Download GG 101L METAMORPHIC ROCKS SUPPLEMENTARY

GG 101L
METAMORPHIC ROCKS SUPPLEMENTARY READING
Metamorphic rocks result from alteration of pre-existing rock types by combinations of heat,
pressure, and the chemical action of fluids and gases. This metamorphosis often takes place deep
within the crust where temperature and pressure are high, and by studying metamorphic rocks we can
make inferences about the conditions under which they formed. We know, for example, that certain
metamorphic minerals only form at particular pressures (equivalent to particular depths). If such
minerals are found in a rock, they indicate that at one time that rock was at that particular depth.
As rocks are metamorphosed, they change mineralogically and texturally to become stable under
the new environment they find themselves in. Minerals contained in the parent rock often are unstable
at temperatures and pressures found deeper in the Earth. With time, therefore, they react in such a way
as to produce new mineral assemblages that are stable under the new conditions; metamorphism is the
collective name for these reactions.
The effects of metamorphism include: 1) chemical recombination and growth of new minerals; 2)
deformation and rotation of mineral grains; and 3) recrystallization of minerals into larger grains. The
net result is a rock of greater crystallinity and hardness, possessing new structural features and textures
which commonly exhibit flowage or other expressions of deformation.
Contact Metamorphism
There are two general types of metamorphism, contact and regional. Keep in mind, however, that
they are not mutually exclusive so that many gradations between them occur. Contact metamorphism is
usually of a local nature (i.e. not over a huge area), and is concentrated near the contacts between
intrusive igneous bodies and the surrounding country rock. Heat and chemical activity are the principal
agents of contact metamorphism, and the rocks affected generally recrystallize into hard, massive
bodies. The effects of contact metamorphism diminish with distance from the intrusion. Indeed, if you
find an area where the amount of contact metamorphism increases in a particular direction, you are
probably heading in the direction of the intrusion that caused the metamorphism (a useful observation
for geological interpretation).
Regional Metamorphism
Regional metamorphism is a result of the combined forces of high temperature and high pressure,
and it is common in areas that have been subjected to large-scale deformational stresses such as the
collision of two tectonic plates. Regional metamorphism therefore often extends across large areas.
The rocks produced in regional metamorphic processes are usually folded, faulted, crushed, sheared, or
stretched (or all of these).
Metamorphic Grade
The degree to which a rock has been metamorphosed is referred to as its metamorphic grade. Each
metamorphic grade corresponds to a particular combination of temperature and pressure, and is
characterized by a particular mineral assemblage. Figure 5.12 (5.10 in 4th edition) of your lab book
illustrates some of the minerals associated with the different metamorphic grades. Note that there are
assemblages of minerals (as opposed to individual minerals) that indicate metamorphic grade.
Regionally metamorphosed areas are exceedingly complex, and metamorphic grades may vary in
all kinds of patterns. It is tempting to say that they show semi-parallel bands of metamorphic grade that
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are roughly perpendicular to the main direction of compression associated with the metamorphism,
however, there are probably as many exceptions to this rule as there are examples that follow it.
Contact metamorphic grade patterns are usually simpler, tending to be roughly concentric around
the intrusion that provided the metamorphic heat. The intrusion itself may not even be exposed, but a
geologist can figure out where it must be (or have been) by mapping the metamorphic rocks nearby.
Foliation
Metamorphic textures hold evidence about the degree of metamorphism that occurred as well as
about the nature of the original rock that was metamorphosed (called the protolith). Foliation develops
when there are directed stresses involved in the metamorphism, meaning that the pressure is greater in
one direction than in others (as opposed to pressure that is the same in all directions). This greatest
pressure direction may be the collision direction of two continents, but as noted above, regionallymetamorphosed areas are terribly complex, so a geologist would not want to jump to such a conclusion
without considerable other evidence. Foliation is a general term for the preferred orientation and
growth of certain minerals, and the type of foliation indicates the degree to which the rock was
metamorphosed. You should note that the development of foliation involves both the actual rotation of
mineral grains as well as their growth in particular orientations. Note also that not all protoliths contain
the right mineral and chemical constituents to develop foliation, even under high directed stresses.
