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EA2010Metamorphic Geology
Johan Lissenberg
Room 2.13
LissenbergCJ@Cardiff.ac.uk
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Course Structure8x1hr Lectures
(including revision)
9x2hr Practicals (duplicated)(including revision & practical exam)
Exploration: 10.00-12.00
Geology: 13.10-15.00
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Course Assessment
Practical Test (40%)During last practical (22 March)
Examination (60%)
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Course Objectives
To recognise and name the common metamorphicminerals in thin section and hand specimen
To interpret metamorphic rocks in terms ofpressure-temperature-deformation time (PTDt)paths
To link metamorphism to geothermal gradients andtectonic setting.
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Metamorphic Geology
Lecture 1Introduction
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Lecture 1 Objectives
To introduce the Metamorphic Geology componentof the Module
To define and explain some basic metamorphic
concepts, including Metamorphic Facies andMetamorphic Facies Series
To introduce Chemographic Diagrams
To illustrate the metamorphic concepts usingmetamorphosed basalts in greenschist, amphiboliteand granulite facies
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Some Key Questions
What is Metamorphism?
Why Study Metamorphism?
What Causes Metamorphism?
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What is Metamorphism?
The IUGS-SCMR has proposed the following definitionof metamorphism:
Metamorphism is a subsolidus process leading tochanges in mineralogyand/or texture (for examplegrain size) and often in chemical composition in arock. These changes are due to physical and/or
chemical conditionsthat differ from thosenormally occurring at the surface of planets and in
zones of cementation and diagenesis below this
surface. They may coexist with partial melting.
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What is Metamorphism?
Thus, in simple terms, metamorphism refers to theThus, in simple terms, metamorphism refers to thechanges in rocks in response to different pressure andchanges in rocks in response to different pressure and
temperaturetemperature
The low-temperaturelimit gradesintodiagenesis
The lowest metamorphic gradeis zeolite facies. However, somezeolites are considereddiagenetic and othersmetamorphic so the boundary
is pretty arbitrary.
Typically, metamorphism beginsin the range of 100-150C.
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What is Metamorphism?
High-temperaturelimit gradesintocrustal melting
Once the rock undergoesmelting, it first becomes amixed igneous-metamorphicrock known as a migmatite.
This process usually begins at
650-850C
Thus, in simple terms, metamorphism refers to theThus, in simple terms, metamorphism refers to thechanges in rocks in response to different pressure andchanges in rocks in response to different pressure and
temperaturetemperature
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Why Study Metamorphic Rocks?
Understand global tectonic environments
Understand geological hazards
Understand mountain building
Understand the origin and evolution ofcontinents
Interpret geothermal fields
Assist in mineral exploration
Quality control of geomaterials
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Example: Mountain Building
Metamorphic maps andstudies of PTDt pathshelp in reconstructing
the history of pastmountain belts
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Example: Geothermal Systems
Understandingmetamorphic reactionshelp to understand andmaximise exploitation ofgeothermal fields
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Example: Seismogenic Zone
Metamorphic reactions determine whether fault zones slipslowly or undergo catastrophic brittle fracture
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Example: Mineral Exploration
Hydrothermally-metamorphosedrocks make a much larger targetthan the hydrothermal ore deposit
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Example: Geomaterials
Many gemstones are located in metamorphic rocks. Metamorphicrocks themselves are common ornamental stones. Production ofceramics, bricks, concretes etc, involves metamorphic reactions.
Kyanite with Ruby
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What Causes Metamorphism?
Metamorphism occurs in response to:
Temperature
Pressure
Fluids
The degree of metamorphism (without specifyingthe cause) is known as the metamorphic grademetamorphic grade.
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What Causes Metamorphism?
Heat
Heat is the most importantvariable.
The variation of temperaturewith depth is the geothermalgradient.
The geothermal gradient =dT/dZFunction of the heat input Q(from the mantle and from
radioactive elements) and thethermal conductivity (k)
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What Causes Metamorphism?
Pressure
LithostaticLithostatic pressurepressure= uniform stress (imparted by rock column= uniform stress (imparted by rock column above) above)
DeviatoricDeviatoric stressstress= unequal pressure in different directions= unequal pressure in different directions
Deviatoric stress causes foliations and/or lineations but does notchange the mineral assemblage
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What Causes Metamorphism?
