Metamorphic Facies and Metamafic
rocks
k (Chapter 25)
Blueschist – Catalina Island, CA. Image: Darrell Henry
Metamorphic Facies Concept
Pentii Eskola (1914) Orijärvi region of southern Finland
Rocks with K-feldspar + cordierite at Oslo
contained the compositionally
p
y equivalent
q
pair
p
biotite + muscovite at Orijärvi
• Eskola: difference must reflect differing
physical conditions between the regions
• Concluded that Finnish rocks equilibrated
at lower
l
temperatures and
d hi
higher
h pressures
than Norwegian ones
Eskola (1915) developed the concept of metamorphic facies:
"In any rock or metamorphic formation which has arrived at a chemical
q
through
g metamorphism
p
at constant temperature
p
and p
pressure
equilibrium
conditions, the mineral composition is controlled only by the chemical
composition. We are led to a general conception which the writer proposes to call
metamorphic facies."
Metamorphic Facies Concept
Dual basis for the facies concept
Descriptive: relationship between rock composition and its
mineralogy
• A metamorphic facies is set of repeatedly associated
metamorphic mineral assemblages
• If we find specified assemblage or group of compatible
assemblages covering range of compositions in the field, then a
certain
t i facies
f i may b
be assigned
i
d tto th
the area
Interpretive: range of T and P conditions represented by each facies
• Eskola was aware of T-P implications of concept and correctly
deduced the relative T and P represented by different facies
• W
We can now assign
i relatively
l i l accurate T and
dP
Pe li
limits
i to
individual facies
Metamorphic Facies Concept
Defined on the basis of mineral assemblages that develop
in mafic rocks
Eskola (1920) proposed 5
original facies:
• Greenschist
• Amphibolite
• Hornfels
• Sanidinite
• Eclogite
Eskola (1939) added
• Granulite
• Epidote-amphibolite
• Blueschist
Later additions
Image: Winter (2001)
•
•
•
•
Zeolite
Prehnite-pumpellyite
Albite-epidote hornfels
Hornblende hornfels
Metamorphic Facies Concept
Defined on the basis of mineral assemblages that develop
in mafic rocks
IUGS-SCMR
IUGS
SCMR version of
facies diagram
Image: IUCS-SCMR (2007)
Metamorphic Facies Concept
Defined on the basis of mineral assemblages that develop
in mafic rocks
Table 25-1 . Definitive Mineral Assemblages of Metamorphic Facies
Facies
Definitive Mineral Assemblage in Mafic Rocks
Zeolite
zeolites: especially laumontite, wairakite, analcime
Prehnite-Pumpellyite
prehnite + pumpellyite (+ chlorite + albite)
Greenschist
chlorite + albite + epidote (or zoisite) + quartz ± actinolite
Amphibolite
hornblende + plagioclase (oligoclase-andesine) ± garnet
Granulite
orthopyroxene (+ clinopyrixene + plagioclase ± garnet ±
hornblende)
Blueschist
glaucophane + lawsonite or epidote (+albite ± chlorite)
Eclogite
pyrope garnet + omphacitic pyroxene (± kyanite)
Contact Facies
After Spear (1993)
Mineral assemblages in mafic rocks of the facies of contact metamorphism do not differ substantially from that of the corresponding
regional facies at higher pressure.
Metamorphic Facies Concept
Metamorphic field gradient - array of peak metamorphic (max T)
experienced in a metamorphic terrain
Typical
Barrovian-type
metamorphic
field gradient
and a series of
metamorphic PT-t paths for
rocks found
along that
gradient in
fi ld
field.
Image: Winter (2001)
Metamorphic Facies Concept
Metamorphic field gradient
Metamorphic field
gradients
(estimated P
P-T
T
conditions along
surface traverses
directlyy up
p
metamorphic
grade) for several
metamorphic
areas.
