metamorphism and crustal evolution summary 2012-13

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ENVS212 Metamorphism Summary Page 1 of 20

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ENVS212 Metamorphism and Crustal Evolution: SUMMARY

2012-2013

Preliminaries Some prior knowledge of Earth structure, igneous and sedimentary rocks, minerals and

rock-forming elements is assumedyou will have taken modules in these subjects already.

In practicals, I will expect you to recall what you have been taught previously

demonstrators and lecturers are there to help but you must take the initiative.

Practicals may contain work which you must finish in your own time. It isyourresponsibility to do this

It isyourresponsibility to make sure you understand the material in this (and everyother) module. To do this, use not just the lecture notes but textbooks, web resources, and

discussions (with your peers, demonstrators and me).

Some key new concepts appear in bold the first time they appear, and each bold word is listed

in the Index at the end of this document; examples of concepts are given in brackets. In the

second part of the module, where more diverse reading is required, yellow boxes indicate

essential reading and white boxes additional reading.

METHOD OF ASSESSMENT - 15 credits total

2012-13

Continuous Assessment 1 under exam conditions (20% of total credits). It will occur during

the practical slot on2:00-4:30 Tuesday 30 October 2012 (Week 6)

This will test your skills in optical microscopy and petrography, and simple paper exercises

concerning PT grids and balancing reactions.

Continuous Assessment 2 under exam conditions (20% of total credits). It will occur during

the practical slot on

2:00-3:30 Tuesday 4 December 2012 (Week 11)

This will test your understanding of compatibility diagrams and thermodynamics.

You will be allowed to take in ANY books and lecture notes: these are open book exams.You will need: Calculator, Ruler, Colour pencils. Mobile phones and laptops NOT allowed

Theory Examination (2 hours, 60% of total credits) to be held in January 2013 (NOT open

bookthis will test knowledge as well as understanding)

Three sections, equally weighted

Section A: One question from a choice of 2: basic definitions

Section B: One question from a choice of 2: metamorphic principles

Section C: One question from a choice of 2: Caledonides case study


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TEXTBOOKS

ESSENTIAL

Winter, J.D., 2009. An Introduction to Igneous and Metamorphic Petrology. 2nd edition. Pearson

Higher Education, 702 pp. This covers much of the material in this module, and powerpoints of the

diagrams are available on the authors website.

Jones, K. & Blake, S. 2003. Mountain Building in Scotland. Open University Worldwide

OTHER TEXTS

Yardley, B.W.D. 1989. An introduction to metamorphic petrology. Longman Earth Science Series.

This is a very good book which contains much material relevant to metamorphic work, but superceded

by Winter.

Miyashiro, A. 1994. Metamorphic Petrology. UCL Press.

Quite detailed on the minutiae of equilibria in various rock types, and has some discussion of the

relation of metamorphism, tectonics and thermal modelling. No reference to kinetics, so book as a whole

is unbalanced. However has useful stuff on "Composition - Paragenesis Diagrams" (compatibility

diagrams).

Deer, W.A., Howie R.A. and Zussman, J.1966. An introduction to the rock forming minerals.

Longman.

McKenzie, W.S. & Guilford, C. 1980. Atlas of rock-forming minerals in thin section. Longman.The last two are to do with mineral identification.

Yardley, B. W. D., MacKenzie, J. K. & Guilford, C. 1990. Atlas of metamorphic rocks and their

textures. Longman Scientific and Technical, London.

Nice pictures of thin sections showing metamorphic minerals and textures.

These other books are largely superceded by Winter, but they still contain much useful information.

Ehlers, E.G. 1972. The interpretation of geological phase diagrams. Dover.

A thorough discussion of phase diagrams of use in both metamorphic and igneous geology, packed with

illustrations. May help to clarify the basic principles.

Wood, B.J. & Fraser, D.G. 1976. Elementary thermodynamics for geologists. Oxford Univ. Press.

Powell, R. 1978 Equilibrium Thermodynamics in Petrology. Harper & Row.

Strachan, R. & Woodcock, N. H. 2000. Geological history of Britain and Ireland. Blackwell

Scientific, Oxford.

There are multiple copies in the Harold Cohen library (7 day loan). Look at chapters 5-7 in relation to

the lecture material.


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Lecture 1 Metamorphism in the field and laboratory

Rocks are found which have unfamiliar minerals and textures not seen in igneous or sedimentary rocks (garnet,

kyanite)

They may have familiar larger-scale features (cross-bedding, pillows)

They may have bulk chemistry similar to known igneous and sedimentary rocks

The distribution of unfamiliar minerals can be mapped

Patterns in a given lithology often vary systematically with position

Some patterns are concentric relative to plutons, suggesting heat may be relevant to the explanation (Fanad, Ireland;

Bergell, Italy)

We can test this idea in lab experiments

Experiments show minerals and textures change as T is raised, without melting (biotite and quartz change to K-

feldspar, orthopyroxene and steam)

Not all of the known minerals can be made this way

High temperatures in the Earth are known to be associated with high pressures

Experiments show that minerals and textures also change as P is raised (albite changes to jadeite and quartz)

A mixture of minerals - a mineral assemblage - may change to another assemblage in an experiment

We interpret unfamiliar mineral assemblages in rocks as being formed by changing P and T conditions, withoutmelting

Metamorphism is defined as the alteration, in the solid state, of the mineral assemblages and textures in rocks as P

and T change and/or the rocks are deformed

A change occurring at a given P and T in one experiment can be reproduced in other experiments

If an assemblage can be left in an experiment for a long time at a certain P and T without changing, it is said to be

stable at those P,T conditions

The set of all P,T conditions for which an assemblage is stable is called the stability field

A picture can be drawn with P and T as axes illustrating the stability field of an assemblage as an area - these

pictures are called PT phase diagrams orPT grids

Stability fields on PT diagrams can be matched to assemblages recorded in rocks, and this implies the PT conditions

to which the rocks were subject

You will sometimes see a textbox like this at the end of the notes which will point to extra information. But, you

should always revise each lecture using textbooks, in particular Winter. You will find the right sections via chapter

headings, figure numbers (when Ive used those figures in lectures), looking up keywords in the index, or just

browsing.


