Download 2.4 1 Temperature, pressure and metamorphism

2.4
1
Key definition
Isochemical means that no elements
are added or removed, with the
exception of volatiles such as water
and carbon dioxide.
Temperature, pressure and
metamorphism
Metamorphism is the isochemical process by which rocks are changed by either heat or
pressure, or both heat and pressure. The chemical composition of the parent rock will be
the same as the metamorphic rock produced.
The rock undergoes the very slow process of solid-state recrystallisation without melting.
Different temperatures and pressures cause new minerals to grow in rocks that have the
same composition. The minerals produced are directly related to pressure and temperature
conditions. The lower temperature limit for metamorphism is between 200 and 150 oC.
Below these temperatures, changes are part of diagenesis. There is no lower pressure limit.
The upper temperature limit is where melting occurs. This happens at around 800 oC.
The process of metamorphism may result in:
• destruction of fossils, beds and sedimentary structures
• hardening of the rock
• change in colour
• alignment of minerals
• growth of new metamorphic minerals.
Temperature /�C
0
200
400
600
800
1000
0
Sedimentary
Case study
Making a brick – it’s metamorphism!
• Take a mass of soft, grey-coloured,
sticky, crushed clay mixed with
water and a little limestone.
• Stir well.
• Press into a brick shape.
• Put in the furnace at 1400 oC for 2
days.
• Cool slowly and you will have a hard,
red brick.
Low grade
Regional metamorphism
600
800
Igneous rocks
Medium grade
20
Depth / km
10
400
Burial
metamorphism
Pressure/MPa
200
Contact metamorphism
High grade
30
1000
Figure 1 Relationship between metamorphism, temperature and pressure
Temperature
Temperature is a key variable in metamorphism:
• High temperatures occur near to igneous intrusions, where the magma heats the
surrounding rocks.
• Temperature also increases with depth, due to the geothermal gradient.
As temperature increases, the rate of metamorphic reactions also increases. This is
because many of the chemical reactions require heat to take place. Higher temperatures
increase the rate at which ions diffuse between minerals, though it is still a slow
process because the ions have to move through solid rock during metamorphism. The
whole process is greatly speeded up by water, which allows the ions to diffuse more
rapidly.
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Module 4
Metamorphic Processes and Products
Pressure
Pressure steadily increases with depth and is applied to rocks in three different ways:
• Pore pressure is the pressure exerted by fluids between the grains in a porous rock.
The presence of water speeds up reactions by acting as a catalyst and increasing the
rate and ease of ion exchange. In an experiment where two dry solids were heated
together for 2 hours at 1300 oC, only 10% reacted. When the same solids were heated
in water for the same time, the reaction was completed at only 600 oC.
• Load pressure is the weight of overlying rocks and physically brings minerals into
contact with each other over very long periods of time.
• Tectonic stress or pressure is caused as the rocks undergo folding or faulting and very
high pressures are exerted, but usually over relatively short periods of time.
Temperature, pressure and
metamorphism
In all cases the higher the pressure, the greater the degree of metamorphism. Reactions
that depend on pressure only are less common than temperature dependent reactions.
Time
Time is very important because metamorphic reactions take place very slowly. These
reactions usually take millions of years to occur. Pressure and temperature conditions
that produce metamorphism have to exist over long periods of time, in order for the
reactions to occur.
Types of metamorphism
Contact metamorphism
Contact metamorphism occurs adjacent to igneous intrusions, which increase the
temperature in the surrounding country rock. The metamorphism is important on a local
scale, producing a metamorphic aureole. Temperatures are generally high but pressure is
low. As pressure is not a significant factor, the minerals are not aligned in contact
metamorphic rocks.
Key definition
Country rock is the rock into which an
igneous rock has been intruded.
Burial metamorphism
Burial metamorphism occurs in conditions of medium to high pressure and relatively low
temperature. To some extent, burial metamorphism overlaps with diagenesis, and grades
into regional metamorphism as temperature increases. It affects rocks deeply buried by
the weight of overlying sediments. It also occurs at subduction zones where sea floor
sediments and basalts are buried. Rocks buried at the deepest levels almost always
contain the blue mineral glaucophane. They are called blueschists.
Regional metamorphism
Regional metamorphism affects larger areas than contact metamorphism, extending over
hundreds or thousands of square kilometres. It is caused by low to high temperature and
low to high pressure at convergent plate margins. It can result from either subduction or
continental collision. Pressure is significant and so minerals have a preferred alignment.
Regionally metamorphosed rocks occur in the cores of fold mountain belts where
mountain ranges have been eroded.
Questions
1 Describe the temperature and pressure conditions associated with each of the three
types of metamorphism.
2 Describe the effects on the surrounding rocks of an igneous intrusion that cooled in
500 years.
3 Explain why it is more difficult to know the pressure at which metamorphic rocks form
than the temperature at which they form.
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2
Identifying regional metamorphic
rocks
All metamorphic rocks are formed from parent rocks, the original rocks that existed prior
to metamorphism. The composition of these rocks affects the mineralogy of the
metamorphic rocks, so they have a kind of ‘family likeness’.
Foliated rocks produced by regional metamorphism
Key definitions
Foliation is the texture in metamorphic
rocks, formed by the preferred
alignment of flat, platy minerals.
Slaty cleavage is the texture in fine
grained rocks formed by low grade
regional metamorphism. Platy minerals
recrystallise perpendicular to the
direction of stress applied during
metamorphism so that the rock splits
into thin sheets.
Figure 1 Shale and slate
These rocks have all been affected by pressure, to some degree, during regional
metamorphism. Any platy minerals they contain take on a preferred alignment known as
foliation. All the rocks described in this spread are foliated. (For an explanation of how
foliated textures form, see spread 2.4.4.) The most common platy mineral is clay so the
rocks described below all have shale as the parent rock.
Slate
The parent rock of slate is shale (for a photo of shale, see spread 2.3.6). Shale is
composed of clay minerals and fine quartz particles. Because clay minerals are rich in
aluminium, so are the metamorphic minerals in slate. This is mainly composed of clay
minerals and mica (although chlorite and quartz may also be present). Shale is fine
grained (grains <1 mm diameter) and shows slaty cleavage. Traces of original bedding
may still be preserved as relict bedding.
