Download Ch 7 - Metamorphic Rocks

Ch 7 - Metamorphic Rocks
Metamorphism
I.
Metamorphism - means "changed form".
a. Agents of Metamorphism - Changes occur because of:
i. Heat
ii. Pressure
iii. Chemical fluids
b. Rocks adjust to become more stable under new, higher temperatures and pressures.
II.
Heat - There are several sources of heat for metamorphism.
a. Geothermal gradient - Temperature increases with depth at a rate of 20 - 30 degrees
C per km in the crust.
b. Ultimate source of the heat? Radioactive decay.
c. Increase of temperature and pressure with depth causes Regional Metamorphism
d. Heat may come from large bodies of molten rock rising under a wide geographic
area.
e. Intrusions of hot magma can bake rocks as it intrudes them. Lava flows can also bake
rocks on the ground surface.
f. Lava or magma in contact with other rock causes Contact Metamorphism.
g. Hornfels is a common contact metamorphic rock.
i. Contact metamorphism along a narrow (approx. 1 meter wide) diabase dike in
the Deep River Basin of North Carolina. Diabase weathers tan. Contact
metamorphic aureole rocks (hornfels) are gray. Host rocks are red siltstones.
III.
Pressure
a. Burial Pressure. Pressure increases with depth due to the weight of the overlying
rocks. A cubic foot of granite weighs 167.9 pounds. Increase of pressure and
temperature with depth causes Regional Metamorphism.
b. Regional metamorphism occurs at depths of 5 - 40 km.
c. Tectonic pressures associated with convergent plate boundaries and continental
collision also cause Regional Metamorphism.
d. Pressure along fault zones causes Dynamic Metamorphism, the crushing and ductile
flow of rock.
e. Rocks formed along fault zones are called mylonites.
IV.
Chemical Fluids
a. In some metamorphic settings, new materials are introduced by the action of
hydrothermal solutions (hot water with dissolved ions). Many metallic ore deposits
form in this way.
b. Hydrothermal solutions associated with magma bodies
c. Black smokers - Sea water percolates through newly formed oceanic crust,
dissolving out metallic sulfide minerals. The hot sea water rises along fractures and
pours from vents in the seafloor as black clouds of dark mineral-rich water. Sulfide
minerals (such as pyrite, sphalerite, and galena) and copper precipitate when the hot
water comes in contact with cold sea water.
How do rocks change?
I.
Metamorphism causes changes in:
a. Texture
b. Mineralogy
II.
Texture
a. The processes of compaction and recrystallization change the texture of rocks
during metamorphism.
b. Compaction
i. The grains move closer together.
ii. The rock becomes more dense.
iii. Porosity is reduced.
iv. Example: clay to shale to slate
c. Recrystallization
i. Growth of new crystals. No changes in overall chemistry. New crystals
grow from the minerals already present.
ii. A preferred orientation of minerals commonly develops under applied
pressure. Platy or sheet-like minerals such as muscovite and biotite
become oriented perpendicular to the direction of force. This preferred
orientation is called foliation.
III.
Metamorphic Textures
a. Foliation is a broad term referring to the alignment of sheet-like minerals. Types
of foliation:
i. Schistosity - alignment of large mica flakes, as in a mica schist derived
from the metamorphism of shale.
ii. Slaty cleavage - alignment of very fine-grained micas, as in a slate
derived from the metamorphism of shale.
iii. Phyllitic structure - alignment of fine-grained micas, as in a phyllite.
iv. Gneissic banding - segregation of light and dark minerals into distinct
layers in the rock, as in a gneiss.
b. Lineation refers to the alignment of elongated, rod-like minerals such as
amphibole, pyroxene, tourmaline, kyanite, etc. Lineation is a texture commonly
seen in the metamorphic rock amphibolite derived from the metamorphism of
basalt.
c. Non-foliated or granular metamorphic rocks are those which are composed of
equidimensional grains such as quartz or calcite. There is no preferred orientation.
The grains form a mosaic.
i. Examples: quartzite derived from the metamorphism of quartz
sandstone, and marble derived from the metamorphism of limestone or
dolostone.
ii. Note: Not all quartzites and marbles are pure. Some contain impurities
that were originally mud interlayered with or mixed with the original
quartz sand or lime mud. These clay impurities metamorphose to layers of
micas or other minerals, which may give marble (in particular) a banded,
gneissic appearance, or which may give a slight foliation to some
quartzites.
d. The foliated metamorphic rocks
i. As shale is subjected to increasing grade of metamorphism (increasing
temperatures and pressures), it undergoes successive changes in texture
associated with an increase in the size of the mica grains.
1. Slate - very fine grained rock. Resembles shale. Has slaty cleavage
which may be at an angle to the original bedding. Relict bedding
may be seen on cleavage planes. Often dark gray in color. "Rings"
when you strike it. (Unlike shale, which makes a dull sound.
Temperature about 200 degrees C; Depth of burial about 10 km.
2. Phyllite - fine-grained metamorphic rock. Has a frosted sheen,
resembling frosted eye shadow. This is no coincidence. Cosmetics
commonly contain ground up muscovite (ground to a size similar
to that occurring naturally in phyllite.)
