Metamorphic Rocks

Metamorphic Rocks
3- Metamorphic Rocks
Rocks that form when a pre-existing rock (protolith)
changes due to temperature or pressure, and/or as a result
of squashing or shearing.
Protolith – the pre-existing rock
Metamorphism doesn’t include weathering, diagenesis, and melting. It is a
solid-state process.
James Hutton, a Scottish doctor became fascinated by metamorphic rocks
and published a book, Theory of the Earth (1795), and outlined many
fundamentals of geology that are still used today. He is referred to as the
father of geology.
Charles Lyell – first proposed the word metamorphism in his book,
Principles of Geology (1833). He was a good friend of Charles Darwin and
his work highly influenced Charles Darwin’s theory of evolution.
How Do We Identify Metamorphic Rocks?
1- Metamorphic Textures – grains are interlocked and grew in place.
Many different types of metamorphic textures
2- Metamorphic Minerals – Certain minerals only grow under
metamorphic temperatures and pressures.
- Called a metamorphic mineral assemblage, or metamorphic facies
3- Foliation – The alignment of platy minerals or alternating layers of
light (felsic) and dark (mafic) minerals.
A foliated
Outcrop
of Gneiss
Formation of Metamorphic Textures
How do metamorphic textures form?
•
•
•
•
•
Recrystalization – changes the shape and size of grains,
but the same mineral remains. E.g. Sandstone may
recystallize into quartzite. See (a)
Phase Change – When a mineral keeps the same
composition but the atoms arrange into a new form
(polymorph). E.g. quartz (SiO2) may change to coesite
(SiO2).
Metamorphic reaction/neocrystallization – The result of
chemical processes that decompose minerals and produce
new minerals. Happens through diffusion of atoms
through solid crystals. Very slow process. See (b)
Pressure Solution – Mineral grains dissolve where their
surfaces are in contact. Occurs when rock is squeezed in
one direction more than the others, at low temps, and
usually in the presence of water. Usually zig-zag shaped
and common in carbonates. See (c)
Plastic Deformation – At high temps, minerals can
behave like soft plastic and become squished or stretched.
Takes place without forming cracks and without changing
the composition of the minerals. See (d)
What Causes Metamorphism?
1.
Heat - Increased heat allows chemical
bonds to break easier.
2.
Pressure – high pressures cause
minerals with ‘open’ lattices to collapse,
forming more dense crystals. Most
metamorphic rocks form at 40-100 km
depth where pressures are 10,000-30,000
times greater than the surface of the
Earth.
3.
A ‘nice’ sample of gneiss
Differential Stress – When forces are
not equal in all directions, minerals may
deform and change shape.
4.
Hydrothermal Fluids – More than
just water, hydrothermal fluids are
solutions that chemically react with
minerals.
Recrystallized limestone becomes marble
Changing Temperature and Pressure
X
Y
•
Minerals have Stability
Fields or regions of pressure
and temperature where they
are stable. Enter a new
stability field and a new
mineral begins to form
•
Al2SiO5 Phase
Diagram
Stability Fields are
characterized by both pressure
and temperature and can be
represented on a Phase
Diagram like the one above.
Differential Stress
• Pressure – A stress that is
the same in all directions.
Can only change size
(volumetric), not shape.
E.g. water pressure,
lithostatic stress.
• Differential Stress –
When the stress is not
equal in all directions.
Can change size and
shape.
• Normal Stress – pushes
or pulls perpendicular
(normal) to a surface. E.g.
crushing a soda can.
• Shear Stress – moves one
part of a material
sideways relative to the
other side. E.g. spreading
out a desk of cards.
Changes in Shape due to Differential Stress
• Differential stresses may cause once equant (~same length in all
dimensions) to become elongate or tabular/platy in shape.
• The preferred orientation of these inequant grains gives the rock a
foliation (a planar fabric)
Tabular /
Formation of Foliation
• Differential stress can result
in the formation of
foliations (planar fabrics) in
a variety of ways.
The Role of Hydrothermal Fluids
• Hydrothermal fluids - Include hot water, steam, and supercritical fluid.
Hydrothermal fluids are chemically-active in that they are able to dissolve certain
minerals, so hydrothermal fluids are solutions, not just water.
• Supercritical Fluid – A substance that forms under high temps and pressures that
has properties of both a gas and a liquid. Supercritical fluids permeate rocks like a
gas and react with minerals like a fluid.
• Where does this fluid come from?
1- groundwater that percolates downward.