Fluids
Many metamorphic reactions involve inputand output of elements (i.e. an open, ratherthan closed, system), which requires theinvolvement of fluids to effect the element
transport. The main fluid is water, butcarbon dioxide may also be important.
Metamorphism involving large chemicalchanges is often termed metasomatismmetasomatism.Metasomatism is a key part of economic
geology.
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What Causes Metamorphism?
Thermodynamics
Why do rocks change when subject to different P and T conditions?
Each mineral has a given energy content; Gibbs free energy (G)G of a mineral changes with P and T, but at different rates for
different mineralsThe system is always looking to be in the lowest energy state possible
Yardley, 1989Al2SiO5
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What Causes Metamorphism?
Thermodynamics
At equilibrium the Gibbs free energy is at a minimum for the given P-T conditions; the atoms in the rock have arranged themselves to yieldthe smallest G possible. !G=0.
Yardley, 1989
Al2SiO5
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What Causes Metamorphism?
Cornerstone of metamorphic petrology is to define the stable phasesfor rocks at different PT conditions
Al2SiO5
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What Causes Metamorphism?
During this module, you will learn to interpret mineral assemblages interms of PT conditions, and relate them to tectonic settings.
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Some Key Metamorphic Concepts
Metamorphic Facies
Metamorphic Facies Series
PTt Paths
Metamorphic Reactions
Chemographic Diagrams
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Metamorphic Facies
A metamorphic faciesis an assemblage of mineralsin metamorphic rocks that is typical of a certainfield in pressure-temperature space.
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The Boundaries betweenThe Boundaries betweenmetamorphic facies aremetamorphic facies arelinked to metamorphiclinked to metamorphic
reactionsreactions
It is not simple, though,It is not simple, though,
because reactions takebecause reactions takeplace at differentplace at different
pressures andpressures andtemperatures according totemperatures according to
compositioncomposition
The bands between theThe bands between thedifferent facies reflectdifferent facies reflectthis variabilitythis variability
Metamorphic Reactions
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Other metamorphicOther metamorphicreactions do not markreactions do not markfacies boundaries butfacies boundaries but
provide importantprovide importantinformation on P and Tinformation on P and T
The most important, andThe most important, andsimplest, involve thesimplest, involve the
polymorphs of Alpolymorphs of Al22SiOSiO55::kyanite (kyanite (KyKy),), sillimanitesillimanite
((SilSil) and Andalusite (And).) and Andalusite (And).
Metamorphic Reactions
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The MetamorphicThe MetamorphicFacies covered inFacies covered inthis course are:this course are:
BlueschistBlueschist
EclogiteEclogite
GreenschistGreenschist
AmphiboliteAmphibolite
GranuliteGranulite
HornfelsHornfels
Metamorphic Facies
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Temperature-pressureTemperature-pressurediagram showing the threediagram showing the threemajor types ofmajor types ofmetamorphic facies seriesmetamorphic facies seriesproposed byproposed by MiyashiroMiyashiro(1973, 1994).(1973, 1994).
Metamorphic Facies Series
Winter (2001)Winter (2001)
The sequence of minerals that develop during progressivemetamorphism (that is, metamorphism at progressivelyhigher temperatures) define a metamorphic facies series
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The MetamorphicThe MetamorphicFacies SeriesFacies Series
covered in thiscovered in thiscourse are:course are:
Medium P/TMedium P/TSeriesSeries
High P/T SeriesHigh P/T Series
Low P/T SeriesLow P/T Series
Metamorphic Facies Series
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The Metamorphic FaciesThe Metamorphic FaciesSeries are linked toSeries are linked tosettingsetting
Medium P/T Series:Medium P/T Series:
Passive Margins;Passive Margins;
Mountain BeltsMountain Belts
High P/T Series:High P/T Series:
SubductionSubduction ZonesZones
(Meteorite Impacts)(Meteorite Impacts)
Low P/T SeriesLow P/T SeriesVolcanically-activeVolcanically-active
terranesterranes; extensional; extensionalsettingssettings
Metamorphic Facies Series
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This in turn is linked to theThis in turn is linked to thegeothermal gradient.geothermal gradient.