Alignment of
minerals has
changed due to
pressure during
metamorphism
Magnification
x50
Shale, composed of clay
minerals aligned by compaction
during diagenesis
Key definitions
A porphyroblast is a large crystal that
has grown during recrystallisation in a
metamorphic rock and is surrounded by
a finer grained groundmass of other
crystals.
Schistosity is the texture in medium
and coarse grained metamorphic rocks
formed by the preferred alignment of
flat/tabular minerals. The alignment is
perpendicular to the direction of stress
applied during metamorphism. No
traces of original bedding remain.
Gneissose banding is the segregation
of light and dark coloured minerals into
layers or bands at the scale of mm to
cm in thickness. The light band is
normally granoblastic (granular) and the
dark band normally shows schistosity.
Slate, composed of clay minerals
and platy metamorphic minerals
mica and chlorite
Slate is a fine grained metamorphic
rock that has slaty cleavage i.e. it
splits into thin sheets
Schist
The parent rock of schist is shale. Schist is produced by higher temperatures and
pressures than those producing slate. It is medium grained (1 to 5 mm) and crystalline.
Although it can occur in a variety of colours, it always has a shiny appearance where the
flat surfaces of muscovite and biotite mica crystals are visible. Schist is typically
composed of mica and garnet. The garnets often form large crystals called
porphyroblasts. The mica crystals are all aligned at right-angles to the maximum
pressure, forming the texture schistosity.
Gneiss
The parent rock of gneiss is shale. Gneiss is formed by the highest temperatures and
pressures during regional metamorphism. It is a coarse grained (>5 mm), crystalline rock
with gneissose banding. Gneiss is typically composed of quartz and feldspar in the light
bands and biotite mica (and other mafic minerals) in the dark bands.
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Module 4
Metamorphic Processes and Products
Thin section
drawing of
schist showing
garnet
porphyroblasts
Thin section
(black) and
drawing of
micas (brown)
Garnet
schist showing
in preferred
porphyroblasts
garnet
Light coloured
alignment
porphyroblasts band
(black) and
micas (brown)
Garnet
in preferred
porphyroblasts
alignment
Mica crystals give
Identifying regional metamorphic
rocks
Feldspar showing two
cleavage directions at
right angles
Quartz
the surface a sheen
0
Dark coloured
band
2mm
0
Mica crystals give
Garnet – mica
the schist
surface a sheen
0 – mica
2mm
Garnet
schist
Thin section drawing of gneiss
Figure 2 Schist
2mm
Biotite mica is dark and
shows preferred orientation
Feldspar showing two
cleavage directions at
right angles
Light coloured
band
Quartz
Quartz rich
band
Dark coloured
band
Biotite mica is dark and
shows preferred orientation
0
Band rich in
biotite mica
2mm
Thin section drawing of gneiss
Gneiss showing gneissose banding
Figure 3 Gneiss
Summary table of rocks produced by regional metamorphism only
Parent Rock
Metamorphic
Colour
Texture
Mineral
composition
Type of
metamorphism
Shale,
composed
of clay
minerals
Slate
Grey or
purple or
green or
black
Slaty
cleavage, fine
grain size
(<1 mm)
Clay minerals and
muscovite mica,
with some chlorite
and quartz
Low grade
regional
Shale
Schist
Quartz rich
rock
band
Band rich in
biotite mica Muscovite and
Silvery
Schistosity,
Gneiss showing
banding
biotite mica
sheen gneissosemedium
grain size
Quartz
(1–5 mm)
Garnet
Medium grade
regional
Kyanite
Shale
Gneiss
Dark and
light bands
Gneissose
banding,
coarse grain
size
(>5 mm)
Biotite mica
Mafic minerals
Quartz
K feldspar
Sillimanite
High grade
regional
Questions
1 Which metamorphic rock would
you associate with high grade
regional metamorphism?
2 With the aid of labelled
diagrams, describe the
differences between schist and
slate.
3 With the aid of labelled
diagrams, describe one similarity
and one difference between
schist and gneiss.
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2.4
3
Identifying metamorphic rocks
The parent rock, for all the metamorphic rocks described in spread 2.4.2, was shale but
this does not mean that shale is the only parent rock. The chemical composition of the
minerals that make up shale is more varied than that of limestones, which are composed
of calcite (CaCO3), or of sandstones, some of which can be almost pure SiO2 in the form
of quartz. There is a much wider range of metamorphic minerals that can recrystallise
from clay minerals. This means that shale can be the parent of different metamorphic
rocks, but limestone and sandstone cannot.
Unfoliated rocks produced by contact or regional
metamorphism
Oolitic limestone – parent
Orthoquartzite – parent
Fossiliferous limestone – parent
Calcite
Quartz grain
Concentric
layers of
calcite
Quartz cement
Quartzite
Calcite
Marble
Nucleus
1mm
Fossil
fragments
0.5 mm
1mm
Interlocking
quartz crystals
1 mm
Figure 1 Thin section drawings of quartzite, marble and their parent rocks
Because these rocks do not contain platy minerals such as mica they do not show
foliation. They can be produced by either regional or contact metamorphism.
Quartzite
Key definition
Granoblastic describes the texture of
metamorphic rocks that contain
interlocking equi-dimensional, crystals.
The parent rock of quartzite is orthoquartzite, sandstone composed of quartz grains held
together by quartz cement. Quartz grains in the sandstone recrystallise, forming
interlocking quartz crystals. The quartz crystals are equidimensional so there can be no
foliation. This texture is described as granoblastic. Any sedimentary structures or fossils
in the parent sandstone are destroyed. The colour of quartzite is white or grey, unless
there were other minerals in the original rock. For example, if any iron oxide was in the
parent rock, there will often be a pink colour.
Marble
Limestones are made essentially of one mineral, calcite, which is stable over a wide
range of temperatures and pressures. As a result, metamorphism of limestone only
causes the original calcite crystals to grow larger. Calcite grains and fossil fragments in
the limestone parent rock recrystallise to form an interlocking mosaic of calcite crystals.
The crystals are equi-dimensional, so there can be no foliation. Marble has granoblastic
texture, but the crystals of calcite make it look sugary in texture. Calcite will react with
dilute HCl. Fossils are destroyed during metamorphism.