2- water and volatiles released from magma
3- water is released during some metamorphic reactions
KAl3Si3O10(OH)2 + SiO2  KAlSi3O8 + Al2SiO5 + H2O
Muscovite
Quartz
K-Feldspar Sillamanite Water
• Hydrothermal fluids speed metamorphic reactions because fluids allow for easy
transport of ions and fluids are consumed in some reactions
• Metasomatism – The process by which a rock’s chemical composition changes
due to reactions with hydrothermal fluids.
• Metasomatism commonly results in the formation of veins, mineral filled cracks.
Veins
• Veins are different than joints
Veins:
– Filled with minerals
– Commonly wavy in shape
Joints:
– Usually planar
– Usually not mineralized,
i.e. just a crack
Types of Metamorphic Rocks
• Metamorphic rocks are grouped into two main
categories:
– Foliated Metamorphic Rocks
– Non-Foliated Metamorphic Rocks
• But what exactly is foliation?
Foliation
• Foliation – The repetition of planar surfaces or layers in a
metamorphic rock. Layers can be paper-thin or meters thick.
– Happens because when rocks are subjected to differential stress, platy
minerals align or alternating light and dark layers form, giving the rock a
planar fabric, called foliation. Note that this is different than bedding.
Slate, a foliated metamorphic rock makes nice
roof shingles because its foliation creates
cleavage planes that easily break
Foliation and Compression Direction
• Slaty Cleavage forms perpendicular to the compression direction,
i.e. a horizontal squish will create vertical cleavage planes.
Compression also commonly results in folding.
Foliated Metamorphic Rocks
Distinguished based on grain size, composition,
and nature of foliation
• Slate – finest grained metamorphic rock.
Foliation occurs because of alignment of
chlorite grains. Protoliths of shale and
mudstone. Lots of slate mines in Vermont!
• Phyllite – Fine-grained, foliation occurs
because of alignment of mica and
occasionally chlorite. Translucent aligned
mica grains give it a silky or waxy sheen
called phyllitic luster. Phyllite forms when
shale/mudstone is metamorphosed at
temperatures high enough to cause
neocrystallization (mica and chlorite form).
Foliation is from aligned mica and chlorite
grains that align due to differential stress.
• Flattened Clast Conglomerate – (aka
metaconglomerate or stretched pebble
conglomerate) forms when conglomerates or
breccias get squished (plastic deformation)
and the once round pebbles become flattened.
Foliation defined by the squished pebbles.
Phyllite
Metaconglomerate
Foliated Metamorphic Rocks
• Schist – A medium to coarse grained
metamorphic rock that possesses
schistosity defined by the preferred
orientation of large mica grains
(biotite/muscovite) as a result of
differential stress. Forms at higher
temps than phyllite and has larger
grains. Also contains a range of
different minerals depending on
composition of the protolith. Can
have a wide range of protoliths so
long as the protolith contains the
elements necessary to make mica.
• Gneiss – a medium to coarse grained
compositionally layered [gneissic
banding] metamorphic rock
consisting of alternating light [felsic]
and dark [mafic] layers.
Schist
Phyllite
Gneiss
Metaconglomerate
How Does Gneissic Banding Form?
• During shearing and compression, minerals become stretched
(plastic deformation) all while recrystallization and
neocrystallization is taking place
– Analogy: think of stirring chocolate fudge into ice cream. The fudge starts
out in a blob but gets deformed and smeared during stirring.
How Does Gneissic Banding Form?
• During shearing and compression, minerals become stretched
(plastic deformation) all while recrystallization and
neocrystallization is taking place
– Analogy: think of stirring chocolate fudge into ice cream. The fudge starts
out in a blob but gets deformed and smeared during stirring.
How Does Gneissic Banding Form?
• During shearing and compression, minerals become stretched
(plastic deformation) all while recrystallization and
neocrystallization is taking place
– Analogy: think of stirring chocolate fudge into ice cream. The fudge starts
out in a blob but gets deformed and smeared during stirring.
How Does Gneissic Banding Form?
• During shearing and compression, minerals become stretched
(plastic deformation) all while recrystallization and
neocrystallization is taking place
– Analogy: think of stirring chocolate fudge into ice cream. The fudge starts
out in a blob but gets deformed and smeared during stirring.
Foliated Metamorphic Rocks
• Migmatite – combination of partially melted metamorphic rocks and igneous
rocks. Typically, a gneiss when subjected to hydrothermal fluids will attain a
lower melting point and begin to partially melt. The felsic stuff melts forming a
new felsic igneous rock surrounded by metamorphic gneiss that is more mafic.
