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Metamorphic Rocks Metamorphic Rocks Metamorphic rocks • Subjected to heat and stress

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Metamorphic Rocks Metamorphic Rocks Metamorphic rocks • Subjected to heat and stress Powered By Docstoc
					Metamorphic Rocks
          Metamorphic rocks
• Subjected to heat (and stress)
• Results in changes in the appearance
  – Mineralogical
  – Textural
• Any rock can be metamorphosed
• But rocks that show metamorphic change
  best are:
  – fine grained clastic sedimentary rocks
  – mafic volcanic rocks
        Metamorphic textures
• Foliated
  – Rocks have some sort of alignment of minerals
  – Physical alignment of platy minerals gives a
    rock that tends to split more easily in one
    direction
  – Foliation may also be just layering of light and
    dark minerals
• Non-foliated
  – Rocks are “massive”
                 Foliated rocks
• Various different kinds of “foliated” rocks
  –   Slate
  –   Phyllite
  –   Schist
  –   Gneiss
• These rocks are of different “grade”
• Major differences are in grain size, but there
  are associated changes in mineralogy and
  the nature of the foliation
              Foliated rocks
• Slate (low grade)
  – Very fine grained (cannot see individual grains
    or identify the minerals)
  – Have “slaty cleavage” – a very smooth surface
    that is not the same as the bedding
  – Slaty cleavage is the result of aligned
    microscopic grains of clay, and possibly
    chlorite or muscovite
                 platy
                mineral




rock cleavage
              Foliated rocks
• Phyllite (low to medium grade)
  – Coarser grained than slate
  – The cleavage is shiny and results from coarser
    grained (than slate) muscovite or chlorite (and
    possibly biotite) grains
  – Cleavage is commonly not as smooth as a slaty
    cleavage
               Foliated rocks
• Schist (medium to high grade)
  – The foliation is clearly the result of aligned
    mineral grains (muscovite, biotite, chlorite, talc,
    some amphibole)
  – Grains are clearly visible
  – The parting (schistosity) is rough due to the
    grain size
              Foliated rocks
• Gneiss (high grade)
  – Layering of minerals (light and dark layers)
  – Dark layers usually of biotite or amphibole and
    grains are also usually aligned
  – The highest grade rocks undergo partial melting
    and the rock usually has lenses of igneous rock
    (granite) and is called a “migmatite”
                 Foliated rocks
• Foliated rocks result from heat and directed stress.
   – Heat causes minerals to grow – either get larger or react
     to form new minerals
   – Stress causes mineral to become oriented
• Because heat and stress cannot be imposed in
  small areas, foliated rocks, where they occur,
  occupy large “regions’
• Hence, the term “Regional Metamorphism” for
  foliated rocks
• The term dynamothermal is also used (dynamo
  meaning pushing or stress, thermal meaning
  heating)
             Non-foliated rocks
• Metamorphism by heat alone produces non-foliated rock
  (rocks that lack a foliation)
• Grade of metamorphism is determined by grain size and
  mineralogy
• Heat alone can be imposed in small areas as long as there
  is a heat source (commonly an igneous body) nearby
• Hence non-foliated rocks are usually the result of “Contact
  Metamorphism”.
• But there are some exceptions…where non-foliated rocks
  are “regional” in most senses of the word
          Non-foliated rocks
• In some cases stress (in regional
  metamorphism) may not produce much of a
  foliation. This commonly occurs in rocks
  consisting of a single mineral.
  – quartz sandstone becomes a quartzite
  – limestone becomes a marble
  – basalt becomes an amphibolite (though
    amphibolite is usually weakly foliated)
                Doing the lab
• Fill in the six columns
  –   F/N
  –   Other textures/structures
  –   Identifiable minerals
  –   Rock name
  –   Grade
  –   Original rock type
                     F/N
• Foliated or non-foliated
• Does the sample show:
  – Tendency to split
  – Layering of minerals
      Other textures/structures
• Relict bedding (in most slates): usually
  visible as colour banding that is different
  from the tendency to split (cleavage)
• Porphyroblasts: larger crystals in a finer
  groundmass (i.e., the metamorphic
  equivalent of the igneous term
  “phenocrysts”).
           Identifiable minerals
• Write down the names of the minerals you can
  identify (visually or by specific properties)
   – Do not use the term “mica”; be specific and write
     muscovite, or biotite
• There are a few new minerals that you have not
  encountered in the minerals lab (lab 1).
   – Andalusite, kyanite, sillimanite (all different forms of
     Al2SiO5, just like graphite and diamond are forms of
     carbon)
   – Staurolite, cordierite
• All are found in metamorphosed clay rich rocks
  and are on display at the back of the lab
Andalusite and staurolite
                Rock name
• Use the handout table
  NOTE: The handout is a modified version
  of Table 8.1, p. 190 (Table 7.1, p. 176, 5th
  ed.) of your textbook.
• Identify the rock on the basis of the grain
  size and mineralogy.
                  Grade
• Table 8.1 (and modified) gives approximate
  grade ranges for the common metamorphic
  rocks
• The second table (back of handout) gives
  slightly more detailed estimates of grade
  depending on mineralogy
               Original rock
• Tables also give likely “protoliths” for the
  common metamorphic rocks.
• However several different rock types have a
  number of possible protoliths. It is possible
  to distinguish which protolith is involved
  from the mineralogy of the rock
  – Gneiss rich in quartz and feldspar (little
    biotite/muscovite) is likely derived from felsic
    igneous rock or arkosic sandstone.
  – Gneiss with abundant biotite/muscovite is
    likely derived from a clay rich sediment
               Some points
• Use the handlens
• Write neatly
• Use a PENCIL (and eraser if necessary)
  – Starting this week, marks will be deducted for
    sloppy work.
Questions?
                   Reminder
Field trip next week.
  – Keep and eye on the weather
  – Often cold (may be rainy)
     • But on and off the bus
  – Dress warmly (layers)
  – Bus leaves promptly at 1:30 pm

				
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