Marble from pure limestone is white. Impurities in the parent limestone give some marble
a range of coloured streaks:
• If there are clay minerals in the limestones, then a number of green or red minerals
such as garnet may form.
• If there are sand grains present, a chemical reaction between calcite and quartz
will produce wollastonite, which can be light green, pinkish, brown, red or yellow.
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Module 4
Metamorphic Processes and Products
Rocks produced by contact metamorphism
Identifying metamorphic rocks
Spotted rock
Because contact metamorphism involves only increased temperatures, it cannot produce
foliation. During contact metamorphism, spots may form in the rock where the heat has
only partially recrystallised the rock. A spotted rock contains the same minerals as shale
or slate. If slate is the parent rock of spotted slate, it will show foliation, but this was
produced due to pressure during regional metamorphism (see spread 2.4.2). The
randomly orientated spots may contain biotite, andalusite and graphite, but they are
usually too indistinct to be identified in hand specimen.
Alignment of micas and
clay minerals
Relict
bedding
Dark
spots
Summary table of unfoliated rocks
Parent rock
Metamorphic
rock
Colour
Texture
Mineral
composition
Type of
metamorphism
Limestone
composed of
calcite
(CaCO3)
Marble
White
Granoblastic
Medium grain
size (1–5 mm)
grain size
increases with
metamorphic
grade
Calcite
(reacts with
dilute HCl)
Contact or
regional
Sandstone
composed of
quartz
(SiO2)
Quartzite
White or grey
Granoblastic
Medium grain
size (1–5
mm), grain size
increases with
metamorphic
grade
Quartz
Contact or
regional
Slate or shale
composed
of some clay
minerals, mica
and quartz
Spotted rock
Grey or
purple or
green or
black with
darker spots
Slaty cleavage if
slate parent rock
Fine grain size
(<1 mm)
Clay minerals
and mica
Poorly formed
minerals (mica,
andalusite,
graphite) in
spots
Contact
Minerals have
preferred alignment
Yes
Medium to
coarse grained
Fine grained
Mica prominent.
May contain garnet
Figure 2 Thin section diagram of spotted
slate showing clay minerals and mica
aligned at 90º to maximum pressure. The
remains of the sedimentary bedding can
just be seen as relict bedding
No
Medium to
coarse grained
Contains dark
and light bands
�10
Composed
of quartz
Fine grained
Composed
of calcite
Contains
dark spots
Questions
Slate
Key
Schist
Produced by regional
metamorphism
Figure 3 Identifying metamorphic rocks
Gneiss
Quartzite
Produced by regional or
contact metamorphism
Marble
Spotted rock
Produced by contact
metamorphism
1 Explain why marble is not always
white.
2 Explain why limestone and
sandstone produce the same
metamorphic rocks in both
contact and thermal
metamorphism.
3 State two pieces of evidence
indicating that quartzite is a
metamorphic rock.
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4
Metamorphic textures
Metamorphic rocks are classified mainly based on their texture. This is because grain
size and orientation tell us a lot about the conditions of metamorphism.
If rocks are subjected to directed pressure, a preferred orientation of the minerals
develops at 90 degrees to the pressure. If the minerals are flat or platy, foliation is
produced (see spread 2.4.2). Because it results from pressure, foliation is a characteristic
of rocks formed by regional metamorphism.
Slaty cleavage
Sandstone no cleavage
Rocks with slaty cleavage will split into thin sheets along the
cleavage planes. It occurs in fine grained rocks formed by
low grade regional metamorphism:
• It can only form in rocks consisting of platy minerals such
as clay minerals, chlorite and micas.
• At the microscopic scale, these minerals become aligned
at 90 degrees to the direction of maximum pressure during
metamorphism.
• Slaty cleavage may be at any angle to bedding, but is
usually parallel to axial planes of the folds.
• It cannot occur in rocks with rounded grains, such as
quartz in sandstones.
Compression
Shale beds
Cleavage planes
Sandstone no cleavage
Foliation produced by the alignment of flat minerals e.g. mica
Direction of maximum
stress during
metamorphism
Direction of
maximum stress
during metamorphism
Flat minerals like mica
align so that their long
axis is at 90° to the
direction of pressure
Schistosity
Slaty cleavage
Relict bedding is at a different angle
from the cleavage
Slaty cleavage
developed at 90°
to maximum
stress
Figure 1 Foliation and slaty cleavage
Bedding and fossils may not be completely destroyed by
metamorphism, leaving traces or relict structures. Fossils
may be deformed due to the high levels of compressive
stress. Slates are common in North Wales and the Lake
District.
Found in schists (medium grained rocks formed by regional
metamorphism), schistosity results from the alignment of flat,
platy minerals, commonly muscovite mica, at 90 degrees to
the direction of maximum pressure during metamorphism.
Light coloured muscovite mica is concentrated into thin
parallel bands, giving the rock a characteristic shiny
appearance (micaceous sheen) where flat surfaces of mica
are visible.
Garnet porphyroblasts are often present and they disrupt the
alignment of mica minerals.
Schists are found in the Highlands of Scotland in Dalradian rocks.
This rock shows porphyroblastic texture in a schist
Key definitions
A relict structure is a structure such as
bedding present in the parent rock,
which is partially preserved in a
metamorphic rock.
Dalradian is the name of a group of
rocks formed in late Precambrian times,
found in Scotland.
Schistosity
caused by
alignment
of micas
Garnet
porphyroblasts
Figure 2 Porphyroblastic texture in a schist
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Module 4
Metamorphic Processes and Products
Gneissose banding
Metamorphic textures
Gneiss – dark and light bands
Found in gneisses (coarse grained
rocks formed by regional
metamorphism), gneissose banding
is formed when light (usually quartz
and feldspar) and dark coloured
minerals (usually biotite mica and
mafic minerals) are separated into
bands. The mica-rich layer is foliated
and the pale layer has granoblastic
texture. The bands may be contorted
or folded but are roughly at 90
degrees to the maximum pressure
direction (for a thin section drawing
and photo showing gneissose
banding, see spread 2.4.2).