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Lecture 2 Controls on metamorphic mineralogyDifferent types of metamorphism can be defined

Contact metamorphism is localised round igneous intrusions which supplied the heat

Regional metamorphism affects large areas of mountain belts, usually associated with deformation

Dynamic metamorphism is alteration of texture due to deformation without the growth of new minerals

Hydrothermal metamorphism involves growth of new minerals by reaction with hot fluids (ocean floor)

The term grade is used to indicate how extreme the PT conditions were to which the rock was subject

In a in a given layer of a layered sequence, different mineral assemblages can be mapped out (Dalradian

metasediments)

Assemblages of different grades in a given layer occur in different regions on a map - these are called metamorphiczones

Zones are named after a characteristic orindex mineral in the assemblage (chlorite zone, biotite zone)

The lines on a map separating different zones are of fixed grade and called isograds

These lines are the map traces of isograd surfaces in 3D which are not necessarily planar or vertical

Isograds are often named after the index mineral which is first found walking up-grade across the isograd (biotite-in

isograd separates chlorite and biotite zones)

The lower grade index minerals does not always disappear when the isograd is crossed

If an index mineral disappears walking up grade, an isograd can be defined by its disappearance (chlorite-out

isograd)

An isograd mapped in one rock type may not be mappable in an adjacent layer of different bulk rock composition

This may be because the conditions were different. More likely is that the rock did not have the right elements to

manufacture the index minerals (pure quartz sandstone cannot make biotite)

Different bulk rock compositions can affect the grade at which index minerals appear (chlorite in metapelites and

metabasites)

The mineral assemblages seen are governed by PT and by bulk rock composition - so its is not straightforward toconstrain P and T from observations

Some minerals are polymorphs, having the same chemical composition but different structure (andalusite, kyanite,

sillimanite). The appearance of one polymorph rather than another in a rock is an indicator of grade regardless of the

other minerals in a rock


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Lecture 3 Metamorphic zonal sequences and equilibrium

Some polymorphs have more open structures than others

The less dense assemblage always appear on the low-pressure side of a reaction line (kyanite high-P relative to

sillimanite, andalusite low-P relative to both, water forms at higher P relative to ice and steam)

Regions may be characterised by a zonal sequence of progressively increasing grades of metamorphism (the

Barrovian sequence has metapelite index minerals chlorite, biotite, garnet, staurolite, kyanite, sillimanite)

Not all zonal sequences are the same (some have andalusite)

PT grids tell us something about the PT conditions reached in a zone and the spatial PT variations in a zonal

sequence

BUT there is a problem: the rocks we sample are at the surface are at very low pressure and temperature, yet the

minerals they contain indicate high pressures and temperatures if we believe the experiments

The missing idea is the effect oftime

If we run an experiment to transform minerals into other minerals, the transformation does not happen

instantaneously

Eventually, in any experiment, all transformations will be complete

The state in which nothing further is happening is called equilibrium

PT grids are used to represent equilibrium assemblages

The approach to equilibrium depends on time

The rate of approach to equilibrium depends on temperature (hotter conditions mean equilibrium is reached more

quickly)

Rocks are likely to record minerals formed at the highest temperatures which have been experienced

The history of any rock can be expressed in a pressure-temperature-time path orPTt path for short, which

describes the changing P and T the rock has experienced (many rocks start cool at the surface, get buried and

heated, then get uncovered and cooled)

Rocks now at the surface often preserve minerals which were in equilibrium at the maximum PT orpeak

metamorphic conditions

The part of the PTt path before the peak is called prograde, that after is retrograde

Growth of minerals on the retrograde path is called retrogression

Because minerals are not in equilibrium it is difficult to determine PTt histories - we need more information (how

can we tell prograde from retrograde minerals?)

http://www.minweb.co.uk/
http://www.minweb.co.uk/http://www.minweb.co.uk/http://www.minweb.co.uk/


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Lecture 4 Textures, fabrics and evolution

Textures may provide that information: texture summarises all the geometric information in a rock (for each

mineral: grain size, grain shape, orientation, spatial distribution etc.)

If some minerals form much larger grains than the rest, and they grew during metamorphism, these are referred to as

porphyroblasts set in a finer grained matrix

If the large grains predated metamorphism and are relics, they are instead porphyroclasts

Some large grains entirely surround inclusions of smaller grains: the large grains are then called poikiloblasts

We can try to interpret any texture in any rock by drawing (or imagining) a sequence of states that led to the final

texture - like making a texture movie

Poikiloblasts are interpreted as large grains that gradually grew to surround the inclusions - hence the inclusions are

older than the poikiloblast, and we obtain information on relative timing

Pseudomorphs are clusters of grains with a single euhedral outline, interpreted as new minerals replacing, and

filling the outline of, an older grain (chlorite after garnet)

A fabric is any oriented texture

A grain shape fabric is any fabric formed by the lining up (preferred orientation) of platy or lathy mineral grains

