Download Introduction to Metamorphism

Introduction to Metamorphism
(C
(Chapter
21)
Metamorphic rocks from the middle of the crust – Quad Creek area, MT. Image: Darrell Henry
Metamorphism - general
IUGS SCMR definition
IUGS-SCMR
d finiti n off metamorphism
t
rphi
“Metamorphism
p
is a subsolidus
process leading to changes in
mineralogy and/or texture (for
example grain size) and often in
chemical composition in a rock.
Folded marble in the Campolungo
Regions of the Alps.
lps. Image
mage source:
Darrell Henry
• These changes
g are due to physical
p
and/or chemical conditions that
differ from those normally occurring
at surfaces of planets and in zones of
cementation and diagenesis below
this surface.
• They may coexist with partial
melting.”
Metamorphism - general
Li it off metamorphism
Limits
t
rphi
Low-temperature limit grades
into diagenesis
g
• Boundary is somewhat arbitrary
• Diagenetic/weathering
processes - indistinguishable
from metamorphic
• Metamorphism begins in range
of 100-150°C for more unstable
types of protolith
Image: Winter (2001)
• Some zeolites considered
diagenetic and others
metamorphic
Metamorphism - general
Li it off metamorphism
Limits
t
rphi
High-temperature limit
grades into mel;ting
g
; g
• Over melting range that solids
and liquids coexist. At what
point in melting process does it
become "igneous"?
• Xenoliths and restites
considered in igneous realm
because melt is dominant, but
distinction is vague.
Image: Winter (2001)
• Migmatites ("mixed rocks") are
gradational
Metamorphism - general
M t
Metamorphic
rphi agents
nt and
nd change
h n
Metamorphic grade: general
i
increase
i degree
in
d
off metamorphism
hi
without specifying exact relationship
between temperature
p
and p
pressure
Temperature: typically most
important factor
• Estimated ranges of oceanic
and continental steady-state
geotherms
h
to depth
d h off 100 k
km
using upper and lower limits
based on heat flows measured
near surface.
Image: Winter (2001)
Metamorphism - general
Increasing temperature has several effects
1. Promotes recrystallization leading to increased grain size.
• Larger surface/volume ratio of mineral results in lower stability
• Increasing temperature eventually overcomes kinetic barriers to
recrystallization,
y
and fine aggregates
gg g
coalesce to larger
g grains
g
Image: Winter (2001)
Metamorphism - general
Increasing temperature has several effects
2) Drive reactions that consume unstable mineral(s) and produces
new minerals that are stable under new conditions
3) Overcomes kinetic barriers that might otherwise preclude
attainment of equilibrium
Image: Winter (2001)
Metamorphism - general
Increasing
g Pressure
• Stress is applied force acting on rock (over particular crosssectional area)
• Strain is response of rock to applied stress (= yielding or
deformation))
Lithostatic pressure is uniform stress (~hydrostatic)
Deviatoric stress = unequal pressure in different directions
Deviatoric stress can be resolved into three mutually
perpendicular stress (s) components:
• σ1 - maximum principal stress
• σ2 - intermediate p
principal
p stress
• σ3 - minimum principal stress
Metamorphism - general
Increasing Pressure
Deviatoric stresses come in three
principal types:
• Tension: σ3 is negative, and
resulting strain is extension, or
pulling apart
• Compression: σ1 is dominant,
results in folding or more
homogenous flattening
• Shear motion occurs along planes at
an angle to σ1
Image: Winter (2001)
Metamorphism - general
Increasing Temperature and Pressure
"Normal" g
gradients may be perturbed in
several ways, typically:
• High T/P geotherms
- in areas of plutonic
activity
i i or rifting
if i
• Low T/P g
geotherms in subduction zones
Image: Winter (2001)
Metamorphic field gradients (estimated P-T
P T conditions along
surface traverses directly up metamorphic grade) for several
metamorphic areas.
Metamorphism - general
Fluids
Evidence for the existence of a
metamorphic fluid:
• Fluid inclusions
• Fluids are required for hydrous or carbonate
phases
• Volatile-involving
g reactions occur at
temperatures and pressures that require
finite fluid pressures
Pfluid means total fluid pressure - sum of partial pressures of each
component (Pfluid = pH2O + pCO2 + …)
May also consider mole fractions of components, which must sum
to 1.0 (XH2O + XCO2 + … = 1.0)
Metamorphism - general
Types
yp of Metamorphism
p
Classification of metamorphism based on setting
( ) Contact
(1)
Metamorphism
Image: Winter (2001)
Temperature distribution within 1-km thick vertical dike and in
country rocks (initially at 0οC) as function of time. Curves are labeled in
years. The model assumes an initial intrusion temperature of 1200oC and cooling by conduction
only.
