GEOL 2312
IGNEOUS AND METAMORPHIC
PETROLOGY
Lecture 19
Metamorphic Facies and
Metamorphosed Mafic Rocks
April 4, 2016
METAMORPHIC FACIES
DEVELOPMENT OF THE CONCEPT
V.M. Goldschmidt (1911, 1912a)
Studied contact metamorphosed pelitic (shales), calcareous (limestones),
and psammitic (sandstone) hornfels in the Oslo region. Found
relatively simple mineral assemblages (< 6 major minerals) in the
inner zones of the aureoles around granitoid intrusives
Determined that equilibrium mineral assemblage related to Xbulk of the
protolith
Also noted that certain mineral pairs (e.g. anorthite + hypersthene)
were consistently present in rocks of appropriate composition,
whereas the compositionally equivalent pair (diopside + andalusite)
was not
If two alternative assemblages are compositional equivalents, we must
be able to relate them by a reaction
In this case the reaction is simple:
MgSiO3 + CaAl2Si2O8 = CaMgSi2O6 + Al2SiO5
En
An
Di
Als
METAMORPHIC FACIES
DEVELOPMENT OF THE CONCEPT
Pentii Eskola (1914, 1915) Orijärvi, S. Finland
Rocks with K-feldspar + cordierite at Oslo contained the
compositionally equivalent pair biotite + muscovite at Orijärvi
Deduced from thermodynamic principles that the Finnish rocks were
more hydrous and lower volume assemblage and they equilibrated at
lower temperatures and higher pressures than the Norwegian ones
2 KMg3AlSi3O10(OH)2 + 6 KAl2AlSi3O10(OH)2 + 15 SiO2 (Orihjärvi)
Bt
Ms
= 3 Mg2Al4Si5O18 + 8 KAlSi3O8 + 8 H2O
Crd
Kfs
(Oslo)
Qtz
METAMORPHIC FACIES
DEVELOPMENT OF THE CONCEPT
Eskola (1915) developed the concept of metamorphic facies:
“In any rock or metamorphic formation which has arrived at a chemical
equilibrium through metamorphism at constant temperature and
pressure 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 the Xbulk & mineralogy
If we find a specified assemblage (or better yet, a group of compatible
assemblages covering a range of compositions) in the field, then a
certain facies may be assigned to the area
Interpretive - the range of temperature and pressure conditions
represented by each facies
Eskola was aware of the P-T implications and correctly deduced the relative
temperatures and pressures of facies he proposed. We can now assign
relatively accurate temperature and pressure limits to individual facies
METAMORPHIC FACIES
DEVELOPMENT OF THE CONCEPT
Eskola (1920) proposed 5 original facies:
 Greenschist
Easily defined on the basis of
 Amphibolite
mineral assemblages that develop in
 Hornfels
mafic rocks, which are abundant in
most terranes and mineral changes
 Sanidinite
define a broad range of P & T
 Eclogite
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
DEVELOPMENT OF THE CONCEPT
Temperature
Sanadinite
Facies
Pressure
Formation of Zeolites
Greenschist
Facies
EpidoteAmphibolite
Facies
Amphibolite
Facies
PyroxeneHornfels
Facies
Granulite
Facies
GlaucophaneSchist Facies
Eclogite
Facies
Winter (2001) Fig. 25-1 The metamorphic facies proposed by Eskola and their relative
temperature-pressure relationships. After Eskola (1939) Die Entstehung der
Gesteine. Julius Springer. Berlin.
METAMORPHIC FACIES
CURRENTLY ACCEPTED FACIES DESIGNATIONS
Boundaries
based on
isograds
(mineral-in)
Winter (2001) Fig. 25-2.
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).
METAMORPHIC FACIES
CURRENTLY ACCEPTED FACIES DESIGNATIONS
Table 25-1. Definitive Mineral Assemblages of Metamorphic Facies
Facies
Zeolite
Definitive Mineral Assemblage in Mafic Rocks
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
FOUR FACIES GROUPS
High Pressure – Subduction Zones
Medium Pressure Orogenic Regions
Low Pressure Contact Auroles
Low Grades –
Burial Metamorphism
METAMORPHIC FACIES
ISOGRADS AND ZONES
An isograd represents the first
appearance of a particular
metamorphic index mineral in the
field as one progresses up
metamorphic grade. When one
crosses an isograd, such as the
biotite isograd, one enters the biotite
zone.
Because classic isograds are based on
the first appearance of a mineral,
and not its disappearance, an index
mineral may still be stable in higher
grade zones
BASED ON METAMORPHIC
REACTIONS IN PELITIC ROCKS
METAMORPHIC FACIES
FACIES AND ZONES
Winter (2001) Fig. 25-9. 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.
METAMORPHIC FACIES
SERIES
Relates
progressive
metamorphism
in a particular
tectonic regime
Winter (2001) Fig. 25-3.
Temperature-pressure
diagram showing the
three major types of
metamorphic facies
series proposed by
Miyashiro (1973, 1994).
METAMORPHISM OF MAFIC ROCKS
GENERAL CONSIDERATIONS

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

Plagioclase: More Ca-rich plagioclases become progressively
unstable as T lowered
General correlation between T and maximum An-content of the stable plagioclase
The excess Ca and Al  calcite, an epidote mineral, sphene, or amphibole, etc.,
depending on P-T-X

Clinopyroxene – Depending on grade CPX breaks down to a
number of mafic minerals - chlorite, actinolite, hornblende,
epidote, a metamorphic pyroxene, etc.
METAMORPHISM OF MAFIC ROCKS
HYDRATION OF MAFIC ROCKS
Subaerial basalts Minimally hydrated
Mafic graywacke –
Strongly hydrated
Submarine basalts Strong hydrothermal alteration
METAMORPHISM OF MAFIC ROCKS
Waterbearing
minerals
LOW GRADE METAMORPHISM
Hydrated protolith
is critical to
initation of low
grade reactions
Zeolites
Zeolite amygdules in North Shore Volcanics
METAMORPHISM OF MAFIC ROCKS
MEDIUM PRESSURE SERIES
Greenschist Facies
Ep+Chl+Act+Alb±Qtz
Water-bearing Phases
Amphibolite Facies
Hb+Pl(>An17)±Gt±Cpx
Amphibolite-Granulite Facies:
Melting possible if
sufficient H2O
Granulite Facies
Opx+Pl(>An35)+Cpx ±Gt
METAMORPHISM OF MAFIC ROCKS
HIGH PRESSURE SERIES
Blueschist Facies
Glaucophane/Jadite (Na)
+ Lawsonite/Epidote (Ca-Al)
± Garnet (Fe-Mg)
(± Aragonite, Paragonite, Chlorite,
Stilpnomelane, Qtz, Albite, Sericite)
Eclogite Facies
Omphacite(Na-Cpx)
+ Pyrope-Almandine
((Fe,Mg)Al Garnet)
± Kyanite
± Opx