Shape fabrics can be planar (as in slates and schists) orlinear or both

A location fabric is formed by the concentration, into layers or rods, of certain minerals - the minerals may

themselves be equant

Location fabrics can be planar (banding, as in gneisses) orlinear (rodding) or both

Grain shape fabrics may be due to strain (shape change) of original mineral grains; growth of new phases or

recrystallisation of existing ones; rotation of original grains; or a combination of all three

Location fabrics may be due to high strain of pre-existing objects so they have become sheet-like; segregation of

minerals during metamorphism; inherited original banding; or a combination of all three

Poikiloblasts may include oriented inclusions indicating that a shape fabric formed before poikiloblast growth

Fabrics usually form during deformation and relate to the strain history of the rock

DIRECTED READING

Find out for yourself the definitions of

hornfels (with "hornfelsic texture")

granulite (with "granulitic texture")

slate, schist (with "schistosity"), gneiss (with "gneissosity")

protolith

Particularly in relation to the points listed above, read and understand ..

Winter Chapter 22.classification


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Winter Chapter 23 .4.1 to 23.4.3origins of textures and fabrics

Winter 23.4.5 to 23.4.6unravelling episodes of deformation and mineral growth

For additional background ..

http://www.bgs.ac.uk/SCMR/

http://www.bgs.ac.uk/SCMR/docs/scmr_how_r4.pdf p. 15
http://www.bgs.ac.uk/SCMR/http://www.bgs.ac.uk/SCMR/http://www.bgs.ac.uk/SCMR/docs/scmr_how_r4.pdfhttp://www.bgs.ac.uk/SCMR/docs/scmr_how_r4.pdfhttp://www.bgs.ac.uk/SCMR/docs/scmr_how_r4.pdfhttp://www.bgs.ac.uk/SCMR/


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Lecture 5 Reactions and metamorphic facies

During metamorphism new mineral grains grow from a supply of the right atoms

The atoms required are given by the mineral formula

Most metamorphic minerals are solid solutions (biotite can have Mg replaced by Fe)

The atoms required for growth must have come from breakdown of other minerals if the bulk composition is fixed

The combined growth and breakdown of phases without atoms added from outside the rock is called metamorphic

reaction

We can guess which minerals broke down by looking at the minerals in the same rock type at different grades (so

infer the Fe in biotite could come from chlorite)

Usually the breakdown of only one mineral is not sufficient to supply all the atoms required for a new mineral(biotite has K which could not come from chlorite)

Several minerals are usually required to break down, and their atoms go into more than one new phase

If a reaction can be written as [list ofreactants] = [list ofproducts] with the same numbers of each element on

either side of the equation it is said to be chemically balanced (1 chlorite + 6 orthoclase = 3 muscovite + 3 biotite +

5 quartz + 3 water)

Only balanced reactions have precisely defined reaction lines on a PT grid which separate the stability field of the

reactants from that of the products

So, we need to know the details of the reaction which grew a mineral to enable us to determine the PT conditions

under which it grew

Balancing often predicts that water would be produced during prograde reactions, because product minerals have

less H - these are dehydration reactions

Mineral assemblages depend not just on grade but on rock type (metabasites have different assemblages to

metapelites)

In the chlorite, biotite and garnet metapelite zones, metabasites often have actinolite, epidote, chlorite, albite and

quartz

In the staurolite, kyanite and sillimanite metapelite zones, metabasites may have hornblende, plagioclase and garnet

In the high grade sillimanite-orthoclase metapelite zone, metabasites may have orthopyroxene, clinopyroxene and

plagioclase (rather like the original igneous mineralogy)

Other rock types such as marbles have yet other mineral assemblages at these grades

We need a general term for the grade of a rock without reference to its particular minerals: this term is

metamorphic facies (the chlorite and garnet metapelite zones are in the greenschist facies)

The metamorphic facies can be visualised as regions in PT space


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Lecture 6 Diversity in metamorphic facies

A particular sequence of metamorphic zones corresponds to a sequence of facies (the Barrovian sequence

corresponds to parts of greenschist, amphibolite and granulite facies)

But not all metamorphic rocks fit in this sequence of conditions (minerals not found in the Barrovian facies

sequence include andalusite, cordierite, jadeite)

Contact metamorphism may give a different zonal sequence (aureole of Skiddaw granite, Cumbria, has cordierite,

then higher grade andalusite)

Different plutons show some variations (Ballachulish pluton, Scotland, has, cordierite, andalusite, then andalusite

plus orthoclase zones)

Regional metamorphism may also be associated with such zones (NE Scotland has regional zones: chlorite, biotite,

cordierite, andalusite, sillimanite, orthoclase-sillimanite; this is called the Buchan zonal sequence)

The last three examples are due to hotter and/or lower pressure metamorphism than is responsible for the Barrovian

sequence and fit into a low-pressure facies sequence (hornblende hornfels, pyroxene hornfels etc.)