Image: Winter (2001)
Metamorphic Facies Concept
Metamorphic series
Miyashiro (1961) initially
proposed five facies series,
most of them named for a
specific representative "type
locality" The series were:
1. Contact Facies Series (very
low-P)
22. Buchan or Abukuma Facies
Series (low-P regional)
3. Barrovian Facies Series
(medium P regional)
(medium-P
4. Sanbagawa Facies Series
(high-P, moderate-T)
5. Franciscan Facies Series
(high-P, low T)
Image: Winter (2001)
Metamorphism of Mafic Rocks
Mineral changes and associations that develop with increasing
metamorphic grade
• Hydrous minerals are not common in highT igneous mafic protolith, so hydration is a
prerequisite for development of
metamorphic mineral assemblages that
characterize most facies
• Unless water is available, mafic igneous
rocks will remain largely unaffected in
metamorphic terranes, even as associated
sediments are completely re
re-equilibrated
equilibrated
• Coarse-grained intrusives are least
permeable, and thus most likely to resist
metamorphic changes, while tuffs and
graywackes are most susceptible
Metamorphism of Mafic Rocks
Mineral changes and associations that develop with increasing
metamorphic grade
Plagioclase
• As T is lowered,
lo ered the more Ca-rich
Ca rich plagioclases become
progressively unstable
• There
e e iss a general
ge e correlation
co e o between
be wee T and
d maximum
u Ancontent of stable plagioclase
• At low metamorphic grades only albite (An0-3) is stable
• In
I upper-greenschist
hi ffacies
i oligoclase
li
l
b
becomes stable.
bl
• Andesine(~An40) and more calcic plagioclases are stable in upper
amphibolite and granulite facies
• Excess Ca and Al released may released from calcite,
calcite an epidote mineral,
mineral
titanite, or amphibole, etc., depending on P-T-X
Clinopyroxene breaks down to a number of mafic minerals,
depending on grade. These include chlorite, actinolite, hornblende, epidote,
a metamorphic pyroxene, etc., and the one(s) that form are commonly
diagnostic of the grade and facies
Metamorphism of Mafic Rocks
Mafic Assemblages at Low Grades
Zeolite and prehnite-pumpellyite facies
•D
Do nott always
l
d
develop
l - typically
t i ll
require unstable protolith
Stilbite on basalt (Poona, India). Image: Darrell
Henry (2007)
• Boles and Coombs (1975)
showed that metamorphism of
their tuffs in NZ was
accompanied by substantial
chemical changes due to
circulating fluids
fluids, and that fluids
played important role in
metamorphic minerals that were
stable
bl ii.e. strong component off
hydrothermal metamorphism.
Metamorphism of Mafic Rocks
Mafic Assemblages of the Medium P/T Series: Greenschist,
Amphibolite, and Granulite Facies (most common)
Metamorphism
p
of mafic rocks is
first evident in greenschist facies
(correlates with chlorite and biotite
zones of associated pelitic rocks)
• Typical minerals include
chlorite,
hl i albite,
lbi actinolite,
i li
epidote, quartz, and possibly
calcite, biotite, or stilpnomelane
ACF diagram
g
- The most characteristic mineral
assemblage of the greenschist facies is: chlorite
+ albite + epidote + actinolite ± quartz
Image: Winter (2001)
• Chlorite, actinolite, and epidote
impart
p the g
green color from
which mafic rocks and facies get
their name
Metamorphism of Mafic Rocks
Mafic Assemblages of the Medium P/T Series: Greenschist,
Amphibolite, and Granulite Facies (most common)
Greenschist to
amphibolite facies
transition involves 2
major
j mineralogical
g
changes
1. Transition from albite
to oligoclase (increased
(in re sed
Ca-content of stable
plagioclase with T)
2 Transition from
2.
actinolite to
hornblende (amphibole
Image: Winter (2001)
becomes able to accept
increasing amounts of
aluminum and alkalis at
higher T)
Metamorphism of Mafic Rocks
Mafic Assemblages of the Medium P/T Series: Greenschist,
Amphibolite, and Granulite Facies (most common)
ACF diagram
• Typically 2-phase Hbl-Plag
• Most amphibolites are
predominantly black rocks
with up to 30% white
plagioclase
l i l
• Garnet occurs in more AlFe-rich and Ca-poor mafic
rocks and clinopyroxene in
Al-poor-Ca-rich
p
ones
Image: Winter (2001)
Metamorphism of Mafic Rocks
Mafic Assemblages of the Medium P/T Series: Greenschist,
Amphibolite, and Granulite Facies (most common)
The transition from amphibolite to granulite facies occurs in the range 650-700oC
• In presence of aqueous fluid, associated pelitic
and quartzo-feldspathic rocks (including
granitoids)) begin
g
g to melt in this range
g at low to
medium pressures , so that migmatites may form
and melts may become mobilized
• Not all pelites and quartzo-feldspathic rocks reach
granulite facies as a result
• Mafic rocks generally melt at somewhat higher T
• If H2O is removed by earlier melts remaining mafic rocks
may become depleted in water
Conversion of felsic gneiss (left)
to charnockite (right -type of the
granulite - opx granitoid).