Slaty cleavage
Slaty cleavage is characterized by closely-spaced fractures that cause the rock to split along parallel
planes. Slaty cleavage results from the parallel orientation of microscopic (i.e., too small to see with
your naked eye) mica minerals. These flat minerals grow with their flat surfaces perpendicular to the
direction of the directed pressure. Because micas have one perfect cleavage direction, the rock ends up
also having a cleavage direction. A rock with slaty cleavage is called slate. It may be difficult to tell
slate from shale, but slates typically are harder and break along flatter planes (in the olden days,
chalkboards were made of slate).
Phyllitic texture
Phyllitic texture develops when the mica minerals have grown a little more than they have in slaty
rocks, and this indicates a slightly greater amount of metamorphism has occurred. These slightly larger
grains are still microscopic, but they are able to reflect light a little better, giving phyllites (the rocks
that show phyllitic texture) an obvious sheen. Phyllites sometimes break along preferred planes similar
to slates.
Schistosity
Schistosity continues the trend in that the individual mica grains are now visible to the naked eye.
A schist thus has a sparkly look to it, with the sparkles coming from the individual mica grains. Schists
tend to break in one direction, but the fracture planes are usually less well-defined than in slates.
Because big mica grains are not very hard, schists can often be pulled apart with your hands or
fingernails.
Gneissic layering
At even higher amounts of metamorphism, rocks are getting pretty close to actually melting. This
allows extreme rearrangement of ions and minerals to take place. Minerals tend to grow in layers along
with others of their same kind. This produces distinct layers of different minerals, also perpendicular to
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the direction of applied pressure. The rock type is called a gneiss (rhymes with nice). The minerals in
a gneiss are often the same minerals that you might find in a plutonic rock (quartz, feldspar, amphibole,
pyroxene, etc.), but the layering is usually the giveaway that it is a metamorphic rock rather than an
igneous rock.
Non-foliated metamorphic rocks
Not all metamorphic rocks are foliated. Instead, they appear massive. This may occur because
there is only one mineral in the protolith (limiting the ability to form layers of different minerals under
gneissic conditions, for example). Non-foliated metamorphic rocks may indicate that the
metamorphism was contact rather than regional, however, as noted above, you wouldn’t want to jump
to this conclusion.
Metaconglomerate
As the name suggests, a metaconglomerate is a conglomerate that has been metamorphosed.
Sometimes it looks just like a conglomerate whereas other times it is obvious that all the pebbles or
cobbles have been stretched and squoshed. In a metaconglomerate the once clay-rich matrix
recrystallizes into other, harder, minerals. Thus unlike a true conglomerate, a metaconglomerate will
fracture across the pebbles just as easily as around the pebbles.
Metabreccia
As the name suggests, a metabreccia is a breccia that has been metamorphosed. Sometimes it
looks just like a breccia whereas other times it is obvious that all the clasts have been stretched and
squoshed. In a metabreccia the once softer matrix recrystallizes into other, harder, minerals. Thus
unlike a true breccia, a metabreccia will fracture across the clasts just as easily as around the clasts.
Quartzite
Quartzite is the result of metamorphosing a quartz-rich protolith, usually quartz sandstone. Other
possible protoliths for quartzite are high-silica igneous rocks such as rhyolite. Because quartz is so
stable at all kinds of temperatures and pressures, there isn’t much that metamorphism can do to it.
About all that happens is that the quartz grains and silica cement (if the protolith is a sandstone) get
pressed together a little bit and start to recrystallize. Thus, what were once individual round sand
particles in a matrix becomes interlocked grains of pure quartz. As with metaonglomerates, quartzites
will fracture right across grains rather than around them.
Marble
Marble is the result of metamorphosing a limestone or dolostone. As with forming a quartzite, the
main process that takes place is the intergrowth of larger and larger calcite crystals. Additionally, any
pore space that was present in the original limestone gets squeezed out, leaving a rather dense, nearly
pure calcite rock. Because it is still calcite, however, marble is no harder than limestone, and it also
fizzes with HCl (although you may have to powder it a little bit first).
Hornfels
Hornfels is a type of non-foliated metamorphic rock that is usually very dark and fine grains. It
consists mainly of amphibole (the most common amphibole is called hornblende, hence the name
hornfels). Hornfels can be really difficult to tell from basalt (in hand sample), but because the two rock
types occur in very different geological settings, you can usually tell which one you are looking at.