Other regional metamorphic terrains have zonal sequences which are yet again different (eastern Japan has

glaucophane, aragonite)

These are due to colder and/or higher pressure metamorphism than is responsible for the Barrovian sequence and fit

into a high-pressure facies sequence (blueschist, eclogite)

A given zonal sequence does not have a fixed P and T but is characterised by a general trend in P and T, or by its

P/T ratio

The general P/T ratio defines the baric type of the metamorphism (the Barrovian sequence is of intermediate

pressure baric type; the blueschist and eclogite facies are of high pressure baric type)

To explain such diverse PT conditions, a model is required for distribution of P and T in the upper Earth

Pressure = Density x Depth x Acceleration of gravity

So pressure can usually be interpreted in terms of burial depth


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Lecture 7: Metamorphism, geotherms and tectonics

Temperature is controlled by the combined effect ofheat production and heat transport

Heat is produced by radioactivity, some in the continental crust, some from deep in the mantle

Heat is transported by conduction and advection

Conduction is the flow of heat from high to low temperatures without any movement of the hot material itself

Different materials conduct heat at different rates (wood, metal)

Stable pieces of lithosphere have a T-depth graph orgeotherm determined by heat conduction

Because depth is related to pressure, a T-depth graph can be turned into a P-T graph

Then, PT conditions in ordinary lithosphere correspond to intermediate pressure baric type

Advection is the transport of heat by the movement of the hot material itself

Advection causes departure from the normal conductive geotherm in the lithopshere

If a pluton is intruded, hot material (magma) has moved to high crustal levels and this gives rise to a low-pressure

baric type of metamorphism (contact aureoles)

During subduction, cold material is carried to depth and this gives rise to a high-pressure baric type of

metamorphism (the Sanbagawa high pressure belt in Japan lies above a present-day subduction zone

Some high-pressure metamorphic belts are not above present-day subduction zones, but are inferred to have formed

above old subduction zones (the Franciscan high pressure belt in California was above a subduction zone, althoughnow the plate boundary is a transform; eclogite-facies rocks in the Alps now form part of a continental collision

zone)


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Lecture 8: Reactions and kineticsOne process in metamorphic reaction must be the transfer of atoms from one place to another without melting and

without physical movement of entire mineral grains

This process is called diffusion and can happen in many materials (sugar will spread throughout a mug of tea,

without stirring, by diffusion)

It takes time to diffuse, so often new minerals grow at positions which minimise the total distance over which atoms

have to move

This can give rise to coronas which form round the edges of reactant minerals

On the atomic scale, diffusion is caused by diffusing atoms bouncing around amongst the other atoms of the phase

through which diffusion is occurring

All atoms and molecules vibrate and bounce faster at higher temperatures, so diffusion is faster at highertemperatures

This is why peak assemblages are often preserved, because the retrograde path is relatively low T and so reactions

can only happen slowly, or not at all

It is important to be able to identify disequilibrium assemblages and textures in rocks

At equilibrium, any particular mineral should be chemically homogeneous

Often we find minerals with colour and birefringence variations due to chemical zoning, which indicate

disequilibrium (actinolite zoned out to hornblende)

Zoning represents growth of grains under changing P and T, and may be used to infer the direction of PT change: in

other words, part of the PTt path (actinolite zoned out to hornblende probably records part of the prograde path)

Some single minerals which are solid solutions at high temperatures prefer to be two separate minerals at low

temperatures (orthoclase at high T, when it can hold Na, is equivalent to orthoclase plus albite at low T)

This can be visualised on a TX phase diagram

The process of breakdown as T decreases is called exsolution

To minimise diffusion, exsolution may happen by many small new grains forming within a single original grain

(orthoclase exsolves albite to give perthitic texture)

As well as diffusion, which helps new grains grow, one process involved in metamorphism is the initiation or

nucleation of entirely new grains

If nucleation is easy, many small new grains will form

If nucleation is difficult, only a few, large, new grains form - this is why some minerals form porphyroblasts

Looking at textures can indicate disequilibrium; it can also be inferred from the number of minerals present

If a rock is in equilibrium, all reactions should have finished, so it should not be possible to write a reaction between

any minerals in the assemblage


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Lecture 9: Metamorphic fluidsBecause water is lost during prograde metamorphism, retrogression cannot always occur unless water is addedto

the rock as it cools

This is why retrogression often occurs only in patches in metamorphic rocks: a reason quite distinct from any kinetic

consideration

Diffusion is slow on geological timescales over distances larger than 10cm to 1m

This is why metamorphism is often isochemical with respect to major oxides such as % Al2O3 on the hand-specimen

scale

In contrast, water formed during prograde reaction must be moved from the rock

Diffusion is too slow to do this, but movement through interconnected pores by permeation is possible, aided by

flow through cracks eventually to the Earth's surface

Metabasic rocks at low grade contain more % H2O than do their igneous protoliths

Often these are interlayered with metapelites which could have been the source

The edges of metabasic bodies are often more hydrated and metamorphosed than the middles, confirming water

came from outside

Marbles lose not only water but also CO2 during prograde metamorphism - these go to form a mixed fluid

Fluids may be trapped inside grains as the grains grow to form fluid inclusions

When observed, fluid inclusions sometimes have a liquid, a gas and a tiny grain of solid inside them (water-rich

liquid, CO2-rich gas and solid NaCl)

These were all dissolved together at the hot metamorphic temperatures when the fluid was trapped

Thus fluids can carry dissolved solids as they move

These solids may be deposited in veins or pores in rock as the flowing fluid cools

Rocks with unusual mineral abundances (e.g. pure garnet) made like this are called skarns, when the unusual

minerals contain rare and/or economically important elements the accumulations are ore deposits

The process where bulk rock chemistry is modified by fluids carrying chemicals in and out of the rock is

metasomatism - this overlaps with hydrothermal metamorphism

Circulating seawater at mid ocean ridges causes large scale hydrothermal metamorphism, hydrating basic rocks to

greenschist facies assemblages and changing bulk chemistry (basalts become enriched in sodium to form spilites)

Such waters emerge in underwater hot springs called black smokers which often precipitate ore deposits

http://www.ocean.udel.edu/kiosk/bsmoker.html

http://www.divediscover.whoi.edu/vents/vent-video.html
http://www.ocean.udel.edu/kiosk/bsmoker.htmlhttp://www.ocean.udel.edu/kiosk/bsmoker.htmlhttp://www.divediscover.whoi.edu/vents/vent-video.htmlhttp://www.divediscover.whoi.edu/vents/vent-video.htmlhttp://www.divediscover.whoi.edu/vents/vent-video.htmlhttp://www.ocean.udel.edu/kiosk/bsmoker.html


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Lecture 10: Eclogites, metamorphism and plate tectonics

Eclogite facies rocks record very high pressures and therefore very deep burial: sometimes as deep as 100 km

They are found in diverse tectonic settings

Group A eclogite facies rocks are as metabasic nodules in basalts (Hawaii) and kimberlites (South Africa).

