Metamorphic FaciesMetamorphic Facies

Best, Chapter 10

Metamorphic GradeMetamorphic Grade

• Controlled by the temperature of metamorphism

• Low-grade rocks contain hydrated and carbonated phases

• High-grade rocks are dehydrated and decarbonized

Metamorphic ZonesMetamorphic 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 ReactionsMetamorphic Reactions• Formation of talc in a siliceous dolomite

Dol + Qtz + H20 = Tlc + Cal + C02• Controlled by the ratio of H20/ C02

Metamorphic FaciesMetamorphic 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 FaciesMain Facies• Zeolite• Prehnite-

pumpellyite• Blueschist• Greenschist• Amphibolite• Granulite• Eclogite

Review of FaciesReview of Facies

• Mineral assemblages in metamorphosed

mafic rocks

• Correlation of Barrow’s zones with facies

from different protoliths

• P, T diagram for various facies

Review of Phase DiagramsReview of Phase Diagrams

• 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

P,T RelationsP,T RelationsJadeite + Quartz = Albite

Open System Models Open System Models -- HH22OO

• Dehydration curves

• Example of the general

case

• Specific minerals

– Breakdown of chlorite,

muscovite, biotite, etc

Open System Models Open System Models -- COCO22

• Dehydration curves

• Example of the general

case

• Specific minerals

– Breakdown of calcite,

dolomite, etc

Univariant CurvesUnivariant 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

PP--T ExamplesT Examples

• Al2O3 phase stability

• Dehydration curves

Stability of Iron OxidesStability of Iron Oxides

• PO2 vs. Temperature diagram

• Main phases

– Hematite

– Magnetite

– Native Iron/Wustite

Miyashiro’s Facies SeriesMiyashiro’s Facies Series• Low geothermal gradient

– Zeolite, pumpellyite-prehnite, blueschist• Intermediate geothermal gradient

– Barrow’s zones• High geothermal gradient

– Andalucite present in pelitic rocks

Relation to SubductionRelation to Subduction

• High pressure series

• Medium P/T series

• High temperature series

PP--T SpaceT Space

• Multiple phase relation fields

• Facies in P-T space

• Miyashiro’s series and facies

Igneous Solidus, Wet/DryIgneous Solidus, Wet/Dry

• Granites

• Basalts

Metamorphic PathMetamorphic Path PolymetamorphismPolymetamorphism

• 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