Download Ch 21 22 Intro Metam and Classif mod 8

Ch. 21 An Introduction to
Metamorphism

Metamorphism is the transformation of
rock by temperature and pressure

Metamorphic rocks are produced by
transformation of:
 Sedimentary and Igneous rocks, and by
the further alteration of other
metamorphic rocks
Kyanite, Sillimanite, and Andalucite, greenschist, amphibolite, glaucophane
Sedimentary
rock
0 km
Metamorphic
rock
Igneous
rock
Sediment
Increasing depth
and temperature
10 km
~200ºC
50 km
~800ºC
Sedimentary
rock
The rocks don’t melt
Metamorphism
Melting
Metamor
-phism
occurs
between
about 10
and 50
km of
depth
Glaciers exposed the
Canadian
Shield
Rocky
Mountains
North
Cascades
Black
Hills
Appalachian
Mountains
Grand
Canyon
Llano
Uplift
Usually buried deep,
metamorphic rocks are seen when erosion
removes covering rocks, and in the cores
of mountains
Best US exposures
in New England
and the South
What causes metamorphism?
1. Temperature Increase

Greater Temps at depth

Radioactive Isotopes

Intruding Magma

Friction Between Moving Bodies of
Rock
Metamorphic Agents and Changes

Temperature: typically
the most important
factor in metamorphism

Continental geotherm is
higher than oceanic due to
concentration of radioactive
(LIL) elements
LESSON: It gets hotter faster with
depth in continental lithosphere
Figure 1-9. Estimated ranges of oceanic and
continental steady-state geotherms to a depth of
100 km using upper and lower limits based on heat
flows measured near the surface. After Sclater et
al. (1980), Earth. Rev. Geophys. Space Sci., 18,
269-311.
Metamorphic Agents and Changes
Increasing temperature has several effects
1) Promotes recrystallization  increased
grain size
2) Drives reactions
3) Overcomes phase stability barriers
What causes metamorphism?
2. Pressure (stress) = Force/meters2
Increases with depth
Pressure can be applied equally in all
directions or differentially
All Directions = “Confining Pressure”
also called “lithostatic pressure”
Differential = “Directed Pressure”
Origin of pressure in
metamorphism
Confining pressure aka “lithostatic”
(due to burial)
(Convergent Margin)
Directed Pressure causes rocks to become folded, and
minerals to reorient perpendicular to the stress: “foliation”
Original bedding is obliterated
Source: Kenneth Murray/Photo Researchers Inc.
Foliation
Minerals Recrystallize Perpendicular to
the Directed Pressure
If the minerals are flat, such as sheetlike Micas, their parallel orientation
gives a layered look; layering unrelated
to the original bedding in the parent
rock.
Main factors causing
metamorphism
3. Parent rock
 Metamorphic rocks typically have the
same chemical composition as the rock
they were formed from.
 Different minerals, but made of the same
atoms.
 Exception: when hot water is involved.
Types of Protolith
Lump the common types of sedimentary and
igneous rocks into six chemically based-groups
1. Ultramafic - very high Mg, Fe, Ni, Cr
2. Mafic - high Fe, Mg, and Ca
3. Shales (pelitic) - high Al, K, Si
4. Carbonates- high Ca, Mg, CO2
5. Quartz - nearly pure SiO2.
6. Quartzo-feldspathic - high Si, Na, K, Al

Chemistry of the metamorphic rock is the most important clue toward deducing
the parent rock
Metamorphic Settings