It is inferred that they are samples of the lower crust and upper mantle as they were at the time the basalt or

kimberlite was generated, with the xenoliths ripped off from the walls of the magma conduits. I find the Hawaii

occurrence challenging to interpret and havent satisfied myself what it means see references below.

They may represent rather cold lower crust and upper mantle samples: diamond-contining varieties come from deep

in the upper mantle

Group B and Group Ceclogite facies rocks are as metagranitoid or metabasic pods and bands a few m to a few kmin size embedded within lower grade rocks, and are usually Phanerozoic

The lower grade of the surrounding rocks implies that the pods were emplaced tectonically after they crystallised, or

that the surroundings were retrogressed, after the pods formed their high-pressure assemblages

Group B and C eclogites originated near the Earth's surface and were deeply buried during subduction and/or

continental collision

The diverse settings of metamorphic rocks can be related to plate tectonics

Atsubduction zones, cool rock is buried deeply, giving rise to high P/T metamorphism

Sometimes belts of contrasting baric type are adjacent and are called paired metamorphic belts (the high-pressureSanbagawa belt in Japan lies east of the low-pressure Ryoke belt, which is related to subduction zone magmas rising

to high levels)

At mid-ocean ridges, hydrothermal circulation metamorphoses ophiolitic rocks

At continental collision zones thickening of continental crust gives rise to conductive heating and deformation

leading to regional metamorphism; associated magmatism leads to contact metamorphism

Coleman, R. G., Lee, D. E., Beatty, L. B. & Brannock, W. W. 1965. Eclogites and eclogites - their differences and

similarities. Geological Society Of America Bulletin 76(5), 483-508.

The original definitions of types A, B, C

Keshav, S., Sen, G. & Presnall, D. C. 2007. Garnet-bearing xenoliths from Salt Lake Crater, Oahu, Hawaii: High-

pressure fractional crystallization in the oceanic mantle. Journal Of Petrology 48(9), 1681-1724.

Yoder, H. S. & Tilley, C. E. 1962. Origin of basalt magmas - an experimental study of natural and synthetic rock

systems. Journal Of Petrology 3(3), 342-532 + 10 plates

These are relevant for my (still incomplete) understanding of why eclogites are present beneath Hawaii


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Lecture 11: Triangular compatibility diagrams 1Compatibility diagrams used to illustrate possible reactions, and compatible equilibrium assemblages.

With just two chemicals, for example Qtz-Jd - plot along a line. Illustrates possible reactions (Alb = Jd + Qtz)

With three chemicals, for example MgO-SiO2-H2O, plot on a triangle.

Tie lines - drawn between minerals to indicate they are stable together at a particular grade

Enclosure reactions: when one mineral (D) plots in the middle of a triangle defined by three others (A,B,C), it is

chemically equivalent to a combination of those three, in other words A+B+C = D

Crossed tie-line reactions: A+B = C+D when tieline AB crosses tieline CD.

Crossed tieline and enclosure reactions are "discontinuous", and happen across a well-defined reaction line in PT

space.

Winter Ch 24

MSH: Yardley 42-46

Learn at least 3 examples of MSH diagrams and be able to explain how they relate to each other

Check out the fundamental principles on

http://serc.carleton.edu/research_education/equilibria/chem_projections.htmland note the links at the bottom, including

http://www.tulane.edu/~sanelson/geol212/triangular_plots_metamophic_petrology.htm

Compatibility diagrams in general: Yardley 29-33

Miyashiro 112-115

Ehlers 167-170

Lecture 12: Triangular compatibility diagrams 2

With more than 3 chemicals we need to "flatten" the diagram into a triangle.

This is done using "projection" - the resulting diagram can only be used when certain phases are present.

AKF diagrams for pelites - assume quartz and water present, "lump together" Fe and MgLimitations of AKF with respect to ferromagnesian minerals (Fe and Mg not distinguished)

Project from muscovite to get Thompson AFM diagram for pelites.

Representation of solid solutions

Limitations of AFM

Winter Ch 24, 28

Learn at least 3 examples of AKF diagrams and be able to explain how they relate to each otherWinter Ch 24, 28

(AFM), 25(ACF)

Learn at least 3 examples of AFM diagrams and be able to explain how they relate to each other. Learn parts of at

least 3 ACF diagrams and be able to explain how they relate to each other.

