Metamorphic Facies and Metamafic

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 yequivalent q pair p 

biotite + muscovite at Orijärvi 

• Eskola: difference must reflect differing 

physical conditions between the regions 

• Concluded that Finnish rocks equilibrated 

at llower temperatures and dhi 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 

equilibrium q through g metamorphism p at constant temperature p and ppressure 

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."


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 ti facies f i may bbe assigned i dtto 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 i l i l T dP li i 

• We can now assign relatively accurate T and Pe limits to 

individual facies


Defined on the basis of mineral assemblages that develop 

in mafic rocks 

Image: Winter (2001) 

Eskola (1920) proposed 5 

original facies: 

• Greenschist 

• Amphibolite 

• Hornfels 

• Sanidinite 

• Eclogite 

Eskola (1939) added 

• Granulite 

• Epidote-amphibolite 

• Blueschist 

Later additions 

• Zeolite 

• Prehnite-pumpellyite 

• Albite-epidote hornfels 

• Hornblende hornfels




IUGS IUGS-SCMR SCMR version of 

facies diagram 

Image: IUCS-SCMR (2007)




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 field gradient - array of peak metamorphic (max T) 

experienced in a metamorphic terrain 

Typical 

Barrovian-type 

metamorphic 

field gradient 

and a series of 

metamorphic P- 

T-t paths for 

rocks found 

along that 

gradient in 

fi field. ld 

Image: Winter (2001)


Metamorphic field gradient 


Metamorphic field 

gradients 

(estimated PP-T T 

conditions along 

surface traverses 

directly y up p 

metamorphic 

grade) for several 

metamorphic 

areas.


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 (medium-P P regional) 

4. Sanbagawa Facies Series 

(high-P, moderate-T) 

5. Franciscan Facies Series 

(high-P, low T)

Metamorphism of Mafic Rocks 

Mineral changes and associations that develop with increasing 

metamorphic grade 

• Hydrous minerals are not common in high- 

T 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


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 eeis sa general gee correlation co e o between beweeT and dmaximum u An- 

content of stable plagioclase 

• At low metamorphic grades only albite (An0-3) is stable 

• IIn upper-greenschist hi ffacies i oligoclase li l bbecomes 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


Stilbite on basalt (Poona, India). Image: Darrell 

Henry (2007) 

Mafic Assemblages at Low Grades 

Zeolite and prehnite-pumpellyite facies 

• DDo not talways l ddevelop l - ttypically i ll 

require unstable protolith 

• 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 of f 

hydrothermal metamorphism.


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 

• Chlorite, actinolite, and epidote 

impart p the ggreen 

color from 

assemblage of the greenschist facies is: chlorite 

+ albite + epidote + actinolite ± quartz 

which mafic rocks and facies get 

their name 

Image: Winter (2001)




Greenschist to 

amphibolite facies 

transition involves 2 

major j mineralogical g 

changes 


1. Transition from albite 

to oligoclase (in (increased re sed 

Ca-content of stable 

plagioclase with T) 

22. Transition from 

actinolite to 

hornblende (amphibole 

becomes able to accept 

increasing amounts of 

aluminum and alkalis at 

higher T)





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 Al- 

Fe-rich and Ca-poor mafic 

rocks and clinopyroxene in 

Al-poor-Ca-rich p 

ones




The transition from amphibolite to granulite facies occurs in the range 650-700oC Conversion of felsic gneiss (left) 

to charnockite (right -type of the 

granulite - opx granitoid). 

Kabbaldurga quarry, India Image: 

Darrell Henry 

• In presence of aqueous fluid, associated pelitic 

and quartzo-feldspathic rocks (including 

granitoids) g ) begin 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 H 2O is removed by earlier melts remaining mafic rocks 

may become depleted in water 

• HHornblende rnbl nd ddecomposes mp and nd orthopyroxene rth p r n + 

clinopyroxene appear 

• This reaction occurs over a T interval of at least 50 o C




Winter (2001) 

The granulite g 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 

• GGarnet t is i also l common, and d 

minor hornblende and/or 

biotite may be present


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 > 700 o C, geothermometry has yielded some very high T, i.e. >1000 o C 

• 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 H 2O 

• 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 H H2O-rich O ih 

• H 2O can be removed by H 2O-undersaturated melts




Winter (2001)


Mafic Assemblages of the High P/T Series: Blueschist and 

Eclogite Facies 

Mafic rocks (and not pelites) develop 

Winter (2001) 

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 

• Great density of eclogites suggests 

that subducted basaltic oceanic 

crust becomes more dense than 

surrounding mantle


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 

jadeitic j py pyroxene + qquartz: 

NaAlSi 3O 8 = NaAlSi 2O 6 + SiO 2 

Ab = Jd + Qtz 

• Assemblage jadeite + 

quartz indicates highpressure 

blueschist facies 

Winter (2001)


Eclogite Facies - mafic assemblage omphacitic pyroxene + 

pyrope-grossular garnet (Christmas-tree rocks) 

Winter (2001) 

• Along higher geothermal 

gradients amphibolite 

facies, or even the granulite 

ffacies, i may llead d to eclogite l i 

facies 

• MMuch hof f blueschist, bl hi and dall ll 

of eclogite facies are 

marked by y high-pressure 

g p 

instability of plagioclase, a 

common phase in 

metabasites of any other 

grade


Metamorphic field gradient (and PTt paths) 

(upper left) crustal thickening 

(clockwise ( (clockwise p path) ) 


(left) shallow magmatism heat heat-flow flow 

(above) ( ) model for some types yp of 

granulite facies metamorphism 

(counter (counter-clockwise clockwise path)


Winter (2001) 

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 

continuously yequilibrate 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 

• Such a cycle is called a pressure- 

temperature-time i path, h or PP-T-t T 

path



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 of f chemically h i ll zoned d 

minerals to document changing P-T 

conditions experienced by a rock 

during their growth 

Winter (2001)



Ch Chemical 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)



Winter (2001) 

Some examples of 

modeled PP-T-t T t paths 

representing common 

types of 

metamorphism 

t hi 

The paths p 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) 



Path (a) is considered typical P-T-t 

path for orogenic belt with crustal 

thickening 

• P increases >> T, because of time lag 

required i dffor hheat t ttransfer f (P equilibrates ilib t 

nearly instantaneously, but heat conducts 

very slowly through rocks) 

• Thickened crustal block quickly 

reaches P max while being relatively cool 

• NNew geotherm h iis hi higher, h bbut transient, i 

and lasts only as long as the thickened 

crust and subduction-related heat 

generation lasts

Winter (2001) 



Path (a) is considered typical P-T-t 

path for orogenic belt with crustal 

thickening 

• Erosion soon affects thickened crust 

and dPb P begins i tto ddecrease bbefore 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 gPmax 

• 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

Winter (2001) 



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) may y occur. 

represents gradation from high-P 

(Barrovian) regional metamorphism to 

“regional-contact g metamorphism” p with 

numerous plutons to local contact 

metamorphism



We may assume that the general form of a path such as (a) 

represents p a typical yp rock during gorogeny g yand regional g 

metamorphism 

1. Contrary to classical treatment of 

metamorphism metamorphism, T and P do not both increase in 

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. 

• Tmax should represent p maximum ggrade 

at which 

chemical equilibrium is "frozen in" and 

metamorphic mineral assemblage is developed 

• This occurs at P well below Pmax which is 

This occurs at P well below Pmax, which is 

uncertain since a mineral geobarometer should 

record the pressure of Tmax

Metamorphic Facies and Metamafic

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