Key definitions
An inclusion is a fragment of an early
formed mineral enclosed by one that
grew later.
Thin section drawing of a garnet porphyroblast
Unfoliated describes the random
orientation of minerals in a
metamorphic rock.
Garnet porphyroblasts
commonly have curved
cracks when seen in
thin section
Porphyroblastic texture
This texture occurs in both regional
Inclusions of early formed
and contact metamorphic rocks.
minerals enclosed by a
Porphyroblasts are large crystals that
garnet porphyroblast
grow during metamorphism and are
2mm
surrounded by a finer grained
groundmass. Metamorphic rocks that contain these large crystals are described as
porphyroblastic. Garnet porphyroblasts found in schists may contain inclusions. Pyrite
porphyroblasts can develop in slate, often forming clear cubic crystals.
Figure 3 Gneissose banding
Examiner tip
Figure 4 Pyrite porphyroblast in slate
Granoblastic texture
This is an unfoliated texture and is formed by thermal metamorphism. Pressure is not a
factor in the formation of a granoblastic texture. The main characteristics are randomly
orientated, equidimensional crystals usually in rocks with few, and sometimes only one,
mineral. Hornfels is an example of a fine grained rock with granoblastic texture. Marble
and quartzite are also granoblastic. Because of their medium grain size and white colour,
their texture is sometimes described as sugary.
Granoblastic texture
Equidimensional crystals
with no preferred alignment
Figure 5 Granoblastic texture
How to tell the difference between the
igneous texture porphyritic and the
metamorphic texture porphyroblastic:
• porphyritic is where large crystals –
phenocrysts – form first in the
magma so grow larger than the
groundmass, which cools later.
• porphyroblastic is where large
crystals such as garnet grow after
the groundmass has developed and
they may distort the groundmass
crystals.
Questions
1 Name two metamorphic rocks
that are unfoliated.
2 With the aid of labelled diagrams
explain how a garnet
porphyroblast affects the
alignment of micas in a schist.
3 Explain why slaty cleavage
commonly has a different
orientation from relict bedding.
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2.4
5
Key definitions
A metamorphic aureole is a region
surrounding an igneous intrusion in
which the country rocks have been
recrystallised and changed by heat from
the intrusion.
A metamorphic grade is a measure of
the intensity of metamorphism. Although
increases in temperature only result in
increasing grade in contact
metamorphism, grade is also used to
describe regional metamorphism where
both temperature and pressure vary.
Andalusite porphyroblasts
Contact metamorphism 1
Contact metamorphism occurs when the country rock is affected by heat from a large
igneous intrusion. Because temperature differences between the surrounding rock and
the intruded magma are greater at shallow levels in the Earth’s crust where pressure is
low, contact metamorphism is described as high temperature, low pressure
metamorphism. High temperature, not pressure leads to the formation of altered,
recrystallised, unfoliated rocks in a zone surrounding the intrusion. This zone is the
metamorphic aureole. Around a large igneous intrusion, such as a batholith, the
metamorphic aureole may be up to 10 km wide. Temperature decreases with distance
from the contact with the intrusion and for this reason the effects of contact
metamorphism are greatest near to the contact and decrease with distance.
Metamorphic grade increases in all directions towards the intrusion.
Contact metamorphism of shale
The chemical composition of minerals in shale is varied and so a range of different
metamorphic rocks is formed, depending on the temperature and therefore the distance
away from the intrusion:
• Close to the contact with the intrusion, temperatures are high and so high grade
metamorphism occurs. Shale is completely recrystallised to form a fine grained, hard,
splintery, granoblastic metamorphic rock called hornfels.
• Further away from the contact, where the heat is less intense, medium grade
metamorphism occurs. Clusters of a new metamorphic mineral andalusite, form
porphyroblasts. This partly recrystallised rock is andalusite slate or rock.
• In the outer part of the metamorphic aureole, temperatures are lower. Some
recrystallisation occurs, causing clusters of dark minerals to grow in separate spots.
Iron, carbon or biotite mica will form the spots. The rock in this outer part of the
metamorphic aureole is called spotted rock and is formed by low grade
metamorphism.
A metamorphic aureole showing contact metamorphism of shale
Edge of
metamorphic
aureole
Shale with spots of partial recrystallisation
Shale country
rock
Granite
Hornfels
0
1
cm
Cross-section of
andalusite crystal
Andalusite
crystal
Andalusite
slate
Spotted rock
1cm
Black spots of iron or carbon
Figure 1 Metamorphic aureole showing
contact metamorphism of shale and
photo of a spotted rock and andalusite
rock
Factors controlling the width of metamorphic aureoles
Volume of the magma
The size of intrusions ranges from batholiths down to minor intrusions (see spread 2.2.9).
Dykes and sills are not large enough and do not produce enough heat to develop a
metamorphic aureole. Because the volume of magma is small it cools quickly and there
is only sufficient heat to change the rock for a few centimetres on either side. This narrow
zone of bleached and hardened rock is known as a baked margin.
Larger intrusions cool slowly and heat the surrounding rocks over long periods of time
(104–106 years), allowing a wide metamorphic aureole to develop.
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Module 4
Metamorphic Processes and Products
Contact metamorphism 1
Temperature of the magma
Composition of the magma
Mafic magma may be intruded at a temperature of 1200 °C, whilst silicic magma may be
intruded at 850 °C. Silicic magmas contain more volatiles. When they enter the country
rock they speed up metamorphic reactions. This compensates for the lower temperature of
the magma, because metamorphic aureoles surrounding silicic intrusions are of similar size
to those around mafic ones.
700
400
300
200
0
Rocks largely composed of one mineral, such as limestone and orthoquartzite, show much
less variation than clay-rich rocks such as shale. Quartzite and marble have larger crystals
the nearer they are to the igneous intrusion and are uniform. Metamorphic aureoles formed
in sandstone country rocks are typically narrower than those formed in clay-rich rocks. If
the country rock is permeable and contains groundwater, heat will be able to move by
convection, allowing a wider aureole to develop.