Check out the fundamental principles onhttp://serc.carleton.edu/research_education/equilibria/chem_projections.html

and note the links at the bottom, including

http://www.tulane.edu/~sanelson/geol212/triangular_plots_metamophic_petrology.htm

To help you understand the link between PT grids and compatibility triangles, look at

http://ees2.geo.rpi.edu/MetaPetaRen/Software/GibbsWeb/Fig04_gridmap.html. When you click on it you get a PT

grid. When you click on a PT point on this grid, you will get an APPROXIMATE compatibility triangle (Thompson

AFM usually) illustrating all the possible assemblages. Experiment by clicking on a PT point. You will get the FM

diagram in a separate window. Now move across ONE reaction line on the PT grid and click again. Compare the

two AFM diagrams. They should be similar except for rearrangements (enclosures or crossed-ties) related to the

reaction marked on the PT grid.
http://serc.carleton.edu/research_education/equilibria/chem_projections.htmlhttp://serc.carleton.edu/research_education/equilibria/chem_projections.htmlhttp://www.tulane.edu/~sanelson/geol212/triangular_plots_metamophic_petrology.htmhttp://www.tulane.edu/~sanelson/geol212/triangular_plots_metamophic_petrology.htmhttp://serc.carleton.edu/research_education/equilibria/chem_projections.htmlhttp://serc.carleton.edu/research_education/equilibria/chem_projections.htmlhttp://www.tulane.edu/~sanelson/geol212/triangular_plots_metamophic_petrology.htmhttp://www.tulane.edu/~sanelson/geol212/triangular_plots_metamophic_petrology.htmhttp://ees2.geo.rpi.edu/MetaPetaRen/Software/GibbsWeb/Fig04_gridmap.htmlhttp://ees2.geo.rpi.edu/MetaPetaRen/Software/GibbsWeb/Fig04_gridmap.htmlhttp://ees2.geo.rpi.edu/MetaPetaRen/Software/GibbsWeb/Fig04_gridmap.htmlhttp://www.tulane.edu/~sanelson/geol212/triangular_plots_metamophic_petrology.htmhttp://serc.carleton.edu/research_education/equilibria/chem_projections.htmlhttp://www.tulane.edu/~sanelson/geol212/triangular_plots_metamophic_petrology.htmhttp://serc.carleton.edu/research_education/equilibria/chem_projections.html


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http://ees2.geo.rpi.edu/MetaPetaRen/Software/GibbsWeb/Fig05_movie.htmlis an example of an actual movie

showing how an AFM diagram changes as conditions change. However, it shows only sliding and not any sort of

discontinuous reactions.

The work is summarised in an electronic paperhttp://gmr.minsocam.org/Papers/v1/v1n3/v1n3.pdfin which more

movies are embedded.

From R. Powell and coworkers, here are some actual Quicktime movies of AFM sliding and discontinuous reactions

as temperature increases, specificallyAFM at 6 kbarsanddetail of AFM at 6 kbars. These animations cover the

garnet, sillimanite and kyanite zones of the Barrovian metamorphic sequence. Bear in mind that metapelites usually

plot below the garnet/chlorite level in AFM diagrams: pick a fairly Fe rich composition and answer: at what

temperature is the staurolite zone entered? At what temperature is the kyanite zone entered?

AKF: Miyashiro 123-124

AFM: Yardley 60-73; 80-85

Miyashiro 125-139; 239-246

ACF: Miyashiro 120-123

Miyashiro is full of examples of these diagrams in Part III (p. 264 onwards)
http://ees2.geo.rpi.edu/MetaPetaRen/Software/GibbsWeb/Fig05_movie.htmlhttp://ees2.geo.rpi.edu/MetaPetaRen/Software/GibbsWeb/Fig05_movie.htmlhttp://gmr.minsocam.org/Papers/v1/v1n3/v1n3.pdfhttp://gmr.minsocam.org/Papers/v1/v1n3/v1n3.pdfhttp://gmr.minsocam.org/Papers/v1/v1n3/v1n3.pdfhttp://mwsdept06/d06/webdocs/earth/johnwh/252/AFM-1.movhttp://mwsdept06/d06/webdocs/earth/johnwh/252/AFM-1.movhttp://mwsdept06/d06/webdocs/earth/johnwh/252/AFM-1.movhttp://mwsdept06/d06/webdocs/earth/johnwh/252/AFM-2.movhttp://mwsdept06/d06/webdocs/earth/johnwh/252/AFM-2.movhttp://mwsdept06/d06/webdocs/earth/johnwh/252/AFM-2.movhttp://mwsdept06/d06/webdocs/earth/johnwh/252/AFM-2.movhttp://mwsdept06/d06/webdocs/earth/johnwh/252/AFM-1.movhttp://gmr.minsocam.org/Papers/v1/v1n3/v1n3.pdfhttp://ees2.geo.rpi.edu/MetaPetaRen/Software/GibbsWeb/Fig05_movie.html


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Lecture 13: Thermodynamics

Thermodynamics: prediction of equilibria Gibbs free energy minimised at equilibrium.

Molar volume is slope on G-P plot - high-P phases always have smaller volume.Entropy is (negative of) slope in G-T plot - high T phases always have more entropy

Physical meaning of entropy (degree of disorderdue to solid/liquid/gas state, temperature in a given phase, and/or

mixing of different chemicals)

Dehydration and melting reactions in relation to entropy

Reaction lines as G = 0

Winter Ch 5, 27

http://serc.carleton.edu/research_education/equilibria/thermodynamics.html

Powell 4-13; 24-36 etc.

Wood & Fraser (Chapter 1) 5-45

Yardley 33-37

Lecture 14: Thermodynamic data

V means change in volume, S change in entropy, H change in enthalpy (heat energy): all are functions of P and

T

V insensitive to P and T for solid-solid reactions

V sensitive to P and T for reactions involving fluids (e.g. water) which are compressible

Reminders of links to triangles, examples of grids

Coesite-quartz example

Grt-cpx reaction rim example (ACF to illustrate reaction; thermodynamics)

Clausius-Approximately straight-line PT graphs for some solid-solid reactions.

