Metamorphic facies

1. Metamorphic Facies
• Barrow was the first person to recognize
zones that were characterized by the
presence of index min :
• Ch bio gr staurolite ky sill
• The boundary b/w each zone was marked by the
app or disapp of one of the index min
• Tilley coined the term isograd to describe the
boundary b/w metamorphic zones bcz he
interpreted it to represent a line of const met
grade

2. 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 (although
kyanite and sillimanite are both polymorphs of Al2SiO5)

3. Isoreactiongrad
• Is defined on the basis of min assem in the
rock, these assem will be determined by
initial bulk comp of the protolith, P,T and
fluid comp during met.
• The boundary b/w met zones is named isograd,
all points along a boundary were exposed to the
same intensity or grade of met.
• Isograds are named with an index min that
appears on the higher grade side of the
boundary.

4. Barrow’s zones
• Barrow interpreted the sequence of index
min from ch to sill as zones of progressive
met, known as Barrow’s zones
• Barrovian met is intermediate P/T met
• Later mapping in Buchan region revealed
a sequence of progressive met that was
different from Barrow’s zones.
• The Buchan zones were:
• bio cordie and sill
• Known as Buchan met (Low P/T type met)

5. The stability field of andalusite occurs at pressures less
than 0.37 GPa (~ 10 km), while kyanite  sillimanite at
the sillimanite isograd only above this pressure
Figure 21-9. The P-T phase diagram for the system Al2SiO5 showing the stability fields for the three polymorphs andalusite,
kyanite, and sillimanite. Also shown is the hydration of Al2SiO5 to pyrophyllite, which limits the occurrence of an Al2SiO5
polymorph at low grades in the presence of excess silica and water. The diagram was calculated using the program TWQ

6. Metamorphic grade
• Metamorphic grade: a general increase
in degree of metamorphism without
specifying the exact relationship
between temperature and pressure

7. Facies concept
• First conceived by Eskola
• Previously, petrologists had considered
individual min as indexes of met grade
(Barrow’s zone)
• There was radical departure from this
concept, and Eskola recognized that it was
the group of min or min assem, that was
the reliable indicator of P-T conditions

8. Facies Def by Turner
• A met facies is a set of met min assem,
repeatedly associated in space and time,
such that there is a const and therefore
predictable relation b/w min comp and bulk
rock ch comp

9. Dual basis for the facies concept : Descriptive and
Interpretive
• Descriptive: relationship between the Xbulk &
mineralogy
– A fundamental feature of Eskola’s concept
– A metamorphic facies is a set of repeatedly
associated metamorphic mineral assemblages
– If we find a specified assemblage in the field,
then a certain facies may be assigned to the
area
Metamorphic Facies

10. • Interpretive: the range of T and P conditions
represented by each facies
– It is now possible to assign relatively accurate
temperature and pressure limits to individual
facies
Metamorphic Facies

11. Met facies
• For rocks metamorphosed under same
physical conditions, different mineral
assemblages represent:
(a) same facies
(b) different facies
(c) same bulk comp
(d) different bulk comp

12. Met facies
• For a given bulk comp, different min ass
indicate :
(a) met under different conditions
(b) same facies
(c) different facies
(d) different P,T conditions

13. Met facies
• Rocks of all bulk comp rexlled at the same
P,T conditions, belong to
(a) same met facies
(b) different facies
(c) will have same mineral ass
(d) may have different min ass

14. Met facies
• Rocks of all bulk composition from a single
small outcrop (met regionally) belong to:
(a) same facies
(b) different facies
(c) same P,T conditions
(d) different P,T conditions

15. facies
• Eskola proposed 08 facies
• Over the yeras, new facies and subfacies
have been added
• Met facies are defined by the min ass that
occur in mafic rocks
• Boundary b/w diff met facies represent the
P,T conditions where major chem
reactions take place in met rocks
• Greenschist (ch+ep) --- breaks to form
amphiboles

16. facies
• Amphibolite (amphiboles)– breaks to form prx
(granulite)
• Facies boundary not absolute, but zone, range
of P,T
• Facies boundary depends on T, Pl, PH20
• Decreasing PH20 relative to Pl - lowering in temp
of the facies boundary
• Facies to which a rock is assigned virtually
always represents the peak met con (max T),
dominant mineralogy is established at Tmax.
• Facies gives peak met condition, not P-T path
(evolution)

17. Eskola (1920) proposed 5 original facies:
–Greenschist
–Amphibolite
–Hornfels
–Sanidinite
–Eclogite
• Easily defined on the basis of mineral
assemblages that develop in mafic rocks
Metamorphic Facies

18. • In his final account, Eskola (1939) added:
–Granulite
–Epidote-amphibolite
–Glaucophane-schist (now called
Blueschist)
... and changed the name of the hornfels
facies to the pyroxene hornfels facies
Metamorphic Facies

19. Metamorphic Facies
Temperature-pressure
diagram showing the
generally accepted limits
of the various facies used
in this text. Boundaries
are approximate and
gradational. The
“typical” or average
continental geotherm is
from Brown and Mussett
(1993). Winter (2001) An
Introduction to Igneous
and Metamorphic
Petrology. Prentice Hall.

