Download Metamorphic Rocks

Chp 7: Metamorphic Rocks
Metamorphism – From the Greek “meta” = to change, and “morpho” = shape.
Metamorphism – “The altering of rock characteristics and mineral compositions due to
heat and/or pressure, or other environmental factors. This changing is a Solid State
Reaction, meaning that the rocks subjected to metamorphic processes do not melt
(otherwise upon cooling, they would form igneous rocks). It is thought to be a relatively
slow geologic process. A great many areas of metamorphism yield abundant mineral
reserves of gold, silver, copper, lead, zinc, and other valuable minerals.
Metamorphic rocks are formed either by being exposed to heat, pressure, or chemically
active fluids, or a combination of these factors to create a rock that has a different texture
and mineral content
The “parent rock” is the term for the rock prior to metamorphism. It may be igneous,
sedimentary, or another metamorphic rock. For example, here are some parent rocks
and the rock that they may metamorphose into under certain conditions:
Limestone – marble
Clay stone – slate
Granite – gneiss, etc.
Specimen of Chrysotile:
fibrous form of
serpentine asbestos
p.186
Chp 7: Metamorphic Rocks
The effect of metamorphism on rocks is analogous to baking a cake:
the resulting cake is dependent upon the ingredients, the amount of
fluids, the temperature, and the length of time it was “baked”.
A great portion of the continents is metamorphic formed during
“continental accretion” during the formation of the Precambrian.
Metamorphics form the stable basement rocks called “continental
shields” upon which surface sedimentary rocks have been deposited.
Metamorphics also comprise a large portion of the crystalline core
of many mountain ranges.
Occurrence of metamorphic rocks: shields, core of mountain ranges
Fig. 7-1, p.185
Chp7: Metamorphic Rocks
Factors Involved in Metamorphism
I.
Heat – The source of heat may be from a large intrusive body such as a pluton, or
heat from activities associated with s plate tectonics.
-At temperatures below 2000 C, only a small amount of fluid is present in most rocks.
As the temperature increases many minerals release pore fluid that was trapped in the
rock or in crystal lattices of its minerals. This pore fluid may become very chemically
reactive, altering the chemistry of the surrounding rocks.
-The Geothermal Gradient – On average the temperature of the rocks in the earth
increase 25C per kilometer of depth. On the continental cratons, the average is
20C/km. On the continental boundaries it is 40C/km. At subduction zones, it is
10C/km because heat is dissipated into the sea.
-At 700C, most rock components become “plastic” where many times the pre-existing
crystals rotate, or twist altering the texture of the rock.
-Under conditions of high heat, pressure, and chemically active fluids, crystal lattices
begin to break down, recreate new types of crystal lattices, rearrange ions, and form new
minerals in the process.
-Some minerals only form at certain temperature and pressures. If these are found in a
metamorphic rock, the temperature of formation can be deduced.
Pressure effects on materials:
a. Lithostatic pressure is applied
equally in all directions to rocks
buried beneath surface of earth.
That pressure increases as depth
of burial increases.
b. Similar to effects on styrofoam
cups lowered to ocean depths
of 750m and 1500m.
Net effect: retain shape but decrease
in volume.
Fig. 7-2, p.187
Chp 7- Metamorphic Rocks
II. Pressure – When rocks are buried, they are subjected to lithostatic pressure the pressures from all sides by the overburden weight of the country rock.
-Differential pressures – may exist whereby the pressures exerted upon the rock are no
equal in all directions. This results in a distortion or twisting effect on the rock.
-Phenocryst rotation or distortion may occur. This can cause grains in the rock to stretc
rotate, bend, line up in rows, become platy, etc. (i.e. micas forming in mica schists)
-Pressure distortion of metamorphic rocks is common around areas of high lithologic
stress such as areas around tectonic boundaries.
III. Chemically Active Fluids – Fluids released from igneous intrusions, or other
metamorphic processes can cause a constant interaction or exchange of ions
altering the rocks.
-Metasomatism – the introduction by fluids of ions from an external source not directly
associated with the intrusion. Hydrothermal Metamorphism – changes due to
migrating superheated water and dissolved ions. Hydrothermal rocks many times
appear “bleached” because of the intense chemical reactions.
Sharp boundary (red line) between intrusion on left and country rock
on the right.
Fig. 7-5, p.190
Differential pressurePressure applied unequally in all directions.
Note garnet in schist
Fig. 7-3, p.188
Chp 7- Metamorphic Rocks
Sources of water –
1. Juvenile Water – water given off by cooling magma.
2. Metamorphic Water – water already present the country rock, which is given off
during metamorphic processes.