0
X
N
Key
5°
Granite
Sandstone
Conglomerate
Quartzite
Shale
Spotted rock
Limestone
Marble
X
Y
Temperature change in intrusions
of different compositions
1000
800
600
400
200
0
1
2
Distance from contact/km
Gabbro
Diorite
Granite
3
Temperature of intrusion Basic 1200°C;
Intermediate 900°C; Acid 800°C;
All intrusions are 5km wide
Figure 2 Graph showing the effects of
temperature and composition of magma
Metamorphic aureole
5°
5
1200
0
Gently dipping contact
produces wider aureole
1
2
3
4
Distance from contact/km
Granite 10km diameter
Granite 5km diameter
Granite 1km diameter
1400
Temperature/�C
The dip of the sides of the intrusion has a major effect on the width of the metamorphic
aureole. A shallow angle of dip gives a wide aureole and a steep angle of dip gives a
narrow aureole. If the sides of the intrusion dip at different angles, then the metamorphic
aureole will be asymmetric.
500
100
Composition of the country rock
Dip of the contact
Decrease in temperature with distance
from intrusions of different size
600
Temperature/�C
The volume of magma in an intrusion affects the maximum temperature reached at any
point and also the time it takes for temperatures to rise in the country rocks.
Metamorphism will not occur unless the temperature rises above 200 °C for an extended
period of time. A small intrusion produces little metamorphic change because the rock
has little time to warm up and there is not enough time for metamorphic reactions to
occur before the rock cools down. With larger intrusions there is time for metamorphic
reactions to take place and for new minerals and recrystallisation to occur, because
temperatures remain high for much longer periods of time.
Y
1 km
Steeply dipping contact
produces narrow aureole
Figure 3 Map of an intrusion with dipping sides
Questions
1 What is the term for the zone surrounding a granite batholith?
2 Hornfels forms at 460 ° C. Using Figure 3, state how far away from each intrusion
hornfels will form.
3 Explain the relationship between metamorphic rocks and metamorphic grade.
147
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2.4
6
Contact metamorphism 2
The thermal gradient and index minerals in a metamorphic
aureole
Unaltered
country rock
Increasing temperature
Granite High Medium Low
batholith grade grade grade
Sillimanite
appears
Shale
Andalusite Biotite
appears appears
Figure 1 Sketch map showing index
minerals and metamorphic grade
Case study
The Skiddaw granite is part of a major
intrusion in the English Lake District.
The intrusion is an oval dome shape
measuring 10 km × 6 km, with a wide
metamorphic aureole.
The zones around the granite are:
• Unmetamorphosed country rock –
Skiddaw slates are fine grained
parent rocks showing slaty cleavage,
formed by regional metamorphism
before the intrusion.
• Outer zone of spotted slate – where
the grain size is slightly coarser than
in the country rocks and small round
dark spots are visible. The spots
contain biotite and organic material.
• Middle zone – Andalusite slate is
medium grained and generally
crystalline, containing andalusite
porphyroblasts.
• Close to the intrusion – the parent
slate has been completely
recrystallised to hornfels that can
contain sillimonite.
Some of the minerals that
7 crystallise at low grades are stable at higher grades, so more
than one index mineral 7can be found in one rock.
Pressure/kbar
Pressure/kbar
Pressure/kbar
Pressure/kbar
Metamorphic aureole
Increasing metamorphic
grade
When a batholith is intruded into beds of shale, increases in metamorphic grade are
marked by the appearance of an index mineral:
• Index minerals are metamorphic minerals, which are stable under specific temperature
and pressure conditions. They indicate the metamorphic grade.
• In contact metamorphism, biotite is the low grade mineral found in spotted rocks.
• The Al2SiO5 polymorph andalusite indicates medium grade and is found in andalusite
rich rocks.
• Sillimanite, another Al2SiO5 polymorph, indicates high grade and is found in hornfels.
• Because contact metamorphism is caused by temperature only, an increase in grade
represents a thermal gradient.
6
7
76
5
6
65
4
5
54
3
4
43
2
3
32
1
2
21
Triple point
Triple point
KYANITE
point
KYANITETriple
Triple point
SILLIMANITE
SILLIMANITE
KYANITE
KYANITE
SILLIMANITE
SILLIMANITE
ANDALUSITE
ANDALUSITE
0
ANDALUSITE
1
ANDALUSITE
10 0
100 200 300 400 500 600 700 800
0
100 200 300Temperature/�C
400 500 600 700 800
0 Temperature and pressure
fields for the Al2SiO5 polymorphs
Temperature/�C
0
0Temperature
100 200
300 400
800
and pressure
fields 500
for the 600
Al2SiO5700
polymorphs
0
100 200
300 400
500
600
700
800
MetamorphicTemperature/�C
path for contact metamorphism
Temperature/�C
Temperature
and pressure
for the metamorphism
Al SiO polymorphs
Metamorphic
pathfields
for contact
Temperature and pressure fields for the Al22SiO55 polymorphs
Andalusite slate
Metamorphic path for contact metamorphism
Andalusite slate
Metamorphic path for contact metamorphism
Relict bedding
Andalusite slate
Relict bedding
Andalusite slate
Dark grey fine
Relict bedding
grained
Dark
greyrock
fine
Relict bedding Crystals of andalusite
grained rock
Crystals of andalusite
Dark
grey
fine
Slaty cleavage
Dark grey fine
grained rock
Slaty cleavage
Crystals of andalusite
grained rock
Crystals of andalusite
Slaty cleavage
Slaty cleavage
0
0
1
1
cm
0 cm 1
0
1
cm
cm
Thin section
drawing
of andalusite
Thin section
crystals
drawing of andalusite
Thin
section
crystals
Thin section
drawing of andalusite
drawing of andalusite
crystals
crystals
Figure 2
Andalusite and sillimanite
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Module 4
Metamorphic Processes and Products
The Al2SiO5 polymorphs in contact metamorphism
Contact metamorphism 2
The Al2SiO5 polymorphs andalusite and sillimanite are found in contact aureoles –
andalusite is the low to medium temperature, low pressure polymorph found in andalusite
slate, whereas sillimanite is the high temperature polymorph found in hornfels. With
increasing metamorphic grade, contact metamorphism follows a path from andalusite to
sillimanite on the Al2SiO5 polymorph phase diagram. Kyanite, the high pressure, low
temperature polymorph, is not found in contact metamorphic rocks due to the lack of
pressure.
