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Metamorphic Grade
Metamorphic Facies
Best, Chapter 10
• Controlled by the temperature of
metamorphism
• Low-grade rocks contain hydrated and
carbonated phases
• High-grade rocks are dehydrated and
decarbonized
Metamorphic Zones
• Mappable metamorphic units of similar
grade in a rock of distinct composition
• Isograds are lines marking the first
appearance of key minerals
• Zones of Barrow
– Biotite, garnet, staurolite, kyanite, sillimanite
Metamorphic Reactions
• Formation of talc in a siliceous dolomite
Dol + Qtz + H20 = Tlc + Cal + C02
• Controlled by the ratio of H20/ C02
Metamorphic Facies
• The set of mineral assemblages occurring in
rocks of diverse composition
• Facies develop under restricted P,T
conditions
• Mineral assemblages may be plotted on
ACF diagrams
Main Facies
• Zeolite
• Prehnitepumpellyite
• Blueschist
• Greenschist
• Amphibolite
• Granulite
• Eclogite
Review of Facies
• Mineral assemblages in metamorphosed
mafic rocks
• Correlation of Barrow’s zones with facies
from different protoliths
• P, T diagram for various facies
P,T Relations
Review of Phase Diagrams
Jadeite + Quartz = Albite
• Solid-solid reactions
• Governed by Clapeyron equation
– dP/dT = 10 ∆H/T ∆V = ∆S/∆V
– ∆H is the heat of reaction
– ∆S is the change in entropy
– ∆V is the change in volume
• The slope of the stability is dP/dT
Open System Models - H2O
Open System Models - CO2
• Dehydration curves
• Dehydration curves
• Example of the general
• Example of the general
case
• Specific minerals
– Breakdown of chlorite,
muscovite, biotite, etc
case
• Specific minerals
– Breakdown of calcite,
dolomite, etc
Univariant Curves
• Curves that define reactions with one
degree of freedom
• In P-T space this means that if T is changed,
than P must also change to maintain
equilibrium
• Many important metamorphic reactions are
defined by these curves
P-T Examples
• Al2O3 phase stability
Stability of Iron Oxides
• PO2 vs. Temperature diagram
• Main phases
• Dehydration curves
– Hematite
– Magnetite
– Native Iron/Wustite
Miyashiro’s Facies Series
Relation to Subduction
• Low geothermal gradient
– Zeolite, pumpellyite-prehnite, blueschist
• Intermediate geothermal gradient
– Barrow’s zones
• High geothermal gradient
– Andalucite present in pelitic rocks
• High pressure series
• Medium P/T series
• High temperature series
P-T Space
Igneous Solidus, Wet/Dry
• Multiple phase relation
fields
• Granites
• Facies in P-T space
• Basalts
• Miyashiro’s series and
facies
Metamorphic Path
Polymetamorphism
• Sometimes there are repeated episodes of
metamorphism
• The last event may be weak or of short
duration
• Polymetamorphism is common in post
tectonic environments and in contact
aureoles