3. Meteoric Water – “groundwater” contained in aquifers encountered in the country
rock during metamorphic processes
Hydrothermal activities – many times form economically rich mineral deposits
of gold, copper, iron, lead, etc. This process is also responsible for the “veining”
(“mother loads”) of gold and other valuable minerals.
Volcanic events such as calderas usually have associated hydrothermal activities
Types of Metamorphism
I. Contact – Effects of Heat and Fluids
Characteristics:
a. “Heat” is the driving force in contact metamorphism.
Common where hot magmatic plutons come into contact with the surrounding country ro
b. The degree of metamorphism is related to the temperature of the magma, the size of the
intrusion, and the chemically active fluid content of the magma involved. Large intrusions such
as batholiths cool for long periods of time -more intense metamorphic change in the country rock.
Chp 7- Metamorphic Rocks
c. Temperatures can reach 9000 degrees C next to the intrusion.
d. As the heat and associated metamorphic changes alter the country rock, the country
rock closest to the intrusion is affected most, furthest from the intrusion is affected least.
e.This sets up a “metamorphic halo” or “aureole” in the country rock around the intrusio
The aureole is a gradation of degrees of metamorphism surrounding the intrusion:
1. Shale – unaltered country rock
2. Slate – low grade metamorphism
3. Phyllite –between low and med. Grade
4. Schist – medium grade metamorphism
5. Gneiss – High grade metamorphism
6. Migmatite – Very high grade metamorphism
7. Melting occurs at 900C,above this temperature = formation of igneous rock.
f. Two types of contact metamorphic rocks are recognized:
1. those resulting from the “baking” of the country rock
2. those resulting from the actions of chemically active fluids
g. Many “baked” types have the texture of porcelain if they contain high amounts of clay
such as shale. This effect is seen in the firing of ceramics in a kiln.
h. Hydrothermal activity is also common with contact metamorphism resulting in an
enrichment of valuable ore deposits.
Metmorphic aureole commonly surrounds many intrusions: this
Model has 3 zones of mineral assemblages around the intrusion, reflec
decreased pressure and temperature effects away from the intrusion
Fig. 7-4, p.189
Table 7-2, p.202
Chp 7- Metamorphic Rocks
II. Regional Burial – Effects of Lithostatic Pressure
Characteristics:
a. Occurs over a very broad area
b. Rocks are altered due to tremendous pressures (and the resulting high temperatures),
resulting in deformation within deeper portions of the crust.
c. Very common along convergent and divergent plate boundaries.
d. Index minerals are minerals that are known to form only under certain temperatures an
pressures. The following is a sequence of known minerals that form from low grade
metamorphism to high grade:
chlorite – (forms around 200 C), muscovite, biotite, garnet, staurolite, kyanite
(forms around 500C)
e. Quartz and feldspars can be present in both igneous and metamorphic rocks, but some
minerals such as andalusite, sillimanite, and kyanite (all 3 minerals are forms of Al2SiO5)
form only from these metamorphic conditions.
f. The presence or absence of these minerals is an indication of the degree of pressure
(and resulting heat) in the formation of the rock in question.
g. Examples of regional burial rocks are: marble from limestone, quartzite from quartz
sandstone, and argillite from clay.
Mineralization due to metamorphism of shale: new minerals form
at different Temperature levels. Progression of minerals indicates
temperature level
Fig. 7-7, p.191
metamorphic aureole in sedimentary rocks
of Michigan
Fig. 7-17, p.200
Chp7- Metamorphic Rocks
III. Dynamic Metamorphism (“Dynamo-thermal”)Characteristics:
a. Usually associated with the pressures around fault zones.
b. “Mylonites” is the term used to describe rocks formed in this way.
c. Typically, the extent of metamorphism is restricted to narrow margins adjacent to faults
d. Myolinites are hard, dense, fine-grained rocks, many of which have laminations or
layerings.
e. These also can be associated with tectonic settings.
Textures of Metamorphic Rocks
I. Foliated Textures Characteristics:
a. Typically associated with contact metamorphism.
b. Minerals are arranged in a platy, parallel fashion.
c. The size and shape of the mineral grains determines if the foliation is fine or coarse.
d. A coarse foliation usually indicates a higher degree of heat such as in gneiss.
e. A fine foliation usually indicates a lower degree of heat such as in schist.
Slate has a very fine foliation exhibiting the lowest grade of contact metamorphism.