Metamorphism - general
Types
yp of Metamorphism
p
Classification of metamorphism based on setting
(1) Contact Metamorphism
• Adjacent to igneous intrusions
• Result of thermal (and possibly
metasomatic) effects of hot
magma intruding cooler
shallow rocks
•O
Occur over wide
id range off
pressures, including very low P
Geologic Map and cross-section of the area
around the Skiddaw granite, Lake District, UK.
Image: Winter (2001)
• Contact metamorphic aureole
Metamorphism - general
Types
yp of Metamorphism
p
Classification of metamorphism based on setting
(1) Contact Metamorphism
Contact metamorphic aureole
Size and shape of aureole
controlled by:
Geologic Map and cross-section of the area
around the Skiddaw granite, Lake District, UK.
Image: Winter (2001)
• Size of pluton
• Shape of pluton
• Orientation of pluton
• Depth and metamorphic
grade prior to intrusion
• Temperature
p
- mafic magmas
g
• Composition
are hotter than silicic magmas
• Fluids and permeability
Metamorphism - general
Types
yp of Metamorphism
p
Classification of metamorphism based on setting
(1) Contact Metamorphism
Spotted phyllite in which small
porphyroblasts of cordierite develop in
a preexisting phyllite.
phyllite
Image: Winter (2001)
Contact metamorphic aureole
Most easily recognized where
pluton is introduced into shallow
p
rocks in static environment
• Rocks near pluton are often
hi h
high-grade
d rocks
k with
ith iisotropic
t i
fabric: hornfelses (or
granofelses) - relict textures
and structures are common
• Polymetamorphic rocks are
common, usually
ll representing
ti
orogenic event followed by
contact one
Metamorphism - general
Types
yp of Metamorphism
p
Contact metamorphism example: Skiddaw Pluton, UK
Ordovician Skiddaw Slates
(English Lake District)
intruded by several granitic
bodies
Intrusions are shallow, and
contact effects overprinted on
an earlier low-grade regional
orogenic
g
metamorphism
p
Geologic Map and cross-section of the area
around the Skiddaw granite, Lake District, UK.
Image: Winter (2001)
Metamorphism - general
Types
yp of Metamorphism
p
Contact metamorphism example: Skiddaw Pluton, UK
• Intermediate zone: slates more
thoroughly recrystallized, contain
biotite + muscovite + cordierite +
quartz
andalusite + q
Geologic Map and cross-section of the area
around the Skiddaw granite, Lake District, UK.
Image: Winter (2001)
• Cordierite-andalusite slate from
intermediate zone of Skiddaw aureole.
Metamorphism - general
Types
yp of Metamorphism
p
Contact metamorphism example: Skiddaw Pluton, UK
• Inner zone: Thoroughly recrystallized
• Lose foliation
Geologic Map and cross-section of the area
around the Skiddaw granite, Lake District, UK.
Image: Winter (2001)
• Andalusite-cordierite schist from
inner zone of Skiddaw aureole. Note
the chiastolite cross in andalusite
Metamorphism - general
Types
yp of Metamorphism
p
Classification of metamorphism based on setting
(2) Regional Metamorphism: metamorphism that affects large
body of rock, and thus covers great lateral extent
Th
Three
principal
i i l types:
• Orogenic metamorphism
• Burial metamorphism
• Ocean-floor metamorphism
Metamorphism - general
Types
yp of Metamorphism
p
Regional Metamorphism
Orogenic Metamorphism
is type of metamorphism
associated with
convergent plate margins
Dynamo-thermal, involving
one or more episodes of
orogeny with combined
elevated
l t d geothermal
th rm l gradients
r di nt
and deformation (deviatoric
stress)
Foliated rocks are a
characteristic product
Metamorphism - general
Types
yp of Metamorphism
p
Regional Metamorphism
Orogenic Metamorphism
Metamorphism
M
hi
• often continues after major
deformation ceases
• associated with intruding plutons
• batholiths commonly in highest
grade region
• Most orogenic belts have several
episodes of deformation and
metamorphism, creating more
complex polymetamorphic
pattern
• T
Transitional
iti
l cases can be
b ttermed
d
regional-contact metamorphism
Metamorphism - general
Types
yp of Metamorphism
p
Regional Metamorphism
Burial metamorphism = low-grade metamorphism
in sedimentary basins due to burial.