Four types of metamorphic settings:
 1. Contact metamorphism – due heat from adjacent rocks
 2. Hydrothermal metamorphism – chemical alterations
from hot, ion-rich water
 3. Regional metamorphism -- Occurs in the cores of
mountain belts and subduction zones (Converging
Margins) . Makes great volumes of metamorphic rock.
Includes:
– a. Burial Metamorphism – e.g. Burial of sediments
deeper than 10 km – non-foliated
– b. Dynamothermal Metamorphism – Directed
pressure in Plate Tectonic Processes – foliated
4. Dynamic. Smearing of minerals as faults slip
Most is Dynamothermal
Contact metamorphism
Produced mostly by local heat source
Baking due to nearby Magma
Effect strongest in rocks in immediate
contact
Hot water facilitates metamorphic reactions by allowing movement of atoms and ions
Hydrothermal Metamorphism
Atoms in the rock may be added or removed by the water
Due circulation of water
near Magma
Important at mid-ocean
ridge, or near
continental volcanoes
Metasomatism
Metamorphism by mobile fluids,
e.g. H2O incl. salines, silica, CO2
The composition changes
•
Evidence for the existence of a
metamorphic fluids: Fluid inclusions
• Fluids are required for hydrous H2O
OH- or carbonate CO3-- phases
• Ex: The fluids expelled from the
subducting slab migrate upwards
and form hot springs, mudvolcanoes or serpentinite Diapirs
within the forearc system. Fluids or
melts released at greater depth will
hydrate and metasomatise the
mantle rocks of the hanging plate
(i.e. the mantle wedge). The source
of melt for the volcanic arcs located
above the subduction zones is this
hydrated mantle wedge above the
subducted slab.
3. Regional Metamorhism
 Most
Dynamothermal metamorphism
occurs along convergent plate boundaries
Example 1: In Subduction Zones
Example 2: Continent-Continent Collisions
 Compressional stresses deforms plate edge
 Continents Collide
 Major Folded Mountain Belts: Alps,
Himalayas, and Appalachian Mts.
Dynamothermal Metamorphism,
Before collision, in subduction zone
Sediments are “unconsolidated”. They will fold if pushed.
Dynamothermal Metamorphism,
After continental collision
Felsic continental materials and sediments are buoyant, they have low density
They float, cannot be subducted, so they get squashed.
Orogenic Regional Metamorphism of
the Scottish Highlands
 George
Barrow studied the metapelites in NE
Scotland from 1884 to 1900.
 He could subdivide the area into a series of
metamorphic zones, each based on the
appearance of a new mineral as metamorphic
grade increased
Figure 21-8. Regional metamorphic
map of the Scottish Highlands,
showing the zones of minerals that
develop with increasing
metamorphic grade. From Gillen
(1982) Metamorphic Geology. An
Introduction to Tectonic and
Metamorphic Processes. George
Allen & Unwin. London.
The sequence of zones now recognized, and the typical
metamorphic mineral assemblage in each, are:
 Chlorite zone. Pelitic rocks are slates or phyllites and
typically contain chlorite, muscovite, quartz and albite
 Biotite zone. Slates give way to phyllites and schists, with
biotite, chlorite, muscovite, quartz, and albite
 Garnet zone. Schists with conspicuous red almandine
garnet, usually with Biotite, chlorite, muscovite, quartz, and
Albite or Oligoclase
 Staurolite zone. Schists with Staurolite, Biotite, Muscovite,
Quartz, garnet, and plagioclase. Some Chlorite may persist
 Kyanite zone. Schists with Kyanite, Biotite, Muscovite,
Quartz, plagioclase, and usually garnet and Staurolite
 Sillimanite zone. Schists and gneisses with Sillimanite,
Biotite, Muscovite, Quartz, plagioclase, garnet, and perhaps
Staurolite. Some Kyanite may also be present. Kyanite and
Sillimanite are both polymorphs of Al2SiO5)
Hi-P
Low-P
Metamorphic Grade and
Index Minerals

Certain minerals, called index
minerals, are good indicators of
the metamorphic conditions in which
they form
Note
Temperature
gradient
Index Minerals in metamorphic rocks
580oC
220oC
Stable temps vary based
on pressure and other
species present
Certain minerals, called
index minerals, are
good indicators of the
metamorphic conditions
in which they form
460oC
Note Quartz and Feldspar are not index minerals: Why?
690oC
Here is an internally heated
pressure vessel at the AMNH
With these you can
study, for example:
1. the temperature and
pressure conditions at
which polymorphs
change from one form to
another.
2. the reactions of
minerals with fluids (for
example salty or alkaline
water) at high
temperatures and
pressures.
3. the conditions
necessary to change one
assemblage of minerals
to another
http://research.amnh.org/earthplan/research/Equipment/Petrology
Thermometers and Pressure Gauges
Sillimanite
Kyanite
Polymorphs of Al2SiO5
Note other minerals in system
Andalusite
CANADA
New England
Dynamothermal
Metamorphism
7_21