Key definition
A polymorph is a mineral that has the
same composition but occurs in
different crystal forms.
Formation of quartzite and marble
When orthoquartzite, a sandstone composed entirely of quartz, is affected by contact
metamorphism, all sedimentary structures including cross bedding and graded bedding
are destroyed. The quartz grains in the sandstone recrystallise to form an interlocking
mosaic of crystals giving it a granoblastic texture. Near to the contact with the igneous
intrusion, in the zone of high grade metamorphism, the crystals are larger than they are
further away from the contact where temperatures are not as high. The resulting rock is
white or pale grey in colour and known as metaquartzite.
Where limestones are affected by contact metamorphism, all sedimentary structures and
fossils are destroyed. The grains and cement composed of calcite will recrystallise to
form an interlocking mosaic of crystals giving it a granoblastic or sugary texture. This
metamorphic rock is called marble. Crystals are larger near to the contact with the
igneous intrusion and smaller further away, due to the thermal gradient. If the parent
limestone is composed purely of calcite, the resulting metamorphic rock is white in
colour. Impurities in the limestone may give streaks of different colours in the marble.
Examiner tip
Make sure that you know the products
of contact metamorphism. Do not write
about slate (unless it is spotted slate or
andalusite slate formed when the
country rock was slate), schist and
gneiss, if you are answering a question
on contact metamorphism.
Edge of metamorphic aureole
Edge of metamorphic aureole
Coarse
marble
Coarse
Marble
marble
Edge
of metamorphic aureole
Marble
Coarse
marble
Igneous
intrusion
Limestone
Limestone
Marble
Limestone
Marble
Igneous
intrusion
Marble
Marble
Igneous
intrusion
Coarse
metaquartzite
Coarse
metaquartzite
Quartzite
Coarse
metaquartzite
Quartzite
Quartzite
Orthoquartzite
Orthoquartzite
Quartzite
Quartzite
100m
Orthoquartzite
Quartzite
m photos showing contact metamorphism of limestone and orthoquartzite
Figure 3 Sketch map100
and
Questions
100m
1 Explain why andalusite is not formed by the contact metamorphism of pure limestone.
2 Draw a cross-section through a metamorphic aureole and through shale country rock.
Label the rock types that would be present on your cross-section.
3 Explain why there are no relict structures in quartzite.
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2.4
7
Regional metamorphism
Most regional metamorphism is accompanied by deformation, so these metamorphic
rocks will have foliated textures.
Regional metamorphism and plate tectonics
Regional metamorphism results from both heat and pressure generated at convergent
plate margins during subduction and continental collision.
The geothermal gradient and plate tectonics
• Along subduction zones magmas are generated, rise and intrude into the crust.
Temperatures are high near the surface result so the geothermal gradient may be in
the range of 50 to 70 °C/km, and contact metamorphism results.
• Compression occurs at a subduction zone where the oceanic crust starts to subduct
and the edge of the non-subducting plate is deformed. The geothermal gradient is
normal at 25 °C/km.
• Along a subduction zone, relatively cool oceanic lithosphere is pushed down to great
depths. This produces a low geothermal gradient of 10 to 15 °C/km.
Case study
Paired metamorphic belts include areas
in New Zealand, Indonesia, Washington
State in the United States, Chile, and the
coast of South America. All these areas
lie around the Pacific at convergent
plate margins, where subduction has
occurred.
Convergent plate margins with subduction zones
• When oceanic and continental plates collide, high pressure is produced as the oceanic
plate is subducted.
• The result is high pressure, low temperature burial metamorphism and the formation of
blueschists.
• Further away from the subduction zone, magma is rising from the melting oceanic
plate and pressures are lower, so high temperature, low pressure metamorphism
occurs.
• High temperatures lead to the formation of igneous intrusions and metamorphic
aureoles.
Paired metamorphic belts will form at convergent margins with subduction zones. The
zone closest to the trench will have high pressure due to compressive stress and low
temperature as no magma is rising. The zone further away has high temperature due to
rising magma and low pressure.
Convergent plate margins continental–continental
Case study
The Dalradian sedimentary rocks were
deposited in late Precambrian and
Cambrian times in an ancient ocean
called Iapetus, which existed between
Scotland and England. Continental–
continental plate movements caused
the ocean to close and the 13 km of
sediments that had been deposited in
the ocean were deformed and regionally
metamorphosed to form the Caledonian
orogenic belt. The area of
metamorphism extends both south and
north of the Great Glen Fault into the
Highlands of Scotland. The metamorphic
zones are displaced by the fault.
Fold mountains form at these margins (see spread 1.3.7) where the Earth’s crust is
deformed, thickened and there is extensive intrusive igneous activity. The Himalayan
mountain range began to form about 50 Ma when India collided with Asia. The
Himalayas are still growing as the plates are still moving towards each other. High
temperatures and pressures acting over such long periods create broad (>100 km2) and
often complex orogenic belts affected by all grades of regional metamorphism.
At the deepest part of the orogenic belt the pressures and temperature will be highest,
giving high grade regional metamorphism. Away from the collision zone and higher in the
crust the grade of metamorphism will be low.
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Module 4
Metamorphic Processes and Products
Regional metamorphism
Paired metamorphic belts in Japan
N
High pressure
low temperature
metamorphism
Honshu
High temperature
low pressure
metamorphism
Kyushu
Shikoku
High temperature
low pressure belt
Key definition
Low temperature
high pressure belt
Volcanoes
Trench
Sediments
Sea level
Crust
Moho
Thrusts
Moho
Oceanic crust
Rising
magma
Partial melting
of subducting
plate
Migmatite is a coarse grained mixed
rock with some of the characteristics of
gneiss and some of the characteristics
of granite, formed by partial melting of
the rock during the highest grade
metamorphism, at the high temperature
boundary between metamorphism and
igneous activity.
Asthenosphere
Lithosphere plate
Figure 1 Paired metamorphic belts in Japan
Figure 3 Migmatite
Grades of regional metamorphic
rocks
Regional metamorphism of orthoquartzite and
limestones produces the same products as
contact metamorphism – quartzite and marble.
