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
• 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)
platy
mineral
rock cleavage
Foliated rocks
• Various different kinds of “foliated” rocks
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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
Pool table tops made of slate
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
Note the glossy sheen typical of phyllite.
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”
Gneissic layering; layers of identifiable
minerals
Dynamic metamorphism
• Change of texture but (often) without
significant mineral growth
• Faults/shear zones
• Foliated but othewise appear unchanged
– E.g., sheared granite
• At most extreme: mylonites, v. fine grained
(can’t identify minerals) and strongly
foliated (though lacking the easy parting of
slaty cleavage)
Foliated granite
Mylonite
So sheared it’s hard to tell what the orignal rock was
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 particularly if
that mineral does not typically grow as
inequant grains.
– 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
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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”).
Relict bedding in slate; colour banding, v.f.g., no
identifiable minerals
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 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 (protolith)
• 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 hand-lens
• Write neatly
• Use a PENCIL (and eraser if necessary)
– Starting this week, marks will be deducted for
sloppy work.
– If we can’t read your work we can’t grade it.
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