MAINE
Augusta

Caused by C-C collision M-P
Most of Appalachians
Montpelier
NEW
VERMONT HAMPSHIRE

Concord

Boston

Albany
NEW YORK
ATLANTIC
OCEAN
MASSACHUSETTS


Binghamton

R.I.
Hartford
Providence
CONNECTICUT


PENNSYLVANIA
Scranton
r
NEW
JERSEY
i
ft
va l
l
ey
Low
grade
Long Medium
Island grade
Newark
High grade
Unmetamorphosed
Chlorite/muscovite zone
Biotite zone
Garnet zone
Staurolite zone
Sillimanite zone
Increasing pressure and temperature
DIAGENESIS
LOW GRADE
HIGH GRADE
INTERMEDIATE GRADE
Chlorite and muscovite
Biotite
Garnet
Staurolite
MELTING
Sillimanite
Metamorphic Facies in Subduction Zones
Metamorphic grade or
Facies: A group of minerals
that form in a particular P-T
environment. Can be used
to deduce T-P conditions of
formation
We can look at minerals in Metamorphic Rocks
and determine where they formed.
Subducted Water, water from hydrated minerals, facilitates metamorphic reactions
by allowing movement of atoms and ions
Greenschist Hand Sample
Chlorite is commonly found in igneous
rocks as an alteration product of mafic
minerals such as pyroxene, amphibole,
and Biotite. In this environment chlorite
may be a retrograde metamorphic
alteration mineral of existing
ferromagnesian minerals, or it may be
present as a Metasomatism product via
addition of Fe, Mg, or other compounds
into the rock mass.
Greenschist Thin Section
Chl-Ep
Blueschist glaucophane
Amphibolite
RINGWOOD, A.E. 1974. The petrological evolution of island arc
systems. Journal of the Geological Society, London 130, 183-204.
Eclogite
Eclogite: red to pink garnet
(almandine-pyrope) in a
green matrix of sodium-rich
pyroxene (omphacite)
Ringwood, (1974) suggested that island arc lavas
(IAT), which are basaltic, could be related to
dehydration of the hydrated ocean crust
(amphibolite) as it transforms to dense eclogite at
depths of ~150km. The hydrous fluids rise up into
the peridotite mantle wedge, promoting partial
melting. Melting occurs at much lower temperatures
in the presence of water.
Figure 10-4 or 10-5 (2nd ed). Dry peridotite solidus compared to several experiments on H2O-saturated peridotites.
1. There are many reactions suggested for the origin near subduction zones of glaucophane, which is a
characteristic component of metabasite blueschists
basaltic
basaltic
Water out
2. Gibbs Free Energy
calcs are an obvious
check, and this drives
research programs to
provide the needed
numbers
experimentally in the
presence of water, etc.
from wet subducted
ocean lithosphere
Non-Foliated Rocks 1: Quartzite
Sample of
quartzite
Thin section
of quartzite
Field Geologists
are grateful for
quartzite. It
doesn’t foliate,
so you can see
folds. Mudrocks
foliate; much
harder to map.
Flattening of quartz grains in quartzite
7_18
Fracture
Sandstone: grains and cement
Fracture
Quartzite: grains interlock
Common metamorphic rocks

Nonfoliated rocks (cont.)
 Marble
– Coarse, crystalline
– Parent rock usually limestone
– Composed of calcite crystals
– Fabric can be random or oriented
Marble (nonfoliated)
Marble
Coarse, crystalline
Parent rock
usually limestone
Composed of
calcite crystals
Fabric can be
random or
oriented
Mudstones are sediments, can be squashed by burial and/or in continent-continent collisions
Change in metamorphic grade with depth
Mudstone is most common sedimentary rock. When metamorphosed, rocks reveal grade:
Increasing Directed Pressure and increasing Temps =>
Common metamorphic rocks