Each of these rocks is composed of only one
mineral, quartz and calcite, respectively. The
minerals are equi-dimensional, so they cannot
align under pressure.
0
Temperature/°C
0 100 200 300 400 500 600 700 800
1
2
Pressure/kbar
Regional metamorphism of shale produces
the following rocks (they are all described in
spreads 2.4.2 and 2.4.3):
• low grade: slate
• medium grade: schist
• high grade: gneiss
Slate
3
4
5
6
7
8
Schist
Gneiss
Migmatite
Increasing metamorphic grade
Figure 2 Regional metamorphic rocks and
their relationship to pressure and temperature
Questions
1 Describe the formation of a
paired metamorphic belt.
2 Describe two general changes
that would occur in a mudstone
during regional metamorphism.
3 Draw up a table with the
following headings: metamorphic
grade; parent rock; metamorphic
rock; mineral composition;
texture. Complete it using the
information in this spread and
spreads 2.4.2 and 2.4.3.
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2.4
8
Regional metamorphic zones
Mapping the Dalradian Supergroup
Key definitions
An index mineral is a metamorphic
mineral that is stable over a particular
temperature and pressure range (e.g.
mica, garnet, Al2SiO5 polymorphs). They
indicate the metamorphic grade (see
spread 2.4.5 for a definition of
metamorphic grade). It is possible to
map metamorphic grade using index
minerals. The first appearance of the
index minerals is mapped as they may
still remain stable at higher
temperatures and pressures.
• In 1893, George Barrow mapped a sequence of highly deformed regionally
metamorphosed rocks in the south-eastern part of the Scottish Highlands. The
metamorphism and deformation occurred during closure of the Iapetus Ocean and the
Caledonian orogeny about 400 Ma ago. These Precambrian rocks are known as the
Dalradian Supergroup.
• As you already know, clay-rich sedimentary rocks such as shale produce a variety of
metamorphic minerals, as temperature and pressure conditions change. When Barrow
mapped rocks like these, he noticed that there was a pattern to the occurrence of
metamorphic minerals. He used the first appearance of some of these minerals, which
he termed index minerals, to draw isograds. Some of the minerals that crystallise at
low grades are stable at higher grades so more than one index mineral can be found
in one rock.
• He was able to map metamorphic zones using index minerals and isograds, which
define the boundaries of the zones. Although he did not do all the mapping personally,
the system he devised was named after him and the zones are called Barrovian zones.
Index minerals
An isograd is a line on a map joining
points of equal metamorphic grade.
They join places where the first
appearance of an index mineral occurs.
S = Sillimanite
K = Kyanite
G = Garnet
B = Biotite
C = Chlorite
A metamorphic zone is the area
between two isograds. The zone is
named after the lower grade isograd. All
locations within a metamorphic zone
experienced the same metamorphic
grade.
A Barrovian zone is a metamorphic
zone mapped using index minerals
identified by George Barrow.
North
C
K
G
B
G
S
K
Isograds
K
0
100 km
G
B
C
Increasing grade
Figure 1 Index minerals, isograds and metamorphic zones
Index minerals and metamorphic zones
Metamorphic grade
low
medium
high
Rock type
Slate
Schist
Gneiss
Index minerals and
metamorphic zones
Chlorite
Biotite
Garnet
Kyanite
Sillimanite
The chlorite zone represents low grade (low pressure and low temperature) regional
metamorphism. The rock is slate where most of the rock has recrystallised but some clay
minerals may still exist.
Schists develop as a result of increasing temperatures and pressures and can be found
in both the biotite and garnet zones. The grain sizes increase with metamorphic grade.
Schists formed at lower temperatures and pressures are composed of quartz, muscovite
mica and biotite mica. Medium grade metamorphism results from higher temperatures
and pressures and many schists formed at this grade contain garnet, and less
commonly, kyanite porphyroblasts.
Kyanite is typically found in gneisses and the kyanite zone represents high grade regional
metamorphism. The sillimanite zone represents high grade regional metamorphism with
very high temperatures and pressures. The rocks are gneisses. Estimates based on the
sillimanite zone indicate a maximum temperature of about 700 oC and maximum
152
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Module 4
Metamorphic Processes and Products
Regional metamorphic zones
pressure of about 7 kb. This pressure exists at a depth of about 25 km below the surface
of the continental crust. It gives a geothermal gradient of about 28 oC km–1.
Quartz and plagioclase feldspar are stable throughout the whole range of grades. This
makes them no use as index minerals.
?
?
?
Thru
st
?
?
ea
t
?
Gl
en
F
au
lt
ne
oi
M
Gr
?
?
7
?
?
6
H
d
lan
igh
ry
nd a
u
o
B
Pressure/kbar
5
lt
Fau
Triple point
4
SILLIMANITE
KYANITE
3
2
Dalradian metamorphic rocks
Barrovian zones
0
Chlorite
50 km
ANDALUSITE
1
0
Biotite
100
200
Garnet
400 500
Temperature/°C
600
700
800
Metamorphic path for regional metamorphism
Kyanite
Sillimanite
300
? Unknown
Figure 2 Regional metamorphic zones
Metamorphic path for contact metamorphism
Figure 3 Kyanite and sillimanite
The Al2SiO5 polymorphs in regional metamorphism
The Al2SiO5 polymorphs kyanite and sillimanite are found in regional metamorphic rocks.
A rock formed at high pressure and low temperature may contain kyanite. A rock formed
at high temperature or at high temperature and high pressure may contain sillimanite,
which can be found in contact and regional metamorphism, both of which can involve
high temperature. With increasing metamorphic grade, regional metamorphism follows a
path from kyanite to sillimanite on the Al2SiO5 polymorph phase diagram.
Questions
1 Describe the rocks found in each of the Barrovian zones.
2 Explain why clay-rich parent rocks are the most useful in mapping metamorphic
zones.
3 Explain the difference between a polymorph and a pseudomorph.
153
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Unit 2
ng
ri
Weathe
C
Erosion
ition
Depos
A
crystallisation
Rocks
Below is a diagram of the rock cycle.
EXTRUSIVE
1
Examination questions
B
Metamorphism
Metamorphic rocks
crystallisation
Igneous
MAGMA
D
Earth’s
surface
Metamorphism
(a) (i) Identify the three rocks A, B and C.