Dehydration line steep with positive slopes on PT graphs; solid/solid lines shallower or negative slopes; dehydration

reactions at high P may bend round and have negative slopes

Solid solutionsgenerally stabilised (i.e. have G reduced) by entropy of mixing

Winter Ch 5, 27

Clausius-Clapeyron relation and application: Miyashiro 75-84Yardley 51-59

Practical 7: Aluminium silicate PT grid construction
http://serc.carleton.edu/research_education/equilibria/thermodynamics.htmlhttp://serc.carleton.edu/research_education/equilibria/thermodynamics.htmlhttp://serc.carleton.edu/research_education/equilibria/thermodynamics.html


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Lecture 15: Thermal modelling

Thermal modelling links small-scale equilibrium and kinetics to larger-scale phenomena.

Heat, heat capacity and temperature (links heat change to temperature change with time).

Heat flow and thermal conductivity (links hear flow to temperature gradient with distance).

Derivation of Fick's law (the diffusion equation) and the thermal diffusivity.

D is roughly 30 km2/Mycan use this to get order of magnitude timescales for heating and cooling of an object

of size L

t ~ L2/D (~ means approximate, or order of magnitude of)

Contact metamorphism thermal model.

Dependence on distance and time.

Lecture 16: Thermal modelling

Initial and boundary conditions.

Peak metamorphism is diachronousdifferent times in different placesExamples: Fanad (from practical in this module), Ballachulish (see earlier lectures and practicals),

Case study: Ross of Mull, illustrating general features

Regional metamorphism

Protolith recognisable from mesoscale structures and composition (In Ross of Mull, strongly banded sequence of

metasandstones and metapelites)

Folding; axial planar fabrics give grade of metamorphism

Different lithologies give different assemblages (In Ross of Mull, metabasic sheet crosscutting metasediments has

grt, plag, hbl whilst nearby metapelites have grt, ky, bt etc.)

Contact aureoles

typically show disequilibrium textures

can give the pressure at the time of intrusion and, hence, the depth of intrusion

commonly overprint regional metamorphic textures, at a lower pressure, implying the regionally metamorphosed

rocks have been partly unroofed by erosion or maybe another process. (In Ross of Mull, pelites were amphibolitefacies and definitely higher pressure than the later contact metamorphism which produced andalusite)

Winter p. 417-418

Winter doesnt have much to say on this aspect of metamorphism, so you need alternative sources like this...

Yardley p. 178-180

Multiple copies in HCL.

Brown, G., Hawkesworth, C. J. & Wilson, R. C. L. 1992. Understanding the Earth: a new synthesis. Cambridge

University Press, Cambridge, 551.

Multiple copies in HCL. Chapter 12: Metamorphism and fluidsFurlong, K. P., Hanson, R. B. & Bowers, J. R. 1991. Modelling thermal regimes. In: Contact Metamorphism (edited

by Kerrick, D. M.).Reviews in Mineralogy 26. Mineralogical Society of America, Washington, 437-506.

Skim this if you want, but it is very detailed.

Wheeler, J., Mangan, L. S. & Prior, D. J. 2004. Disequilibrium in the Ross of Mull contact metamorphic aureole,

Scotland: a consequence of polymetamorphism.Journal of Petrology45, 835-853.PDF

Examples of the use of compatibility triangles and grids in a contact aureole
http://pcwww.liv.ac.uk/johnwh/WheelerEtAl2004Mull.pdfhttp://pcwww.liv.ac.uk/johnwh/WheelerEtAl2004Mull.pdfhttp://pcwww.liv.ac.uk/johnwh/WheelerEtAl2004Mull.pdfhttp://pcwww.liv.ac.uk/johnwh/WheelerEtAl2004Mull.pdf


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Lecture 17: Caledonian plate tectonic and structural framework

From north to south, main tectonic units and faults separating them: foreland, Moine Thrust Zone, NW Highlands,

Great Glen Fault, Grampian Mountains, Highland Border Fault, Midland Valley, Southern Uplands Fault, Southern

Uplands, Iapetus suture.

Plate tectonic setting: Laurentia (northern continent), Iapetus (ocean, formed in late Precambrian/Cambrian), Baltica

(southern continent).

General point: collisional orogens are built on sites of subducted oceans

Dalradian: pelites, psammites, limestones, lavas. Commonly shallow water but 30 km thickness in totalformed by

stretching prior to/ during Iapetus formation. Much thinner succession in Cambro-Ordovician on foreland.

General point: margins of ocean are often stretched and have thick sedimentary sequences

Shortening of crust as evidenced by folds, thrusts, recumbent fold nappes.

Hunter, A. & Easterbrook, G. 2004. The Geological History of the British Isles. Geological Society, Bath.

18.95 on Amazon.co.uk (Autumn 2008).This is briefer than Strachan and Woodcock, and is lavishly illustrated.

Harris, A. L. 1985. The nature and timing of orogenic activity in the Caledonian rocks of the British Isles. In:Memoir9. The Geological Society.

This is where the maps used in practicals are from. NOT in the library but JW has copies to loan.

Lecture 18: Caledonian metamorphic patterns in space and time

Smooth variations in metamorphic gradeisogradsmark gradational differences in metamorphic history (e.g.

Barrovian), such as spatial variations in amount of erosion since peak metamorphism

Abrupt variations in metamorphic grademetamorphic breaksrepresent deformation afterthe metamorphic

pattern was established(e.g. Highland Boundary Fault, Moine Thrust Zone)

Other abrupt differences are due simply to younger sediments being deposited unconformablyRegional metamorphismcould be due to multiple intrusions close enough together than their thermal effects

overlap (there wouldnt be deformation in this scenario)

Regional metamorphismcould be due to crustal thickening followed by thermal re-equilibration

PTt pathswe cannot ignore time, and need to acknowledge the existence of successive metamorphic events

(overprinting)

Western Ireland Dalradian as an example of clockwise PT path.