Foliated rocks
Slate
–Very fine-grained
–Excellent rock cleavage, often perp.
to original
–Made by low-grade metamorphism
of shale
Example of slate
Slate
Very fine-grained
Excellent rock cleavage, often perp. to original
Made by low-grade metamorphism of shale
Phyllite also
lacks visible mineral grains
Phyllite
Grade of metamorphism
between slate and schist
Made of small platy
minerals
Glossy sheen with rock
cleavage
Composed mainly of
muscovite and/or chlorite
Common metamorphic rocks

Foliated rocks
Schist
–Medium- to coarse-grained
–Comprised of platy minerals (micas)
–The term schist describes the texture
–To indicate composition, mineral names
are used (such as mica schist)
Schist
Foliated rocks
Schist
Medium- to coarse-grained
Comprised of platy minerals (micas)
The term schist describes the texture
To indicate composition, mineral names are used, e.g. Garnet Schist
Common metamorphic rocks

Foliated rocks
Gneiss
–Medium- to coarse-grained
–Banded appearance
–High-grade metamorphism
–Composed of light-colored feldspar
layers with bands of dark mafic
minerals
Gneiss displays bands of light
and dark minerals
Foliated rocks
Medium- to coarsegrained
Banded appearance
High-grade
metamorphism
Composed of lightcolored feldspar
layers with bands of
dark mafic minerals
What are metamorphic textures?

Texture refers to the size, shape, and
arrangement of mineral grains within a
rock

Ex: Foliation – planar arrangement of
mineral grains within a rock,
perpendicular to the directed pressure,
common near convergent margins
Development of lineation or
foliation due to directed pressure
Migmatites- When Partial Melting Starts
Heat the rock, when the minerals with the lowest
melting points (Qtz, Feldspar) at that pressure
melt then recrystallize, we get separate bands of
Metamorphic and Igneous rock
Chapter 22: A Classification of
Metamorphic Rocks
►Metamorphic
rocks are classified on the basis
of texture and composition (either
mineralogical or chemical)
►Unlike igneous rocks, which have been
plagued by a proliferation of local and specific
names, metamorphic rock names are
surprisingly simple and flexible
►May choose some prefix-type modifiers to
attach to names if care to stress some
important or unusual textural or
mineralogical aspects
►Foliation: any
planar fabric element
►Lineation: any linear fabric elements
 Cleavage
• The property of a rock to split along a
regular set of sub-parallel, closely-spaced
planes
• any type of foliation in which platy
Phyllosilicates are aligned (parallel) but are
too fine grained to see without a
microscope
 Schistosity
◦ A preferred orientation of mineral grains or
grain aggregates produced by metamorphic
processes
◦ Aligned minerals are coarse grained enough
to see with the unaided eye
◦ The orientation is generally planar, but linear
orientations are not excluded