Medium P/T Series:Medium P/T Series:
20-5020-50C/kmC/km
High P/T Series:High P/T Series:
50C/kmC/km
Metamorphic Facies Series
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An important principle of metamorphism is that differentAn important principle of metamorphism is that different
rocks give different metamorphic assemblages at the samerocks give different metamorphic assemblages at the sameP and TP and T i.ei.ethe mineral assemblages depend on P, T and Xthe mineral assemblages depend on P, T and X(composition)(composition)
Chemographic Diagrams
1.Ultramafic -very high Mg, Fe, Ni, Cr2.Mafic -high Fe, Mg, and Ca3.Shales (pelitic) -high Al, K, Si4.Carbonates-high Ca, Mg, CO2
5.Quartz-nearly pure SiO2.6.Quartzo-feldspathic -high Si, Na, K, Al
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Two diagrams you will encounter in this course are the ACF and AKFdiagrams. They are simplifications, in that they replace the 8 major
elements by three variables, but they can be used to:1) Identify the protolith (the type of rock before metamorphism)
given the mineral assemblage
2) Predict the mineral assemblage given the whole-rock composition
Chemographic Diagrams
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Chemographic Diagrams
For the ACF diagram, the three pseudo-components are allcalculated on an atomic basis:
A = Al2O3+ Fe2O3- Na2O - K2O
C = CaO - 3.3 P2O5
F = FeO + MgO + MnO
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Chemographic Diagrams
Minerals plot at differentpoints on the diagram. Forexample
Anorthite CaAl2Si2O8
A= 1 + 0 - 0 - 0 = 1, C= 1 - 0 =1, and F=0
Provisional values sum to 2, sowe can normalize to 1 bymultiplying each value by #,resulting in
A=0.5C=0.5
F=0
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Chemographic Diagrams
Note:SiO2is not represented in theACF diagram, so anyassemblage may have quartz aswell
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Chemographic Diagrams
Identifying the protolith (the type of rock before metamorphism) giventhe mineral assemblage:
Connect the observed minerals by tie lines. The bulk composition will liein the triangle represented by minerals stable in a particular facies e.g. most metabasic rocks will lie within the red triangle
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Chemographic Diagrams
Predict the mineral assemblage given the whole-rock composition:
Plot the bulk composition. The tie lines will show the minerals stable inthis rock in a particular facies e.g. most metabasic rocks ingreenschist facies will have chlotite + epidote + actinolite
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Chemographic Diagrams
Note:There are generally 3 stable phases. You will encounter rockswith more than 3 phases, but this may mean that one of themis in the process of being reacted out
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Example (Practical 1): ProgressiveMetamorphism of Basalt at Medium P/T
Zeolite Facies
Prehnite-Pumpellyite Facies
Greenschist
Amphibolite
Granulite
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Example (Practical 1): ProgressiveMetamorphism of Basalt at Medium P/T
Zeolite Facies: zeolites, especially laumontite, wairakite, analcime
Prehnite-Pumpellyite Facies: prehnite + pumpellyite (+ chlorite +albite)
Greenschist Facies: chlorite + albite + epidote (or zoisite) + quartz actinolite
Amphibolite Facies: hornblende, plagioclase (oligoclase-andesine) garnet
Granulite Facies: orthopyroxene (+ clinopyroxene + plagioclase garnet hornblende)
Definitive Mineral Assemblages of Metamorphic Facies
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Greenschist Facies
Key Minerals
Albite
Quartz
Chlorite
Epidote
Actinolite
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Greenschist Facies
Key Minerals
Albite
Quartz
Chlorite
Epidote
Actinolite
+ quartz + albite
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Amphibolite Facies
Key Minerals
PlagioclaseHornblende
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Amphibolite Facies
Key Minerals
Plagioclase
Hornblende
(garnet)
+ quartz + albite
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Granulite Facies
Key Minerals
Orthopyroxene
Clinopyroxene
Calcic plagioclase
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Granulite Facies
Key Minerals
Orthopyroxene
Clinopyroxene
Calcicplagioclase
(garnet)
+ quartz + albite
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Metabasalt: Metamorphic Changes