Kabbaldurga quarry, India Image:
Darrell Henry
• H
Hornblende
rnbl nd d
decomposes
mp
and
nd orthopyroxene
rth p r n +
clinopyroxene appear
• This reaction occurs over a T interval of at least 50oC
Metamorphism of Mafic Rocks
Mafic Assemblages of the Medium P/T Series: Greenschist,
Amphibolite, and Granulite Facies (most common)
The g
granulite facies is
characterized by the
presence of a largely
anhydrous mineral
assemblage
IIn metabasites
b i di
diagnostic
i
mineral assemblage is
orthopyroxene +
clinopyroxene +
plagioclase + quartz
Winter (2001)
• G
Garnett is
i also
l common, and
d
minor hornblende and/or
biotite may be present
Metamorphism of Mafic Rocks
The origin of granulite facies rocks is complex and controversial.
There is general agreement, however, on two points
1) Granulites represent unusually hot conditions
• T > 700oC, geothermometry has yielded some very high T, i.e. >1000oC
• Average geotherm T for granulite facies depths should be ~500oC,
suggesting that granulites are products of crustal thickening and excess
heating
2) Granulites are dry
• These rocks didn’t melt due to lack of available H2O
• Granulite facies terranes represent deeply buried and
dehydrated roots of continental crust
• Touret: fluid inclusions in granulite facies rocks of S.
Norway are CO2-rich, while those in amphibolite facies
rocks
k are more H2O-rich
O i h
• H2O can be removed by H2O-undersaturated melts
Metamorphism of Mafic Rocks
Mafic Assemblages of the Medium P/T Series: Greenschist,
Amphibolite, and Granulite Facies (most common)
Winter (2001)
Metamorphism of Mafic Rocks
Mafic Assemblages of the High P/T Series: Blueschist and
Eclogite Facies
Mafic rocks (and not pelites) develop
conspicuous and definitive mineral
assemblages under high P/T
conditions
• High P/T geothermal gradients
characterize subduction zones
• Mafic blueschists are recognizable
by color - useful indicators of
ancient subduction zones
Winter (2001)
• Great density of eclogites suggests
that subducted basaltic oceanic
crust becomes more dense than
surrounding mantle
Metamorphism of Mafic Rocks
Blueschist Facies - characterized in metabasites by presence
of sodic blue amphibole stable only at high P (e.g.
glaucophane)
• Association of glaucophane
+ lawsonite is diagnostic.
• Albite breaks down at high
pressure by reaction to
jjadeitic py
pyroxene + q
quartz:
NaAlSi3O8 = NaAlSi2O6 + SiO2
Ab
= Jd
+ Qtz
• Assemblage jadeite +
quartz indicates highpressure blueschist facies
Winter (2001)
Metamorphism of Mafic Rocks
Eclogite Facies - mafic assemblage omphacitic pyroxene +
pyrope-grossular garnet (Christmas-tree rocks)
• Along higher geothermal
gradients amphibolite
facies, or even the granulite
f i may llead
facies,
d to eclogite
l i
facies
•M
Much
h off blueschist,
bl
hi and
d all
ll
of eclogite facies are
marked byy high-pressure
g p
instability of plagioclase, a
common phase in
metabasites of any other
grade
Winter (2001)
Metamorphism of Mafic Rocks
Metamorphic field gradient (and PTt paths)
Image: Winter (2001)
(upper left) crustal thickening
((clockwise p
path))
(left) shallow magmatism heatheat-flow
((above)) model for some types
yp of
granulite facies metamorphism
(counter--clockwise path)
(counter
Metamorphism of Mafic Rocks
Pressure-Temperature-Time
Pressure
Temperature Time (P
(P-T-t)
T t) Paths
Temporal implication of
progressive metamorphism:
that rocks pass through series
of mineral assemblages upon
continuouslyy equilibrate
q
to
increasing metamorphic grade
• Consider complete
p
set of T-P
conditions that rock may
experience during metamorphic
cycle from burial to metamorphism
(and orogeny) to uplift and erosion
Winter (2001)
• Such a cycle is called a pressuretemperature-time
i
path,
h or P-T-t
PT
path
Metamorphism of Mafic Rocks
Pressure-Temperature-Time
Pressure
Temperature Time (P
(P-T-t)
T t) Paths
Metamorphic P-T-t paths may be addressed by:
1) Observing partial overprints of one
mineral assemblage upon another Relict minerals may indicate a portion of
either prograde or retrograde path (or both)
depending upon when they were created
2) Apply geothermometers and
geobarometers to core vs. rim
compositions
iti
off chemically
h i ll zoned
d
minerals to document changing P-T
conditions experienced by a rock
during their growth
Winter (2001)
Metamorphism of Mafic Rocks
Pressure-Temperature-Time
Pressure
Temperature Time (P
(P-T-t)
T t) Paths
Chemical
Ch
i l zoning
i profiles
fil across a
garnet from Tauern Window.