[3]
(ii) Describe with the aid of a sketch the term flow
banding.
[2]
(iii) Explain why igneous rock B has no crystals.
[1]
(iv) Define the term conchoidal fracture.
[1]
(b) Plagioclase feldspar, augite and hornblende are all part
of Bowen’s Reaction Series. They have been entered on
the reaction series diagram below.
D
Ca rich plagioclase feldspar
Augite
Products
Hornblende
Biotite
Partial melting
Processess
E
From mantle
F
Figure 1
G
(a) (i) Complete the table below using the diagram of the
rock cycle.
Location
Process or product
A
B
C
D
[4]
(ii) Name two processes that occur after deposition to
produce rock group D.
[2]
(b) Explain how the crystal grain size of igneous rocks is
related to the depth at which they crystallised.
[2]
(c) Explain the difference between an era and a system.
Give one example of each.
[2]
Total 8
(OCR 2832 May 06)
2
Na rich plagioclase feldspar
Descriptions of three igneous rocks are given in the table
below.
Description
• Flow banded
Rock
• Light grey or red or brown colour
A
• Very fine crystals <1 mm
• Conchoidal fracture
Rock
• Black colour
B
• No crystals
• Coarse crystal grain size
• Greenish black crystals of augite and
Rock
homblende
C
• White crystals of plagioclase fedspar
• White crystals of potash fedspar
Figure 2
(i) Name the minerals D, E, F and G from Bowen’s
Reaction Series.
[4]
(ii) Explain the relationship of Bowen’s Reaction
Series to temperature.
[2]
(iii) Name the minerals that form the discontinuous part
of Bowen’s Reaction Series.
[1]
(OCR 2835 June 06)
(c) The table below shows the chemical composition by
percentage of oxides of four igneous rocks H, J, K and L.
(i) To which igneous rock groups do H, J, K and L
belong?
[4]
(ii) Describe the changes in the % of oxides of silicon
and sodium compared to iron and magnesium
across the four rock groups.
[2]
Oxide %
A
B
C
D
SiO2
46.0
73.0
60.0
43.5
Al2O3
15.0
13.0
17.0
4.0
Fe oxides
12.0
2.0
6.0
12.5
MgO
9.0
0.5
3.5
34.0
CaO
9.0
1.5
7.0
3.5
Na2O
3.5
4.0
3.5
0.5
K2O
1.5
4.0
1.5
0.3
others
4.0
2.0
1.5
1.7
Total 20
(OCR 2835 June 06)
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Unit 2
Rocks
Practice questions
3 (a)The table below shows the results of a student’s research
into the world’s top 12 most deadly volcanic eruptions.
Rank
Volcano
Location
Year of
eruption
Death
Major cause of
death
 1
Tambora
Indonesia
1815
92 000
Ash fall, starvation
 2
Krakatau
Indonesia
1883
36 417
Ash fall, tsunami
 3
Mount Pelée
Martinique
1902
29 025
Pyroclastic flows
 4
Ruiz
Colombia
1985
25 000
Lahars
 5
Unzan
Japan
1792
14 300
Volcano collapse,
tsunami
 6
Laki
Iceland
1783
  9 350
Starvation
 7
Kelut
Indonesia
1919
  5 110
Lahars
 8
Galunggung
Italy
1882
  4 011
Lahars
 9
Vesuvius
Italy
1631
  3 500
Lava flows, lahars
10
Vesuvius
Indonesia
   79
  3 360
Ash falls,
pyroclastic flows
11
Pandayan
Indonesia
1772
  2 957
Pyroclastic flows
12
Lamington
Papua New
Guinea
1951
  2 942
Pyroclastic flows
(i)Explain why Indonesia has so many volcanic
eruptions.
[2]
(ii)Using the table calculate the percentage of
eruptions that had starvation as a major cause of
death. Show your working.
[2]
(iii)Suggest reasons why the global summer of 1816
was very cold.
[3]
Total 7
(OCR 2832 May 07)
4The graphic log below shows a commonly found sequence
of sedimentary rocks.
E
D
C
B
A
50 cm
ay l
Cl Si
f mc
t Sand
G
s
ule
n
ra
Figure 3
(a) (i)Using the graphic log, name and explain the
formation of the sedimentary structure shown by
the change in grain sizes in bed A.
[3]
(ii)Flute casts are found at the base of bed A. Draw a
labelled diagram of a flute cast. Explain how a flute
cast is formed.
[3]
(OCR 2835 June 06)
(b)The diagram below shows thin section drawings of two
metamorphic rocks and their sedimentary parent rocks.
Cement
0 1 2
mm
Quartz
cement
Quartz
Calcite
Rock L
Rock M
Rock N
Rock O
Figure 4
(i)Complete the sentences below by entering the
correct rock letters. Rock…………is the parent of
rock………….
Rock…………is the parent of rock…………. [2]
(ii) Describe how rock L forms.
[2]
(iii)Rock N has symmetrical ripple marks on the
bedding planes. Describe the environment in which
rock N was deposited.
[2]
(c)i)Describe one mechanical weathering process,
operating in a cold climate, that affects limestone.
[2]
(ii) State the shape of the scree fragments.
[1]
(iii)Describe one chemical weathering process that
affects the limestone.
[2]
Total 17
(OCR 2832 May 07)
5 (a)Describe how the following factors control
metamorphism.
(i) temperature
[2]
(ii) pressure
[2]
(b)Regional metamorphic rocks form as a result of changes
in both temperature and pressure.
(i)Name the rock type that is formed as a result of the
regional metamorphism of pure limestone and pure
sandstone.
[2]
(ii)Explain why shales give rise to a wide variety
of new metamorphic minerals when regionally
metamorphosed.
[2]
(iii)Define the following terms:
• index mineral
[1]
• isograd
[1]
(OCR 2835 June 06)
(c)Explain why rocks formed by contact metamorphism
lack any foliation.
[2]
Total 13
(OCR 2832 May 06)
6Using diagrams explain the differences between sills and
lava flows.
[8]
(OCR 2832 May 06)
7Describe with the aid of diagrams, the processes of
compaction and cementation.
[8]
(OCR 2832 May 07)
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