Yardley, B. W. D., Barber, J. P. & Gray, J. R. 1987. The metamorphism of the Dalradian rocks of Western Ireland

and its relation to tectonic setting.Philosophical Transactions of the Royal Society of London Series a-

Mathematical Physical and Engineering Sciences321(1557), 243-&.PDF
http://pcwww.liv.ac.uk/earth/johnwh/212/papers/Yardley%20et%20al.%201987.pdfhttp://pcwww.liv.ac.uk/earth/johnwh/212/papers/Yardley%20et%20al.%201987.pdfhttp://pcwww.liv.ac.uk/earth/johnwh/212/papers/Yardley%20et%20al.%201987.pdfhttp://pcwww.liv.ac.uk/earth/johnwh/212/papers/Yardley%20et%20al.%201987.pdf


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Lecture 19: The Grampian Orogeny

Dalradian metamorphism at 470 Ma

What do isograds really mean? Spatialvariations in peak P and T, hence spatial variations in amount of burial

and/or amount of unroofing (erosion)

What do spatial variations in baric type mean?

Barrovian metamorphismthickening of crust by shortening and nappe emplacement, followed by some thermal re-

equilibration and erosion

Buchan metamorphismenhanced heat flow may be related to Newer Gabbros

470 Ma predated collision considerably

Collision of some kind of island arc is proposed to explain Grampian orogeny, together with obduction of an

ophiolitic nappe over the entire Grampians, providing some of the pressure

Southern Upland accretionary prism carries detrital metamorphic minerals

Oliver, G. J. H. 2001. Reconstruction of the Grampian episode in Scotland: its place in the Caledonian Orogeny.

Tectonophysics332(1-2), 23-49.PDFStudy in particular Fig. 12.

Lecture 20: The Scandian Orogeny

435 Ma

Metamorphic breaksoften relate to tectonics postdating metamorphic peak, e.g. thrusting

Deformation and metamorphism in Northern Highlands, north of Great Glen Fault

None to SE of GGF

Postulate huge slip (700 km) on GGF to shuffle bits of orogen (terranes) sideways

Granites of age 430-405 Ma in ScotlandBallachulish etc.

Contact metamorphism much later than regional metamorphism, and also registers lower pressures, because theprevious Grampian metamorphic rocks have been unroofed to shallower depths by erosion

Dewey, J. F. & Strachan, R. A. 2003. Changing Silurian-Devonian relative plate motion in the Caledonides: sinistral

transpression to sinistral transtension. Journal Of The Geological Society 160, 219-229.PDF

Examine in particular the arguments (evidence?) for 700 km of movement on the Great Glen Fault
http://pcwww.liv.ac.uk/earth/johnwh/212/papers/Oliver%202001.pdfhttp://pcwww.liv.ac.uk/earth/johnwh/212/papers/Oliver%202001.pdfhttp://pcwww.liv.ac.uk/earth/johnwh/212/papers/Oliver%202001.pdfhttp://pcwww.liv.ac.uk/earth/johnwh/212/papers/Dewey%20&%20Strachan%202003.pdfhttp://pcwww.liv.ac.uk/earth/johnwh/212/papers/Dewey%20&%20Strachan%202003.pdfhttp://pcwww.liv.ac.uk/earth/johnwh/212/papers/Dewey%20&%20Strachan%202003.pdfhttp://pcwww.liv.ac.uk/earth/johnwh/212/papers/Dewey%20&%20Strachan%202003.pdfhttp://pcwww.liv.ac.uk/earth/johnwh/212/papers/Oliver%202001.pdf


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Index to key concepts (page numbers not quite right due to the wonders of Microsoft Word program design)

Advection, 10

Assemblage, 3

Balanced, 8Baric type, 9

Black smokers, 12

Compatibility diagrams, 14

Conduction, 10

Contact metamorphism, 4

Coronas, 11

Dehydration, 8

Diffusion, 11

Dynamic metamorphism, 4

Equilibrium, 5

Exsolution, 11

Fabric, 6

Fluid inclusions, 12

Geotherm, 10

Gneiss, 6

Grade, 4

Grain shape fabric, 6

Granulite, 6

Heat production, 10

Hornfels, 6

Hydrothermal metamorphism, 4

Inclusions, 6

Index mineral, 4

Isograds, 4

Location fabric, 6Matrix, 6

Metamorphic facies, 8

Metamorphic reaction, 8

Metamorphism, 3

Metasomatism, 12

Nucleation, 11

Ore deposits, 12

Paired metamorphic belts, 13

Peak, 5

Permeation, 12

Perthitic texture, 11

Poikiloblasts, 6

Polymorphs, 4Porphyroblasts, 6

Porphyroclasts, 6

Pressure-temperature-time path, 5

Products, 8

Prograde, 5

Protolith, 6

Pseudomorphs, 6

PT phase diagrams, 3

Reactants, 8

Reaction lines, 8

Regional metamorphism, 4

Retrograde, 5

Retrogression, 5

Slate, 6Solid solutions, 8

Spilites, 12

Stability field, 3

Stable, 3

Strain history, 6

Texture movie, 6

Textures, 6

Thermal modelling, 17

Thermodynamics, 16

Time, 5

TX phase diagram, 11

Veins, 12

Zonal sequence, 5

Zones, 4


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metamorphism and crustal evolution summary 2012-13

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