Conventional P-T diagram
g
(pressure
(p
increases
upward) showing three modeled "clockwise"
P-T-t paths computed from the profiles
Winter (2001)
Metamorphism of Mafic Rocks
Pressure-Temperature-Time
Pressure
Temperature Time (P
(P-T-t)
T t) Paths
Some examples of
modeled P
P-T-t
T t paths
representing common
types of
metamorphism
t
hi
The p
paths illustrated are
schematic, and numerous
variations are possible,
depending upon style of
deformation and the rates
of thickening, heat
transfer, magmatism, and
erosion,
i
etc.
t
Winter (2001)
Metamorphism of Mafic Rocks
Pressure-Temperature-Time
Pressure
Temperature Time (P
(P-T-t)
T t) Paths
Path (a) is considered typical P-T-t
path for orogenic belt with crustal
thickening
• P increases >> T, because of time lag
required
i d ffor h
heatt ttransfer
f (P equilibrates
ilib t
nearly instantaneously, but heat conducts
very slowly through rocks)
• Thickened crustal block quickly
reaches Pmax while being relatively cool
Winter (2001)
• N
New geotherm
h
i hi
is
higher,
h b
but transient,
i
and lasts only as long as the thickened
crust and subduction-related heat
generation lasts
Metamorphism of Mafic Rocks
Pressure-Temperature-Time
Pressure
Temperature Time (P
(P-T-t)
T t) Paths
Path (a) is considered typical P-T-t
path for orogenic belt with crustal
thickening
• Erosion soon affects thickened crust
and
dPb
begins
i tto d
decrease b
before
f
rocks
k
can equilibrate with higher orogenic
geotherm
• T still increasing due to slow heat
transfer so that P-T-t path has a
negative
g
slope
p following
g Pmax
Winter (2001)
• Reach Tmax when cooling effect of
uplift and erosion catches up to the
increased
d geotherm, so that thermal
perturbation of crustal thickening is
dampened and begins to fade
Metamorphism of Mafic Rocks
Pressure-Temperature-Time
Pressure
Temperature Time (P
(P-T-t)
T t) Paths
Path (b): rocks heated and cooled
at virtually constant pressure by
magmatic intrusion at shallow
levels
• P-T-t path for contact
metamorphism
• Depending upon extent of magmatic
activity and its contribution to crustal
mass, any path transitional between (a)
and (b) mayy occur.
Winter (2001)
represents gradation from high-P
(Barrovian) regional metamorphism to
g
metamorphism”
p
with
“regional-contact
numerous plutons to local contact
metamorphism
Metamorphism of Mafic Rocks
Pressure-Temperature-Time
Pressure
Temperature Time (P
(P-T-t)
T t) Paths
We may assume that the general form of a path such as (a)
represents
p
a typical
yp
rock during
g orogeny
g y and regional
g
metamorphism
1. Contrary to classical treatment of
metamorphism T and P do not both increase in
metamorphism,
unison as a single unified "metamorphic grade."
2. Pmax and Tmax do not occur at the same time
• In usual case of "clockwise" P-T-t paths, Pmax
occurs much earlier than Tmax.
p
maximum g
grade at which
• Tmax should represent
chemical equilibrium is "frozen in" and
metamorphic mineral assemblage is developed
Pmax which is
• This occurs at P well below Pmax,
uncertain since a mineral geobarometer should
record the pressure of Tmax