MetPet 1 Intro

7/26/2019 MetPet 1 Intro

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EA2010Metamorphic Geology

Johan Lissenberg

Room 2.13

[email protected]


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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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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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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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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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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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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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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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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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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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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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Cornerstone of metamorphic petrology is to define the stable phasesfor rocks at different PT conditions

Al2SiO5


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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



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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).



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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).


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



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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



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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



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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)


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



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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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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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Note:SiO2is not represented in theACF diagram, so anyassemblage may have quartz aswell


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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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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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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

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MetPet 1 Intro

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