Mylonite from Adirondack Mtns…note thin laminations
Fig. 7-6, p.190
When rocks are subjected to differential pressure, mineral grains
Typically align in parallel fashion-producing a ‘foliated’ structure.
b. Foliated metamorphic rock…
Fig. 7-8, p.192
Fig. 7-8b, p.192
Chp 7- Metamorphic Rocks
Examples of Foliated Textured Metamorphic Rocks:
Slate –has a very fine foliation due to it having formed at the lowest grade of contact
metamorphism. It possesses a slaty cleavage, easily cleaving or parting along the axis of
layering. It is used for pool tables, chalkboards, and building tiles for this reason. The
different colors of slates are due to the presence of minerals such as chlorite (green),
graphite (black), or iron oxide (red).
Phyllite – similar to slate but coarser grained. It is more lustrous or glossy due to tiny
mica minerals. Grains are too small to be identified with the unaided eye.
Schist – is most commonly produced by regional burial metamorphism. It can also be
produced by medium grade contact metamorphism. Metamorphosed clay rich
sedimentary rocks typically produce schists (although other rocks may also produce them)
All schists contain more than 50% platy and elongated minerals all of which large
enough to identify. The degree of schistosity reflects the temperature of formation: the
greater the temperature, the greater the degree of schistosity. Schists are common in low
to medium grade metamorphic environments. Schists are named as to the most abundant
mineral: mica schist, talk schist, biotite schist, chlorite schist, etc.
Hand specimen of slate
Fig. 7-9a, p.194
Large sheet of slate-note the original bedding: up right to lower
left
Fig. 7-9b, p.194
Phyllite: note lustrous sheen and bedding at angle to cleavage
Fig. 7-10, p.194
Garnet-mica schist
Fig. 7-11a, p.195
Hornblende-mica-garnet schist
Fig. 7-11b, p.195
Chp 7 Metamorphic Rocks
Texture continued….
Gneiss – is a streaked or has segregated bands of alternating light and dark minerals.
Quartz and feldspar are the major light colored minerals and biotite and hornblende are
the principle dark colored minerals. Gneiss typically forms from regional metamorphism
of clay-rich sedimentary rocks, from contact metamorphism of granites, or from metamorphism of older metamorphic rocks.
Amphibolite – a dark-colored, slightly foliated rock consisting primarily of hornblende
and plagioclase. The metamorphism of mafic rocks such as basalt produce amphibolites.
Migmatites – “mixed metamorphics” – These have characteristics of both igneous and
metamorphic rocks indicating very high heat and pressure. Examples include the rocks
touching an intrusion: the very highest grade contact metamorphism. Most contain
granite components, or lenses (small pieces of other rocks), and appear to have been
twisted or wavy. This may be due to partial melting of the country rock.
Gneiss-recognized by segregated bands of light and dark minerals.
This gneiss has been folded…
Fig. 7-12, p.195
Migmatites-high grade metamorphic rock, with streaks or lenses of
granite. Picture from Lake Huron, Ontario, Canada
Fig. 7-13, p.198
Chp 7 Metamorphic Rocks
II. Nonfoliated Textures Characteristics:
These textures result from the metamorphosing of rocks whose minerals do not show a
preferred orientation, and therefore are not foliated.
Most non-foliated rocks result from contact or regional burial of rocks that are devoid
of platy or elongated crystals.
Two Types of Nonfoliated rocks:
those composed of mainly one mineral (marble or quartzite)
those composed of mineral grains that are too small to be seen- hornfels or greenstones.
Examples of Non-foliated Textured Metamorphic Rocks:
Marble – the parent rock is a limestone (mostly calcite) or dolostone (mostly dolomite)
that was subjected to contact or regional burial. It may be fine-grained to coarse-grained.
Color variation is due to impurities in the parent rock. Because of its texture and softness
marble has been used extensively for sculpturing.
Quartzite – the parent rock is a quartz sandstone subjected to medium to high grade
contact or regional burial resulting in a hard, coarse-grained compact rock. Pure quartzite
is white but impurities may alter the color. Since it is so hard from the re-crystallization
of the quartz, it is commonly used for the bases of roads and buildings.
Chp 7- Metamorphic Rocks
Greenstone – this is the name given to any compact, dark green, altered, mafic igneous
rock that formed under low to high grade metamorphic conditions. The green color is due
to the minerals chlorite, epidote, and hornblende. These are commonly the rocks found
in “greenstone belts” along the transitional zones of sialic continental plates to mafic
oceanic plates.