20. The definitive mineral assemblages that characterize each
facies (for mafic rocks).
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)
Table 25-1. Definitive Mineral Assemblages of Metamorphic Facies
Mineral assemblages in mafic rocks of the facies of contact meta-
morphism do not differ substantially from that of the corresponding
regional facies at higher pressure.

21. It is convenient to consider metamorphic facies in 4
groups:
1) Facies of high pressure
– The blueschist and eclogite facies
– Blueschist facies occurs in areas of low T/P
gradients, characteristically developed in
subduction zones
– Eclogites are stable under normal geothermal
conditions
May develop wherever mafic magmas solidify in the
deep crust or mantle: crustal chambers or dikes, sub-
crustal magmatic underplates, subducted crust that is
redistributed into the mantle

22. 2) Facies of medium pressure
– Most metamorphic rocks now exposed belong to
the greenschist, amphibolite, or granulite facies
– The greenschist and amphibolite facies conform
to the “typical” geothermal gradient
Metamorphic Facies

23. 3) Facies of low pressure
– Albite-epidote hornfels, hornblende hornfels, and
pyroxene hornfels facies: contact metamorphic
terranes and regional terranes with very high
geothermal gradient.
Metamorphic Facies
– Sanidinite facies is
rare- limited to
xenoliths in basic
magmas and the
innermost portions
of some contact
aureoles adjacent to
hot basic intrusives

24. Metamorphic Facies
– Zeolite and prehnite-
pumpellyite facies
are thus not always
represented, and
the greenschist
facies is the lowest
grade developed in
many regional
terranes
4) Facies of low grades
– Rocks often fail to recrystallize thoroughly at very
low grades, and equilibrium is not always attained

25. Typical mineral changes that take place in metabasic rocks during progressive metamorphism in the medium P/T
facies series. The approximate location of the pelitic zones of Barrovian metamorphism are included for comparison.
Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

26. Refers to a sequence of facies observed in a transect
across a met belt.
So Barrow’s sequence: ch-bio-gr-stau-ky-sill represent
a facies series
Or green sch-ep amph- amp – gra : is facies series
Barrow’s sequence ch-bio-gr-sta-ky-sill was thought to
be typical of all orogenic belt.
Myashiro in Japan found evidence of high P and low P
type met == introduced the concept of barric type
Facies Series

27. Myashiro’s facies series
• Originally he recognized three different
facies series:
• 1. zeo-pre-pump-blue-eclogite = high P-T
or glau – jadeite (blue-eclogite)
• 2. green-ep amph-amp-granulite (typical
Barrovian sequence) = intermed P-T or ky-
sill
• 3. green-amp-granulite (Abakuma type,
similar to Buchan type) = low P-T or and to
sill

28. • Miyashiro (1961) proposed five facies series, most of
them named for a specific representative “type
locality” The series were:
1. Contact Facies Series (very low-P)
2. Buchan or Abukuma Facies Series (low-P
regional)
3. Barrovian Facies Series (medium-P
regional)
4. Sanbagawa Facies Series (high-P,
moderate-T)
5. Franciscan Facies Series (high-P, low T)
Facies Series

29. Contact series (very low P)
Buchan or Abukuma series (low P, regional)
Barrovian series (medium P, regional)
Sanbagawa series (high P, moderate T)
Franciscan series (high P, low T)

30. Figure 21-13. Some of
the paired metamorphic
belts in the circum-
Pacific region. From
Miyashiro (1994)
Metamorphic Petrology.
Oxford University Press.

31. Figure 21-1. Metamorphic field gradients (estimated P-T conditions along surface traverses directly up
metamorphic grade) for several metamorphic areas. After Turner (1981). Metamorphic Petrology: Mineralogical,
Field, and Tectonic Aspects. McGraw-Hill.

32. Plate tectonics and Facies Series
• Discovery of plate tectonics in 1960s
provided a new framework to look at met
• Myashiro observed :
facies series = f (tectonics)
Different Facies series represents different
tectonic conditions

33. Temperature-
pressure diagram
showing the three
major types of
metamorphic
facies series
proposed by
Miyashiro (1973,
1994). Winter
(2001) An
Introduction to
Igneous and
Metamorphic
Petrology.
Prentice Hall.