• Southland Syncline in New Zealand:
thick
hi k pile
il ((> 10 k
km)) off M
Mesozoic
i
volcaniclastics
• Mild deformation and no igneous
intrusions discovered
• Fine-grained, high-temperature
phases,, g
p
glassyy ash: veryy susceptible
p
to
metamorphic alteration
• Metamorphic effects attributed to
increased pressure and temperature
due to burial
Image: Winter (2001)
Metamorphism - general
Types
yp of Metamorphism
p
Regional Metamorphism
Burial metamorphism
• Range from
diagenesis to
formation of zeolites,
prehnite, pumpellyite,
laumontite, etc
• Coombs ((1961)) also
proposed
hydrothermal
metamorphism,
caused by hot H2Orich fluids and usually
involving
metasomatism
t
ti
Image: Winter (2001)
Metamorphism - general
Types
yp of Metamorphism
p
Regional Metamorphism
Burial metamorphism
p
= low-grade
g
metamorphism
p
in sedimentaryy
basins due to burial.
• Burial
B i l metamorphism
hi
occurs iin areas that
h h
have not
experienced significant deformation or orogeny
• Restricted
R
i d to llarge, relatively
l i l undisturbed
di
b d
sedimentary piles away from active plate margins
e g The Gulf of Mexico?
e.g.
• Well into metamorphic range, and the weight of the
overlying sediments sufficient to impart a foliation
at depth
Metamorphism - general
Types
yp of Metamorphism
p
Regional Metamorphism
Ocean-Floor Metamorphism
p
affects the oceanic crust
at ocean ridge spreading centers
• Wide range of T at relatively low P
• Considerable metasomatic alteration notably loss of Ca and Si and gain of Mg and Na
• Changes correlated with exchange between
basalt and hot seawater
• Example
E
pl off h
hydrothermal
dr th r l metamorphism
t
rphi
• Highly altered chlorite-quartz rocks distinctive high
high-Mg,
Mg, low
low-Ca
Ca composition
http://www.whoi.edu/page.do?pid=10655
Metamorphism - general
Types
yp of Metamorphism
p
Impact
Metamorphism
(dynamic)
Impact metamorphism
("shock metamorphism")
occurs at meteorite
impact craters
• dominant type of
metamorphism
t
rphi
on
n
Moon.
Metamorphism - general
Types
yp of Metamorphism
p
Fault-Zone Metamorphism (dynamic) – primarily
Metamorphism - general
Progressive Nature of Metamorphism
Prograde metamorphism: changes in rock that
accompany
p y increasing
g metamorphic
p
grade
g
• High grade metamorphic rocks progress through sequence of
mineral assemblages
g rather than hopping
pp g directlyy from an
unmetamorphosed rock to the current metamorphic rock
• Prograde
g
reactions are endothermic and driven byy increasing
gT
• Devolatilization is easier than reintroducing volatiles
• Preserved zonal distribution of metamorphic rocks suggests that
each rock preserves conditions of peak metamorphic grade (T)
• Geothermometry indicates that the mineral compositions
commonly preserve the maximum temperature
Metamorphism - general
Progressive Nature of Metamorphism
Retrograde: decreasing grade as rock cools and recovers from
a metamorphic or igneous event
• All rocks that we now
find must also have
cooled to surface
conditions
• Retrograde
metamorphism is of
only minor
significance
Image: Winter (2001)
At what point on its cyclic P-T-t path did its present mineral assemblage
last equilibrate?
Metamorphism - general
Types of Protoliths
Lump common types of
sedimentary and igneous rocks
into six chemically based-groups
1 Ultramafic - very high Mg,
1.
Mg Fe,
Fe Ni,
Ni
Cr
Chromite-bearing serpentinite
2. Mafic - high
g Fe,, Mg,
g, and Ca
3. Shales (pelitic) - high Al, K, Si
g Ca,, Mg,
g, CO2
4. Carbonates- high
5. Quartz - nearly pure SiO2.
Garnet amphibolite
6. Quartzo-feldspathic
p
- high
g Si,, Na,,
K, Al
Metamorphism - general
Types of Protoliths
Lump common types of
sedimentary and igneous rocks
into six chemically based-groups
1 Ultramafic - very high Mg,
1.
Mg Fe,
Fe Ni,
Ni
Cr
Garnet-staurolite schist
2. Mafic - high
g Fe,, Mg,
g, and Ca
3. Shales (pelitic) - high Al, K, Si
g Ca,, Mg,
g, CO2
4. Carbonates- high
5. Quartz - nearly pure SiO2.
6. Quartzo-feldspathic
p
- high
g Si,, Na,,
K, Al
Layered vesuvianite-diopside-grossular carbonate-silicate rock
Metamorphism - general
Types of Protoliths
Lump common types of
sedimentary and igneous rocks
into six chemically based-groups
1 Ultramafic - very high Mg,
1.