The resulting mixture of these two rock types is called a migmatite and has a
marble cake kind of look.
Migmatite
Nonfoliated Metamorphic Rocks
Metamorphic rocks that have recrystallized and/or neocrystallized but do not
typically have a foliation (usually because grains are not sufficiently elongated).
Distinguished based on composition, but may be foliated if subjected to
significant differential stress
• Hornfels – Rock that undergoes heating in the absence of significant differential
stress. Typically hornfels form when rocks are baked by igneous intrusions
(contact metamorphism). No foliation is present because crystals grow in random
orientations due to a lack of significant differential stress. Composition varies and
depends on composition of protolith.
• Amphibolite – Metamorphosed
mafic rock (basalt, gabbro) can’t
form felsic minerals, so they tend
to form amphibolites, which are
dominantly made of visible
crystals of hornblende and
plagioclase (Ca-feldspar). Can
often be foliated.
Amphibolite
Nonfoliated
Metamorphic Rocks
Quartzite
• Quartzite – Metamorphosed quartz
sandstone with larger interlocking
quartz crystals. Matrix material and
pore space is eliminated. Sandstone
looks grainy, Quartzite looks glassier
or more crystalline.
• Marble – limestone and other
carbonate rocks recrystallize into
interlocking grains of calcite. Pore
space and much of the original grain
form is destroyed. Impurities may
form compositional bands
occasionally because marble flows at
relatively low temperatures.
Marble
Metamorphic Compositions
• Occasionally, for simplicity, geologists will simply refer to the
composition of a metamorphic rock
– Mafic (or Basic) Metamorphic Rock – lots of mafic minerals
– Calcareous Metamorphic Rock – Calcite-bearing protoliths (limestone)
– Quartzo-Feldspathic (i.e. felsic) Metamorphic Rocks – form from
protoliths than contain a lot of feldspar and quartz (e.g. granite, diorite)
• Occasionally geologists will simply refer to metamorphic rocks by
their protolith
– Metasedimentary rock
– Metaigneous rock
– Pelitic Metamorphic Rock
Sedimentary protolith
Metaconglomerate is a metasedimentary rock
Metamorphic Grade
• Not all metamorphism occurs under the same conditions, so geologists classify the
metamorphic grade, or specific set of conditions under which certain
metamorphic rocks form
• Metamorphic Facies – groups of metamorphic minerals that form under similar
temperature and pressure conditions.
• Low-Grade – rocks that form under low temperatures (200-320o C)
• Intermediate-Grade – rocks that form under temperatures (320-600o C)
• High-Grade – rocks that form above ~600o C.
Prograde & Retrograde Metamorphism
• Prograde Metamorphism – Metamorphism that occurs while
temp and pressure progressively increase. They form minerals
that are stable at higher temp and pressure. Neocrystallization
commonly releases water in the host rock, so high grade rocks
tend to be drier (little no OH-) than low grade rocks. So, schist
loses its schistosity at high grades and may form gneiss.
– Biotite: K(Mg,Fe)3AlSi3O10(F,OH)2
– Muscovite: KAl2(AlSi3O10)(F,OH)2
– Chlorite: (Fe, Mg, Al)6(Si, Al)4O10(OH)8
• Chlorite is common in retrograded rocks
• Retrograde Metamorphism – Metamorphism that occurs when
temp and pressure decreases. For metamorphic reactions to occur
in these conditions, water must be added to the rock
(hydrothermal fluids). Without water, high grade rocks cannot be
retrograded. This is why very old (billions of years) high grade
rocks are exposed at the surface of the Earth in certain places.
Metamorphic Grade: Graphical View
Shale: Diagenesis to High-Grade
Metamorphism
• A single protolith (shale shown below) can
form a variety of metamorphic rocks
depending on the grade of metamorphism
incurred after burial. Certain mineral
assemblages reflect the grade of
metamorphism
Metamorphic Facies
• A given P-T
horizon has a
characteristic set
of minerals that
form. Which ones
form depend on
protolith
composition
• If you know the
P-T conditions
and the protolith
composition, you
can predict the
mineralogy of the
resultant
metamorphic
rock
P-T Paths
• The metamorphic rocks that we see are now exposed at the surface of the
Earth, so we can describe the life of a metamorphic rock with a P-T path.
• The life cycle of a rock can be plotted on a pressure temperature plot.