3. Schist - metamorphic rock containing abundant obvious micas,
several millimeters across. Several types of schist may be
recognized, based on minerals which may be present:
a. mica schist
b. garnet schist
c. chlorite schist
d. kyanite schist
e. talc schist
4. Gneiss - (pronounced "nice") - a banded or striped rock with
alternating layers of dark and light minerals. The dark layers
commonly contain biotite, and the light layers commonly contain
quartz and feldspar.
5. Migmatite - a very high grade metamorphic rock that has been
subjected to such high temperatures that it has partially melted. It
is intermediate between the metamorphic and the igneous rocks.
Look for swirled banding. The light colored minerals have
undergone melting and flow. The dark colored minerals have been
contorted by flow. Example - the Lithonia Gneiss in the area
southeast of Atlanta.
e. The non-foliated (and weakly foliated) metamorphic rocks
i. Marble - fizzes in acid because its dominant minerals is
calcite (or dolomite). The parent rock is limestone (or
dolostone).
ii. Quartzite - interlocking grains of quartz. Scratches glass. The
rock fractures through the grains (rather than between the grains
as it does in sandstone). The parent rock is quartz sandstone.
iii. Metagraywacke - metamorphosed graywacke or
"dirty sandstone".
iv. Metaconglomerate and stretched pebble
metaconglomerate - the parent rock is conglomerate. The clasts are fairly
easily recognized. May be more difficult to recognize if the clasts have
been stretched.
v. Hornfels - A fine-grained, tough, dense, hard, massive rock. Usually (but
not always) dark in color. Finer grained than basalt, which it may
superficially resemble. This rock forms through contact metamorphism.
The parent rock is commonly siltstone or basalt, but may be other types of
rock.
vi. Serpentinite - A dark green, dense, tough, massive, hard rack. May
contain veins of asbestos. The parent rock is peridotite, an ultramafic
rock.
vii. Soapstone (sometimes called steatite) - a soft, easily carved rock with a
slippery feel because it contains talc and chlorite. The parent rock is
peridotite (ultramafic), probably with more water associated with it than
in the formation of serpentinite. Example - Soapstone Ridge southeast of
Atlanta.
viii. Metabasalt (sometimes called greenstone if massive and green, or
greenschist if foliated and green) - the green color comes from chlorite
(soft and bluish green) and epidote (pea green). The parent rock is basalt.
The grade of metamorphism is LOW.
ix. Amphibolite - Abundant amphibole is present; may be lineated. Usually
black. The parent rock is basalt. The grade of metamorphism is HIGH.
Has been subjected to higher temperatures and pressures than metabasalt,
greenstone, or greenschist.
x. Mylonite - A dynamic metamorphic rock which forms along fault zones.
Mylonite along the Linville Falls Fault, Linville Falls, NC. Relatively undeformed
conglomeratic quartzite lies above the layered mylonite zone.
IV.
Mineral changes in metamorphic rocks
a. Recrystallization - rearrangement of crystal structure of existing minerals.
Commonly many small crystals merge to form larger crystals, such as the
clay in shale becoming micas in slate, phyllite, and schist.
i. Note the chemical compositions of clay and muscovite.
ii. Also, fine-grained calcite in limestone recrystallizes to the coarsegrained calcite mosaic in marble.
b. Formation of new minerals - there are a number of metamorphic minerals
which form during metamorphism and are found exclusively (or almost
exclusively) in metamorphic rocks:
i. Garnet - dark red dodecahedrons (12 sides)
ii. Staurolite - brown lozenge-shaped minerals, commonly twinned to
form "fairy crosses".
iii. Kyanite - sky blue bladed minerals with differential hardness. Scratch
lengthwise with a knife or nail, but not sideways.
iv.
v.
vi.
vii.
V.
Chlorite - dark bluish green, soft. Fe, Mg
Talc - white or pale green and soft.
Graphite - metamorphosed carbon
Tourmaline - commonly black. Forms elongated crystals with a rounded
triangular cross-section. Can see at Stone Mountain.
viii. Asbestos - fibrous mineral. Commonly light greenish. Occurs in veins
(seems to fill a crack) with the fibers oriented perpendicular to the edged
of the vein. Associated with lung diseases. Mesothelioma and asbestosis.
Found in serpentinite. "Serpent rock" name due to snake-like veins of
asbestos.
ix. Micas - muscovite (silvery), biotite (dark brown), phlogopite (light
brown)
Metamorphic index minerals
a. In regional metamorphic terranes, the temperature and pressure regime is
indicated by the distribution of metamorphic minerals across a large area.
b. Low metamorphic grade (low temperatures and pressures) - about 200 degrees C
Slate and phyllite
i. chlorite
ii. muscovite
iii. biotite
c. Intermediate metamorphic grade
i. Schist
ii. garnet
iii. staurolite
d. High metamorphic grade - 800 degrees C (verging on melting)
i. Gneiss and migmatite
ii. sillimanite
e. Know which minerals are the metamorphic index minerals. Know which ones are
characteristic of low, intermediate, and high metamorphic grades.