Metamorphic_Rocks

8/17/2019 Metamorphic_Rocks

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Metamorphic Rocks3- Metamorphic Rocks

Rocks that form when a pre-existing rock (protolith )

changes due to temperature or pressure, and/or as a resultof squashing or shearing.

Protolith – the pre-existing rock

Metamorphism doesn’t include weathering, diagenesis, and melting. It is asolid-state process.

James Hutton , a Scottish doctor became fascinated by metamorphic rocks

and published a book, Theory of the Earth (1795), and outlined many

fundamentals of geology that are still used today. He is referred to as thefather of geology.

Char les Lyell – first proposed the word metamorphism in his book,

Principles of Geology (1833). He was a good friend of Charles Darwin and

his work highly influenced Charles Darwin’s theory of evolution.


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How Do We Identify Metamorphic Rocks?

1- Metamorphic Textures – grains are interlocked and grew in place.

Many different types of metamorphic textures

2- Metamorphic Minerals – Certain minerals only grow under

metamorphic temperatures and pressures.

- Called a metamorphic mineral assemblage, or metamorphic facies

3- Foliation – The alignment of platy minerals or alternating layers of

light (felsic) and dark (mafic) minerals.

A foliated

Outcrop

of Gneiss


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Formation of Metamorphic Textures

• Recrystalization – changes the shape and size of grains,

but the same mineral remains. E.g. Sandstone mayrecystallize into quartzite. See (a)

• Phase Change – When a mineral keeps the samecomposition but the atoms arrange into a new form(polymorph). E.g. quartz (SiO2) may change to coesite(SiO2).

• Metamorphi c reaction/neocrystall ization – The result ofchemical processes that decompose minerals and producenew minerals. Happens through diffusion of atomsthrough solid crystals. Very slow process. See (b)

• Pressure Solution – Mineral grains dissolve where theirsurfaces are in contact. Occurs when rock is squeezed inone direction more than the others, at low temps, andusually in the presence of water. Usually zig-zag shapedand common in carbonates. See (c)

• Plastic Deformation – At high temps, minerals can behave like soft plastic and become squished or stretched.Takes place without forming cracks and without changingthe composition of the minerals. See (d)

How do metamorphic textures form?


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What Causes Metamorphism?1. Heat - Increased heat allows chemical

bonds to break easier.

2. Pressure – high pressures causeminerals with ‘open’ lattices to collapse,

forming more dense crystals. Most

metamorphic rocks form at 40-100 km

depth where pressures are 10,000-30,000

times greater than the surface of the

Earth.

3. Dif ferential Stress – When forces arenot equal in all directions, minerals may

deform and change shape.

4. Hydrothermal F luids – More than just water, hydrothermal fluids are

solutions that chemically react with

minerals.

A ‘nice’ sample of gneiss

Recrystallized limestone becomes marble


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Changing Temperature and Pressure

• Minerals have StabilityFields or regions of pressureand temperature where theyare stable. Enter a new

stability field and a newmineral begins to form

• Stabil ity F ields arecharacterized by both pressureand temperature and can berepresented on a Phase

Diagram like the one above.

X

Y

Al2SiO5 PhaseDiagram


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Differential Stress

• Pressure – A stress that is

the same in all directions.Can only change size(volumetric), not shape.

E.g. water pressure,lithostatic stress.

• Differential Stress – When the stress is notequal in all directions.Can change size andshape.

• Normal Stress – pushesor pulls perpendicular(normal) to a surface. E.g.

crushing a soda can.• Shear Stress – moves one

part of a materialsideways relative to theother side. E.g. spreadingout a desk of cards.


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Changes in Shape due to Differential Stress

• Differential stresses may cause once equant (~same length in all

dimensions) to become elongate or tabular/platy in shape.

• The preferred orientation of these inequant grains gives the rock a

foliation (a planar fabric)

Tabular /


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Formation of Foliation

• Differential stress can resultin the formation of

foliations (planar fabrics) in

a variety of ways.


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The Role of Hydrothermal Fluids• Hydrothermal fluids - Include hot water, steam, and supercriti cal f lu id .

Hydrothermal fluids are chemically-active in that they are able to dissolve certain

minerals, so hydrothermal fluids are solutions, not just water.

• Supercr iti cal F lu id – A substance that forms under high temps and pressures that

has properties of both a gas and a liquid. Supercritical fluids permeate rocks like a

gas and react with minerals like a fluid.

• Where does this fluid come from?

1- groundwater that percolates downward.

2- water and volatiles released from magma

3- water is released during some metamorphic reactions

KAl3Si3O10(OH)2 + SiO2 KAlSi3O8 + Al2SiO5 + H2O

Muscovite Quartz K-Feldspar Sillamanite Water

• Hydrothermal fluids speed metamorphic reactions because fluids allow for easy

transport of ions and fluids are consumed in some reactions

• Metasomatism – The process by which a rock’s chemical composition changes

due to reactions with hydrothermal fluids.