Hornfels – fine-grained, nonfoliated rock formed from contact metamorphism. The
grains are equidimensional with its composition dependent upon the composition of the
parent rock. Most are formed from contact metamorphism of clay-rich sedimentary rocks
or impure dolomites.
Anthracite – is a black, lustrous, hard coal that is high in carbon and low in volatiles.
Its parent rock is bituminous coal that was subjected to regional burial.
Marble results from the metamorphism of sedimentary rocks known
Limestone or dolostone
Fig. 7-15, p.199
Photomicrograph of marble-note the mosaic of
roughly equidimensional minerals-indicates nonfoliated texture
Fig. 7-14, p.198
Quartzite results from the metamorphism of quartz sandstone
Fig. 7-16, p.199
Chp 7- Metamorphic Rocks
Metamorphic Zones or Facies –
a. A “metamorphic facies” is a group of metamorphic rocks characterized by particular
mineral assemblages (more than one mineral is present) under the same broad
temperature/pressure conditions.
b. Each facies is named after its most characteristic rock or mineral.
c. Metamorphic facies are usually are applied to areas whose parent rocks were original
clay-rich. Metamorphic facies cannot be applied to areas where the parent was pure
limestone or pure quartz sandstones because they would produce only marbles and
quartzites respectively.
Examples of Metamorphic Facies:
a. Greenschist Facies – forms whenever the rock is rich in the mineral chlorite and is
subjected to relatively low temperatures and pressures.
b. Granulite Facies and Amphibolite Facies – form under similar chemistries but the
pressures are significantly greater.
c. Blueschist Facies – form at subduction zones where, due to the presence of seawater,
the temperature is low, but because of the tectonic activity, the pressure is high.
This results in an abundance of a blue-colored amphibole mineral named glaucophane.
The presence of a blueschist facies indicates to the geologist the presence of ancient
subduction zones.
Metamorphic facies produced along oceanic-continental boundary
Fig. 7-19, p.201
Chp 7- Metamorphic Rocks-Summary
Metamorphic Rocks form as a result of ‘metamorphism’…an alteration of rock characteristics and chemical composition due to application of heat and/or pressure, or chemically active fluids.
“Parent rock” is term applied to the rock being metamorphosed-it may be igneous, sedimentary or even another metacmorphic rock.
Metamorphic rocks commonly occur in-core of mountain ranges
-continental shields (sedimentary rocks commonly deposited on top of them…)
-original continental accretion in PreCambrian
Factors applied during metamorphism:
-Heat
-Pressure
-Chemically active fluids
Table 7-1, p.192
Chp 7- Metamorphic Rocks-Summary
Types of Metamorphism:
A. Contact metamorphism: results from heat and fluids
-metamorphic ‘halo’ known as aureole is generated (shale)
-baked zones common
-hydrothermal effects occur..
B. Regional burial: occurs over large area
-gradation of minerals common as a result of high pressure
-specific minerals indicate different levels of pressure/temperature
C. Dynamic metamorphism: usually associated with fault zones
- mylonites
Economic uses- mining slate, hydrothermal minerals suggest proximity
to gold or silver??
Slate mine in Wales, England. Formed by mountain building process
dated at 400 to 440 million years ago (MYA)
Fig. 7-21, p.202
Chp 7- Metamorphic Rocks-Summary
Metamorphic textures:
A. Foliated-results from contact metamorphism
-varies from coarse to fine
slate, phyllite, schist, gneiss, amphibolite, migmatite
B. Non-Foliated- no preferred orientation to minerals
-2 types: single mineral, grains too small to be seen with naked eye
marble, quartzite, greenstone, hornfels, anthracite
Metamorphic Zones/Facies: metamorphic rocks characterized by
specific mineral assemblages that reflect pressure-temperature regime
rock experienced:
1. Greenschist: contain chlorite, low temperature, lo pressure
2.Granulite/Amphibolite: similar but higher pressure
3. Blueschist: fairly low temperature, high pressure. Indicative of
subduction zones. Glaucophane mineral….
Pressure-temperature diagram showing where metamorphic facies occur.
Each facies is named after its most characteristic mineral.
Fig. 7-18, p.200
Chp 7- Metamorphic Rocks
Fig. 7-CO, p.182
Fig. 7-8a, p.192
Different colored slates used for roof in Michigan
p.193
Slate roof in Switzerland
Fig. 7-9c, p.194
Franciscan Complex in CA: low temperature, high pressure subduction
Fig. 7-20, p.201
Fig. 7-20a, p.201
Fig. 7-20b, p.201