Gneissose AKA gneissic structure
◦ Segregated into mafic and felsic layers by
metamorphic processes
◦ Gneissose rocks usually have visible grains
b. Phyllite: a rock with a
schistosity in which very fine
phyllosilicates (sericite
and/or chlorite), although
rarely coarse enough to see
unaided, cause a reflective
foliation surface.
a. Slate: compact, very finegrained, metamorphic rock
with a well-developed
cleavage. Freshly cleaved
surfaces are dull
a
b
Figure 22-1. Examples of foliated metamorphic rocks. a. Slate. b. Phyllite. Note the difference in reflectance on the foliation surfaces
between a and b: phyllite is characterized by a satiny sheen. Winter (2001) An Introduction to Igneous and Metamorphic Petrology.
Prentice Hall.
Foliated Metamorphic Rocks
Schist: metamorphic rocks in
which the foliated minerals
are coarse enough to see
easily in hand specimen.
Figure 22-1c. Garnet muscovite schist. Muscovite crystals are visible and silvery, garnets occur as large dark porphyroblasts.
Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Foliated Metamorphic Rocks
Gneiss: a metamorphic rock
displaying gneissose
structure. Gneisses are
typically layered (also called
banded), generally with
alternating felsic and darker
mineral layers. Gneisses may
also be lineated, but must
also show segregations of
felsic-mineral-rich and darkmineral-rich concentrations.
Figure 22-1d. Quartzo-feldspathic gneiss with obvious layering. Winter (2001) An Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
Non-Foliated Metamorphic Rocks
Granofels: a comprehensive term for any rock
with no preferred orientation
 An outdated alternative to granofels is
granulite, but this term is now used to denote
very high grade rocks (whether foliated or not),
and is not endorsed here as a synonym for
granofels.
Hornfels is a subset,
preferably for fine grained,
contact metamorphosed
rocks
Non-Foliated Metamorphic Rocks
Marble: a metamorphic rock composed
predominantly of calcite or dolomite. The
protolith is typically limestone or
dolostone.
Non-Foliated Metamorphic Rocks
Quartzite: a metamorphic rock composed
predominantly of quartz. The protolith is
typically sandstone.
Metamorphic Rock Facies
Greenschist/Greenstone: a low-grade
metamorphic rock that typically contains
chlorite, actinolite, epidote, and albite.
Note that the first three minerals are green,
which imparts the color to the rock. Such a
rock is called greenschist if foliated, and
greenstone if not. The protolith is either a
mafic igneous rock or a graywacke.
Metamorphic Rock Facies
Amphibolite: a metamorphic rock
dominated by hornblende + plagioclase.
Amphibolites may be foliated or nonfoliated. The protolith is either a mafic
igneous rock or graywacke.
Serpentinite: an ultramafic rock
metamorphosed at low grade, so that it
contains mostly serpentine.
Blueschist: a usually blue, usually
glaucophane bearing metamorphosed
mafic igneous rock or mafic graywacke.
This term is even applied to non-schistose
rocks.
Eclogite: a green and red metamorphic rock that
contains the green clinopyroxene Omphacite and
pink garnet Pyrope. The protolith is typically basaltic.
It forms via the dehydration of Amphibolite at about
150km down a subduction zone.
Granulite: medium to coarse–grained
metamorphic rocks that have experienced
high-temperature metamorphism,
composed mainly of feldspars sometimes
associated with quartz and anhydrous
ferromagnesian minerals, with granoblastic
(anhedral phaneritic equi-granular) texture
and gneissose to massive structure.
Metagabbro showing
granoblastic texture
Note how the interlocking
plagioclase grains in this rock
meet at ~120 degree triple
junctions. This feature is
characteristic of granoblastic
texture.
Skarn: a contact metamorphosed and silicametasomatized carbonate rock containing
calc-silicate minerals, such as grossular,
epidote, tremolite, vesuvianite, etc. Tactite
is a synonym.
Migmatite: a composite silicate rock that is
heterogeneous on the 1-10 cm scale,
commonly having a dark gneissic matrix
(melanosome) and lighter felsic portions
(leucosome). Migmatites may appear
layered, or the leucosomes may occur as
pods or form a network of cross-cutting
veins.
Additional Modifiers
Porphyroblastic means that a metamorphic
rock has one or more metamorphic
minerals that grew much larger than the
others. Each individual crystal is a
porphyroblast
Some porphyroblasts, particularly in lowgrade contact metamorphism, occur as
ovoid “spots”
e.g. spotted hornfels, or spotted phyllite
Augen
Some gneisses have large eye-shaped grains
(commonly feldspar) that are derived from preexisting large crystals by shear (as described in
Section 23.1). Individual grains of this sort are
called auge (German for eye), and the (German)
plural is augen. An augen gneiss is a gneiss with
augen structure (Fig. 23-18).
Additional Modifying Terms:
Other modifying terms that we may want to
add as a means of emphasizing some aspect
of a rock may concern such features as grainsize, color, chemical aspects, (aluminous,
calcareous, mafic, felsic, etc.). As a general
rule we use these when the aspect is unusual.
NOT a calcareous marble or mafic greenschist,
as these are redundant, as is a fine grained
slate.
Additional Modifying Terms:
Ortho- a prefix indicating an igneous parent,
and
Para- a prefix indicating a sedimentary parent
The terms are used only when they serve to
dissipate doubt. For example, many quartzofeldspathic gneisses could easily be derived from
either an impure arkose or a granitoid rock. If some
mineralogical, chemical, or field-derived clue permits
the distinction, terms such as orthogneiss, paragneiss,
or orthoamphibolite may be useful.