metamorphic rocks

1. Metamorphic Rocks

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Rock Cycle

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METAMORPHISM
 Meta- From the Greek meta meaning ‘with’ or ‘after’,
a prefix implying change and meaning ‘behind’ or
‘after’.
 Metamorphism describes the process of changing
the characteristics of a rock.
 The solid-state change in composition, mineralogy,
or texture of pre-existing rocks due to change in
temperature, pressure, or chemical components.
 Metamorphic rocks formed as a solid not a magma.
 If the rocks melt and form magma, upon cooling they
will be igneous rocks.

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Metamorphic Processes
 Original rock + changed P/T = new
metamorphic rock
 From change in pressure (P), temperature
(T), or both.
 Change in T and P  new texture, new
minerals.
 Recrystallization in solid state without
melting
 Rock composition stays about the same,
except for water loss.

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Metamorphic Processes cont…
Controls on metamorphism:
 Temperature: (up to 900º C in crust)
 Heat from sedimentary and tectonic
burial
 Heat from magma near intrusions
 Pressure: (from 0 to 20 kb at base of
crust, about 65 km down) from tectonic
loading and/or sedimentary burial
 Directed force: Plate collisions cause
deformation

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Role of plate tectonics
Where does most metamorphism
occur?
 Convergent plate boundaries!
 Why?

7. Metamorphism
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Temperature: (in crust up to ~750o C)
heat as burial depth increases
heat from hot magma near intrusions
Pressure with increasing depth
From tectonic loading or sedimentary
burial
Directed stress
Plate collisions cause deformation

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Mineral Changes
 Mineral assemblage of the original rock
changes because original minerals become
unstable
 Recrystallization in solid state without
melting
 Minerals with greater densities become stable
with more pressure
 Exchange of ions between minerals  new
minerals
 Clay minerals  muscovite, chlorite, epidote,
garnet, staurolite, kyanite, sillimanite

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garnet
kyanite
staurolite
Metamorphic
Minerals

10. Changes in minerals:
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Sedimentary and igneous minerals are unstable
under new T and P conditions.
Minerals in the original rock change and new
group becomes stable:
exchange of ions between minerals  new
minerals
solid state recrystallization (without
melting)
new mineral usually have different
compositions from original mineral
minerals with higher densities more stable
at greater pressures

11. Mineral Changes
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Examples of mineral changes:
Clay minerals  Muscovite, other micas
Ca, Mg minerals  Garnet
Granite becomes gneiss
Minerals in some metamorphic rocks do not
change:
Pure limestone and dolomite becomes Marble
crystals become coarser (larger)
original grains become coarse polygons

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Mineral Changes
 Recrystallization to larger crystals
 Quartz sandstones
 Quartz  recrystallized quartz
 Originally round grains become coarse
polygons
 Quartz sandstone  quartzite
 Carbonate rocks
 Stays calcite and dolomite at high grade
 Crystals become coarser
 Limestone  marble

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Draw contour lines
showing equal
metamorphic grade
(isograds). Draw the
contours between where
index minerals are found,
as shown for the minerals
sillimanite, kyanite, and
muscovite and chlorite.

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limestone with fossils >> recrystallized marble
Textural change cont…

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Textural change
 Pressure from regional metamorphism and high
pressure metamorphism changes rock texture
 Foliation – It is the parallel alignment of
recrystallized minerals typically the result of
Regional Metamorphism

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Mineral & Textural Changes

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Examples of Foliated*
Metamorphic Rocks
 Slate
 Phyllite
 Schist
 Gneiss
*typically the result of
Regional Metamorphism

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Textural change cont…
 Slaty (foliated)- tendency to split along
parallel planes
 Parallel orientation of microscopic grains
 Formed at low metamorphic grade
 Typical minerals: chlorite and clay minerals)

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p. 108
Development of Slaty Cleavage

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Textural change cont…
 Phyllitic (foliated)
- phyllite
 Parallel arrangement of
platy minerals (mainly
micas)
 Most grains barely
visible with naked eye
 Rock with this texture
is called phyllite

