  rock) intruding
  into a limestone.
  You can see that
  it has been
  metamorphosed
  into marble near
  the contact with
  the basalt.
                    Mylonite
• Intensely sheared
  rock. Note the
  recrystallization of
  feldspar (lightest
  crystals) and the
  ribbons of quartz
  (grey ) in a dark,
  biotite-rich matrix.
• Difficult to
  determine the
  protolith!
Tectonic Settings of Metamorphism
        Metamorphic Rocks
• Sandstone can
  be
  metamorphosed
  to quartzite.
• Sutured quartz
  boundaries
  (changes
  mechanical
  properties of the
  rock)
         Metamorphic Rocks
• A limestone
  protolith can be
  metamorphosed
  into a marble (this
  example is quite
  pure, i.e. white).
       Metamorphic Rocks
• Mudstone
  (pelite)
         Metamorphic Rocks
• Phyllite - is the
  metamorphic
  step beyond
  slate. New
  layers of mica
  minerals give
  phyllite a shiny,
  “wavy”
  appearance.
         Metamorphic Rocks
• Schist - heavily
  foliated rock in
  which all the
  original clay
  minerals are fully
  transformed
        Metamorphic Rocks
• Greenschist –
  minerals such as
  epidote, chlorite
  and actinolite
        Metamorphic Rocks
• This is a
  blueschist. It was
  formed in a
  subduction zone in
  high pressure and
  relatively low
  temperature
• Blue mineral =
  glaucophane
         Metamorphic Rocks
• Migmatite
Partial melting of
  protolith
Dark parts =
  restite
Light parts =
  leucosome
       Metamorphic Rocks
• Garnet Gneiss
         Metamorphic Rocks
• This is an eclogite.
  You can see the
  presence of
  garnet (pink) and
  pyroxene (green)
         Metamorphic Rocks
• This image
  shows chert
  (red), which is a
  sedimentary
  rock being
  replaced by
  silica (SiO2)
  veins.
        Metamorphic Rocks
• This shows a
  basalt (igneous
  rock) which has
  been
  metamorphosed
  into an
  amphibolite (i.e.
  pyroxene
  changed to
  amphibole)
       Metamorphic Rocks
• This shows an
  olivine-rich
  igneous rock
  (dunite) which
  has been
  metamorphosed
  into an
  serpentinite

				
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