These plots outline the P-T (pressure, temperature) history of a rock
Isograds – Zones of Similar P-T Conditions
• Since we can determine
the P-T conditions of a
metamorphic rock based
on its metamorphic
facies, we can map out
regions of similar
metamorphic grade.
• These regions are
separated by lines called
isograds, which are lines
that delineate the first
appearance of a mineral
from a new metamorphic
facies
Metamorphic Environments
• The geothermal gradient varies throughout various tectonic
environments, so it stands to reason that metamorphic processes
will vary depending on tectonic environment.
• In general, heat flow is HIGH in:
– Near magma bodies
– Rifts (rising magma)
– Young mountain belts (faults bring up warm rocks)
• Heat Flow is LOW in:
– Stable continents (called cratons).
Burial Metamorphism
• As sediments are buried in a sedimentary basin…
– P increases because of the weight of the overburden.
– T increases because of the geothermal gradient.
• Requires burial below diagenetic effects.
– This is ~ 8–15 km depending on the geothermal gradient.
Contact Metamorphism
• Heat flows from hotter to
colder materials, so when
a hot igneous intrusion
(magma) comes into
contact with cold country
rock, it creates a
metamorphic aureole /
contact aureole or baked
zone.
• This results in contact
metamorphism, whereby
rocks undergo
metamorphic reactions
due to heating (little or no
pressure change)
• Contact metamorphism
typically produces
hornfels, a nonfoliated
metamorphic rock.
Dynamic Metamorphism
• Although at shallow
depths faults break rock
and slide past each other,
in the asthenosphere,
rocks don’t break, rather
they flow past each other.
• Rock in the deep portions
of faults undergoes
dynamic metamorphism
and creates a fine-grained
metamorphic rock called a
mylonite
• Mylonites are thus found
at all plate boundaries at
depth.
Fault Breccia
• At shallow depths, faults break rocks into angular
pieces forming a rock called fault breccia
Mylonite
• An exposure where a
gneiss once met a
ductile shear zone.
• Same composition, but
grain size is greatly
reduced.
• Grain size change,
causes the color to
change.
Gneiss
Mylonite
Regional Metamorphism
• Mountain belts commonly produce a range of metamorphic rocks.
• When subduction eats up all available oceanic crust, collisional
orogens (mountain building events) happen.
• Mountains get eroded and expose the once deep metamorphic rocks
(e.g. most of the High Country)
Regional Metamorphism
• Regional metamorphism creates foliated rocks.
• This type of metamorphism is, by far, the most
important in terms of the amount of rock altered.
– Collisional belts are often…
• 1000’s of km long.
• 100’s of km wide.
Hydrothermal Metamorphism
• Alteration by hot, chemically aggressive water.
• A dominant process near mid-ocean ridge magma.
– Cold ocean water seeps into fractured crust.
– Heated by magma, this water then reacts with mafic rock.
– The hot water rises and is ejected via black smokers.
Subduction Metamorphism
• Subduction creates the unique blueschist facies.
• Trenches and accretionary prisms have…
– Low temperature (low geothermal gradient)
– High pressures
• High P & Low T favor
glaucophane, a blue
amphibole mineral.
Blueschist from Shell Beach, CA
• Meteor impacts can generate a large amount of heat from the
conversion of kinetic energy to heat
• This heat may melt or even vaporize rock
• Causes quartz to change phase to coesite or stishovite
(polymorphs)
• sometimes referred to as shocked quartz.
• Meteor Crater, AZ has abundant shocked quartz
• Chicxulub Crater also has shocked quartz!
Exhumation
• How do metamorphic rocks return to the surface?
• Exhumation is due to...
– Uplift – Compression squeezes deep rocks upward.
• Faults bring up deep rocks
– Erosional unroofing – Weathering and erosion removes vast
amounts of rock.
San Gabriel Mountains: Los Angeles, CA
Deep rocks brought up by the Sierra Madre Fault.
Where Can You Find Metamorphic Rocks Today?
• Shield – Older portions of the continental crust where large
amounts of metamorphic rock crop out at the surface of the Earth.
The Rock Cycle
• Once a rock is formed it may be changed into a new type
of rock by various processes…
• The Rock Cycle is a mass transport cycle that outlines
the progressive transformation of Earth materials from
one rock type to another.
• The Earth is affected by various cycles
– Temporal Cycle – time dependant, such as lunar cycles,
seasons, etc…
– Mass-Transfer Cycle – involving the transport of materials
Weathering
Granite
Metamorphism
Arkose
Gneiss