Mg Fe,
Fe Ni,
Ni
Cr
2. Mafic - high
g Fe,, Mg,
g, and Ca
3. Shales (pelitic) - high Al, K, Si
g Ca,, Mg,
g, CO2
4. Carbonates- high
5. Quartz - nearly pure SiO2.
6. Quartzo-feldspathic
p
- high
g Si,, Na,,
K, Al
Granitic gneiss
Metamorphism - general
Example: Orogenic Regional Metamorphism of the
Scottish Highlands
George
g Barrow (1893,
(
,
1912) - SE Highlands
of Scotland Caledonian orogeny ~
500 Ma
• Regional
g
metamorphic
p
map of Scottish
Highlands, showing
zones of minerals that
develop with increasing
metamorphic grade.
• Barrow studied both
intercalated pelitic and
mafic rocks
Image: Winter (2001)
Metamorphism - general
Example: Orogenic Regional Metamorphism of the
Scottish Highlands
Subdivide area into
metamorphic zones within
metapelites, based on
appearance of new mineral
as metamorphic grade
increased.
Sequence of zones and typical
metamorphic mineral
assemblage
g are:
• Chlorite zone. Pelitic rocks
are slates or phyllites
• chlorite,
chlorite muscovite,
muscovite quartz
and albite
Image: Winter (2001)
Metamorphism - general
Example: Orogenic Regional Metamorphism of the
Scottish Highlands
• Bi
Biotite
i zone. Slates
Sl
give
i way
to phyllites and schists
• biotite, chlorite, muscovite,
quartz and albite
quartz,
• Garnet zone. Schists
• conspicuous red almandine
garnet, usually with biotite,
chlorite, muscovite, quartz,
and albite or oligoclase
g
Image: Winter (2001)
Metamorphism - general
Example: Orogenic Regional Metamorphism of the
Scottish Highlands
• Staurolite zone. Schists
• staurolite, biotite, muscovite,
quartz, garnet, and
plagioclase. Some chlorite
may persist
• Kyanite zone. Schists
• kyanite,
y
, biotite,, muscovite,,
quartz, plagioclase, and
usually garnet and staurolite
zone. Schists and
• Sillimanite zone
gneisses
• sillimanite, biotite, muscovite,
qquartz,, plagioclase,
p g
, garnet,
g
,
and perhaps staurolite.
Image: Winter (2001)
Metamorphism - general
Example: Orogenic Regional Metamorphism of the
Scottish Highlands
• An isograd represents first
appearance of particular
metamorphic index mineral in field
as one progresses up metamorphic
hi
grade (Barrow’s definition)
• When one crosses an isograd,
g , such as the
biotite isograd, one enters the biotite zone
• Zones thus have same name as isograd that
forms low-grade boundary of that zone
Image: Winter (2001)
• Because classic isograds are based on first appearance of a
mineral, and not its disappearance, an index mineral may still be
stable in higher grade zones
Metamorphism - general
Example: Orogenic Regional Metamorphism of the
Scottish Highlands
A variation occurs in the area just
j
to the north of Barrow’s, in the
Banff and Buchan district
Image: Winter (2001)
Metamorphism - general
Example: Paired metamorphic belts - Japan
NW belt ("inner" belt,
inward,, or awayy from trench))
is Ryoke-Abukuma Belt
• Low P/T Buchan-type
yp of
regional orogenic metamorphism
• Dominant meta-pelitic
sediments, and isograds up to the
sillimanite zone have been
mapped
• High-temperature-low-pressure
belt, and granitic plutons are
common
Image: Winter (2001)
Metamorphism - general
Example: Paired metamorphic belts - Japan
Outer belt, called the
Sanbagawa
g
Belt
• It is high-P/low-T
• Only reaches garnet zone in
pelitic rocks
• Basic rocks are more common
than in Ryoke belt, however, and
in these glaucophane may be
developed
• Rocks are commonly called
blueschists
Image: Winter (2001)
Metamorphism - general
Example: Paired metamorphic belts - Japan
suggests that
h 600oC
isotherm, for
example, could be as
deep as 100 km in the
trench-subduction
zone area, and as
shallow as 20 km
beneath volcanic arc
Image: Winter (2001)
Metamorphism - general
Examples: Paired metamorphic belts - Japan
Miyashiro (1961,
1973) suggested
d
that coeval
occurrence
• metamorphic belts,
an outer, high-P/T
belt, and an inner,
lower-P/T belt
• ought to be common
occurrence in
subduction zones,
either modern or
ancient
Image: Winter (2001)