Metamorphic Zones, Index Minerals,
Isograds, Facies and Facies Series
the onslaught of terminology to understand how we categorize
metamorphic rocks and their conditions of formation!
What textures do you see?
Review: Types of Protolith
1.Pelitic/mudrocks - high Al, K, Si
2. Quartzo-feldspathic - high Si, Na, K, Al
3. Calcareous- high Ca, Mg, CO2
4. Mafic- high Ca, Mg, Fe
5. Ultramafic- very high Mg, Fe, low Si, Al
WHY IS CHEMICAL COMPOSITION OF
PROTOLITH IMPORTANT?
Metamorphic Grade
Refers to maximum T or P of
metamorphism
What grades have we talked about?
The idea of grade is general. We can better
express the maximum P, T constraints
using the concepts of metamorphic zone
and facies. More on this!
Orogenic Regional Metamorphism of
the Scottish Highlands: Development of
the Index Mineral Concept
• George Barrow
(1893, 1912)
• SE Highlands of
Scotland Caledonian Orogeny
~ 500 Ma
• Lots of folding
• Granites
Barrow’s
Area
Figure 21-8. Regional metamorphic
map of the Scottish Highlands,
showing the zones of minerals that
develop with increasing
metamorphic grade. From Gillen
(1982) Metamorphic Geology. An
Introduction to Tectonic and
Metamorphic Processes. George
Allen & Unwin. London.
Orogenic Regional Metamorphism of
the Scottish Highlands
• Barrow studied pelitic rocks
• Could subdivide the area into a series of
metamorphic zones, each based on the
appearance of a new mineral as metamorphic
grade increased
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)
Each of these minerals is an
INDEX mineral.






Chlorite zone
Biotite zone
Garnet zone
Staurolite zone
Kyanite zone
Sillimanite zone
WHAT IS AN INDEX MINERAL
• Sequence = “Barrovian zones”
• The P-T conditions referred to as “Barrovian-type”
metamorphism (fairly typical of many belts)
• Now extended to a much larger area of the Highlands
• ANOTHER DEFINTION:
• Isograd
line that separates the zones (a line in the field
of constant metamorphic grade). Also reflects
the FIRST APPEARANCE of the index
mineral.
Figure 21-8. Regional
metamorphic map of the
Scottish Highlands, showing
the zones of minerals that
develop with increasing
metamorphic grade. From
Gillen (1982) Metamorphic
Geology. An Introduction to
Tectonic and Metamorphic
Processes. George Allen &
Unwin. London.
To summarize:
• 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
• Zones thus have the same name as the isograd that forms
the low-grade boundary of that 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
A variation occurs in the area just to the north of
Barrow’s, in the Banff and Buchan district
• Pelitic compositions are similar, but the sequence
of isograds is:
 chlorite
 biotite
 garnet
 andalusite
 sillimanite
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
1 GPa = 10kbars
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 (Berman, 1988, 1990, 1991).
Metamorphic Facies
• Eskola (1915) developed the concept of
metamorphic facies:
• What is a metamorphic facies?
Metamorphic Facies
Fig. 25-2. Temperaturepressure 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.
Metamorphic Facies
• The range of temperature and pressure conditions
represented by each facies
 Eskola aware of the P-T implications and correctly
deduced the relative temperatures and pressures of
facies he proposed
 Can now assign relatively accurate temperature and
pressure limits to individual facies
Metamorphic Facies
Eskola (1920) proposed 5 original facies:
 Greenschist
 Amphibolite
 Hornfels
 Sanidinite
 Eclogite
• Easily defined on the basis of mineral
assemblages that develop in mafic rocks
• More facies have been added since original
designations
Metamorphic Facies
Temperature
Sanadinite
Facies
Pressure
Formation of Zeolites
Greenschist
Facies
EpidoteAmphibolite
Facies
Amphibolite
Facies
PyroxeneHornfels
Facies
Granulite
Facies
GlaucophaneSchist Facies
Eclogite
Facies
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
Fig. 25-2. Temperaturepressure 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.
Metamorphic Facies defined for
mafic protolith
• The definitive mineral assemblages that characterize each facies
(for mafic rocks).
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.
It is convenient to consider metamorphic facies in 4 groups:
1) Facies of high pressure



The blueschist and eclogite facies: low molar volume
phases under conditions of high pressure
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
Metamorphic Facies
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
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. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Metamorphic Facies
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.
Sanidinite facies is
rare- limited to
xenoliths in basic
magmas and the
innermost portions of
some contact aureoles
adjacent to hot basic
intrusives
Metamorphic Facies
4) Facies of low grades

Rocks often fail to recrystallize thoroughly at very low
grades, and equilibrium is not always attained

Zeolite and prehnitepumpellyite facies are
thus not always
represented, and the
greenschist facies is
the lowest grade
developed in many
regional terranes
Metamorphic Facies
Review
• Metamorphic zone (e.g., chlorite zone)
• Index Mineral
• Isograd
• Metamorphic Facies
Facies Series/Field Gradient
A traverse up grade through a metamorphic terrane should
follow one of several possible metamorphic field
gradients, and, if extensive enough, cross through a
sequence of facies
Field gradient
Fig. 25-3.
Temperaturepressure 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.
Pressure-Temperature Time Paths
•Facies concept leads to idea that
metamorphic petrologists try to reconstruct
CONDITIONS of metamorphism.
•Also important is TIME. Time tells us about
the RATES of processes.
Regional Metamorphism
3 stages:
Burial/crustal thickening-why does trajectory have
steep slope?
Heating stage
Uplift stage
Regional Metamorphism
What are prograde vs.
retrograde metamorphic
paths or reactions?
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. McGrawHill.
Example of Contact Metamorphism
What does this diagram
show? ------------------>
Explain how the
metamorphic grade and
assemblages MIGHT change
with distance from this dike.