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Textural change cont…
 Schistosity (foliated) - Schist- elongate and
platy minerals recrystallize along parallel planes
 Aligned mineral crystals visible
 Abundant mica minerals, other common minerals:
quartz, amphiboles
 Layers up to a few millimetres thick

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Textural change cont…
 Gneissic banding (foliated) - Gneiss- dark and
light-colored minerals, less platy minerals,
segregate into compositional bands
 Mineral crystals visible
 Layers up to several cm thick
 Separation into layers with
different compositions

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Textural change cont…
 Gneissic banding (foliated) – Gneiss
 Light-colored bands usually include quartz and feldspar
 Dark bands commonly composed of hornblende and biotite

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 Metamorphic rocks with no
preferred orientation of
mineral grains
 commonly have a single
mineral such as quartz,
calcite or dolomite
 Examples: marble*,
quartzite*, hornfels* and
anthracite
*typically the result of Contact Metamorphism
Non-Foliated Metamorphic Rocks

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Metamorphic grade
 Maximum P and T indicated by mineral
assemblages and mineral assemblage:
 Mudstone, shale  slate phyllite 
schist  gneiss
 New minerals and textures depend on
temperature and pressure conditions.

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Metamorphic grade cont…
 Metamorphic grade:
 Low, medium, high
grades
 Crystal size usually
gets larger as T
goes up (higher
grade)
 Foliation usually
gets coarser
(thicker) at higher
grade

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Metamorphic grade cont…
 Low grade
metamorphis
 150° - 400°C
 New minerals –
microscopic
chlorite, muscovite,
Na-plagioclase,
quartz
 New foliation –
slaty cleavage
 Rock name – slate
Example of regionally metamorphosed mudstone

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Metamorphic grade cont…
 Medium grade
metamorphism
 350° - 600°C
 New foliation –
schistosity
 New minerals –
muscovite, biotite,
Ca-plagioclase,
garnet, quartz
 Rock name – schist
Example of regionally metamorphosed mudstone

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Metamorphic grade cont…
 High grade
metamorphism
 T > 600°C
 New foliation –
gneissic banding
 New minerals – Ca-
plagioclase, garnet,
staurolite, kyanite,
sillimanite, quartz
 Rock name – gneiss
 Partial melting at
highest grade
Example of regionally metamorphosed mudstone

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Metamorphic Types
 Contact
Metamorphism
 High Pressure
Metamorphism
 Regional
Metamorphism

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Contact Metamorphism
(local heating only)
 Changes caused by the
high temperature of
an intruding magma,
which can “bake” the
surrounding rock.
 High T around pluton
(igneous intrusion)
 Rocks near pluton are
heated as pluton cools
 Low stress environment
 New minerals, but no
new foliation
 Is local around the
pluton

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High Pressure Metamorphism
(mainly P rise with stress)
 Ocean sediment
dragged down
subduction zone
 Is fast, so pressure
increases quickly, T
increases more
slowly
 Material makes high
P minerals
 Quickly pushed back
to surface

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Regional Metamorphism
(caused by plate collisions)
 Have slow tectonic
burial
 T and P increase
together
 Compression
directed stress
(rocks are
“squeezed”)
 Rocks change shape,
micas become
parallel

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Regional Metamorphism
(caused by plate collisions)
 Foliation (schist) is
common
 Perpendicular to
maximum stress
 Best developed at
continent – continent
collisions
 Crust is thick, so
have mountain ranges
at surface
 Happening now in
Himalaya Mts, Zagros
Mts. in Iran

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Example Sheet
 Sedimentary rock (mudstone) deposited in an
ocean
 Sedimentary burial
 Ocean closes in continent – continent collision
 Rock is buried deeper, stressed and heated
 New minerals form (biotite, muscovite, garnet)
 Micas are parallel because of directed stress
  new layering
 Later (20+ million years) erosion brings
metamorphic rocks to surface

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