34. Temperature-
pressure diagram
showing the three
major types of
metamorphic
facies series
proposed by
Miyashiro (1973,
1994). Winter
(2001) An
Introduction to
Igneous and
Metamorphic
Petrology.
Prentice Hall.
SUBDUCTION
Cont Coll / Orogenic bel
Island Arc and MOR

37. The high P/T series, for example, typically occurs in subduction zones
where “normal” isotherms are depressed by the subduction of cool
lithosphere faster than it can equilibrate thermally
The facies sequence here is (zeolite facies) - (prehnite-pumpellyite
facies) - blueschist facies - eclogite facies.
The medium P/T series is characteristic of common orogenic belts
(Barrovian type)
The sequence is (zeolite facies) - (prehnite-pumpellyite facies) -
greenschist facies -amphibolite facies - (granulite facies)
Crustal melting under water-saturated conditions occurs in the upper
amphibolite facies.
The granulite facies, therefore, occurs only in water-deficient rocks,
either dehydrated lower crust, or areas with high XCO2 in the fluid
The low P/T series is characteristic of high-heat-flow orogenic belts (Buchan
or Ryoke-Abukuma type), rift areas, or contact metamorphism
The sequence of facies may be a low-pressure version of the medium
P/T series described above (but with cordierite and/or andalusite), or
the sequence (zeolite facies) - albite-epidote hornfels facies -
hornblende hornfels facies - pyroxene hornfels facies

38. • Mineral changes and associations along T-P gradients
characteristic of the three facies series
– Hydration of original mafic minerals generally required
– If water unavailable, mafic igneous rocks will remain largely
unaffected, even as associated sediments are completely re-
equilibrated
– Coarse-grained intrusives are the least permeable and likely
to resist metamorphic changes
– Tuffs and graywackes are the most susceptible
Metamorphism of Mafic Rocks

39. Plagioclase:
• More Ca-rich plagioclases become progressively unstable as
T lowered
• General correlation between temperature and maximum An-
content of the stable plagioclase
– At low metamorphic grades only albite (An0-3) is stable
– In the upper-greenschist facies oligoclase becomes stable. The An-
content of plagioclase thus jumps from An1-7 to An17-20 (across the
peristerite solvus) as grade increases
– Andesine and more calcic plagioclases are stable in the upper
amphibolite and granulite facies
• The excess Ca and Al  calcite, an epidote mineral, sphene,
or amphibole, etc., depending on P-T-X
• The principal mineral changes are due to the breakdown of the
two most common basaltic minerals: plagioclase and
clinopyroxene
Metamorphism of Mafic Rocks

40. • Greenschist  amphibolite facies transition
involves two major mineralogical changes
1. Albite oligoclase (increased Ca-content
with temperature)
2. Actinolite hornblende (amphibole accepts
increasing aluminum and alkalis at higher T)
• Both transitions occur at approximately the
same grade, but have different P/T slopes
Greenschist, Amphibolite, Granulite
Facies

41. • Mafic rocks generally melt at higher
temperatures
• If water is removed by the earlier melts the
remaining mafic rocks may become depleted
in water
• Hornblende decomposes and orthopyroxene
+ clinopyroxene appear
• This reaction occurs over a T interval > 50oC
Greenschist, Amphibolite, Granulite
Facies

42. Origin of granulite facies rocks is complex and
controversial. There is general agreement, however,
on two points
1) Granulites represent unusually hot conditions
– Temperatures > 700oC (geothermometry has
yielded some very high temperatures, even in
excess of 1000oC)
– Average geotherm temperatures for granulite
facies depths should be in the vicinity of 500oC,
suggesting that granulites are the products of
crustal thickening and excess heating
Greenschist, Amphibolite, Granulite
Facies

43. 2) Granulites are dry
– Rocks don’t melt due to lack of available water
– Granulite facies terranes represent deeply buried and
dehydrated roots of the continental crust
– Fluid inclusions in granulite facies rocks of S. Norway are
CO2-rich, whereas those in the amphibolite facies rocks
are H2O-rich
Greenschist, Amphibolite, Granulite
Facies

44. It is a long-held tenet that granulite facies terranes represent
the deeply buried and dehydrated roots of the continental
crust
Most exposed granulite facies rocks are found in the deeply
eroded areas of the Precambrian continental shields
Touret showed that the fluid inclusions in granulite facies
rocks of S. Norway are CO2-rich, while those in the
amphibolite facies rocks are more H2O-rich
This may imply that dehydration of at least some
granulite facies terranes may be due to the infiltration of
CO2 replacing H2O, rather than the elimination of fluids
altogether

45. Ocean floor met
• Met near the MOR ??
• Ocean floor spread, it is possible that
entire ocean crust is amph to granulite
• But that is not the case
• Problem? Water required to met
basalt/gabbro to convert to green/ amphi
• May be restricted to shear/fracture zone
• Spilites? Plag is albite/oli, sea water and
soda rich sed
• Spilites = low grade met : albite, ch, epido

46. Paired Metamorphic Belts of Japan
Figure 21-12. The Sanbagawa and
Ryoke metamorphic belts of Japan.
From Turner (1981) Metamorphic
Petrology: Mineralogical, Field, and
Tectonic Aspects. McGraw-Hill and
Miyashiro (1994) Metamorphic
Petrology. Oxford University Press.

47. Paired Metamorphic Belts of Japan

48. Convergent
• Paired met belt
• High P/T and Low P/T
• Subduction : zeo-pre-pump-blue-eclo
• Arc : zeo-pre-pump-green-amp-gra
• If not inverted, ancient sub zone can be
detected

50. Facies/facies series
• Facies : Static view of met, peak met
conditions, not P-T path (evolution)
• Facies series : reflect only the sequence of
peak met condition