• Metasomatism commonly results in the formation of veins , mineral filled cracks.


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Veins

• Veins are different than joints

Veins:

– Filled with minerals

– Commonly wavy in shape

Joints: – Usually planar

– Usually not mineralized,

i.e. just a crack


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Types of Metamorphic Rocks

• Metamorphic rocks are grouped into two main

categories:

– Foliated Metamorphic Rocks

– Non-Foliated Metamorphic Rocks• But what exactly is foliation ?


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Foliation• Foliation – The repetition of planar surfaces or layers in a

metamorphic rock. Layers can be paper-thin or meters thick.

– Happens because when rocks are subjected to differential stress, platyminerals align or alternating light and dark layers form, giving the rock a

planar fabric, called foliation. Note that this is different than bedding.

Slate, a foliated metamorphic rock makes nice

roof shingles because its foliation createscleavage planes that easily break


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Foliation and Compression Direction

• Slaty Cleavage forms perpendicular to the compression direction,

i.e. a horizontal squish will create vertical cleavage planes.Compression also commonly results in folding.


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Foliated Metamorphic RocksDistinguished based on grain size, composition,

and nature of fol iation

• Slate – finest grained metamorphic rock.Foliation occurs because of alignment ofchlorite grains. Protoliths of shale andmudstone. Lots of slate mines in Vermont!

• Phyllite – Fine-grained, foliation occurs because of alignment of mica andoccasionally chlorite. Translucent alignedmica grains give it a silky or waxy sheencalled phyllitic luster. Phyllite forms whenshale/mudstone is metamorphosed attemperatures high enough to causeneocrystallization (mica and chlorite form).Foliation is from aligned mica and chlorite

grains that align due to differential stress.• F lattened Clast Conglomerate – (aka

metaconglomerate or stretched pebbleconglomerate) forms when conglomerates or breccias get squished (plastic deformation)and the once round pebbles become flattened.

Foliation defined by the squished pebbles.

Phyllite

Metaconglomerate


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Foliated Metamorphic Rocks

• Schist – A medium to coarse grained

metamorphic rock that possesses

schistosity defined by the preferredorientation of large mica grains

(biotite/muscovite) as a result of

differential stress. Forms at higher

temps than phyllite and has larger

grains. Also contains a range ofdifferent minerals depending on

composition of the protolith. Can

have a wide range of protoliths so

long as the protolith contains the

elements necessary to make mica.• Gneiss – a medium to coarse grained

compositionally layered [gneissic

banding ] metamorphic rock

consisting of alternating light [felsic]

and dark [mafic] layers.

Phyllite

Metaconglomerate

Schist

Gneiss


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How Does Gneissic Banding Form?

• During shearing and compression, minerals become stretched

(plastic deformation) all while recrystallization and

neocrystallization is taking place

– Analogy: think of stirring chocolate fudge into ice cream. The fudge starts

out in a blob but gets deformed and smeared during stirring.


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Foliated Metamorphic Rocks• Migmatite – combination of partially melted metamorphic rocks and igneous

rocks. Typically, a gneiss when subjected to hydrothermal fluids will attain a

lower melting point and begin to partially melt. The felsic stuff melts forming anew felsic igneous rock surrounded by metamorphic gneiss that is more mafic.

The resulting mixture of these two rock types is called a migmatite and has a

marble cake kind of look.

Migmatite


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Metamorphic rocks that have recrystall ized and/or neocrystall ized but do not

typical ly have a foliation (usually because grains are not suf f icientl y elongated).

Distinguished based on composition, but may be foli ated if subjected to

signif icant differential stress

• Hornfels – Rock that undergoes heating in the absence of significant differential

stress. Typically hornfels form when rocks are baked by igneous intrusions

(contact metamorphism). No foliation is present because crystals grow in random

orientations due to a lack of significant differential stress. Composition varies and

depends on composition of protolith.

Nonfoliated Metamorphic Rocks

• Amphibolite – Metamorphosed

mafic rock (basalt, gabbro) can’t

form felsic minerals, so they tendto form amphibolites, which are

dominantly made of visible

crystals of hornblende and

plagioclase (Ca-feldspar). Can

often be foliated.Amphibolite


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Nonfoliated

Metamorphic Rocks

• Quartzite – Metamorphosed quartzsandstone with larger interlocking

quartz crystals. Matrix material and

pore space is eliminated. Sandstone

looks grainy, Quartzite looks glassier

or more crystalline.

• Marble – limestone and other

carbonate rocks recrystallize into

interlocking grains of calcite. Pore

space and much of the original grainform is destroyed. Impurities may

form compositional bands

occasionally because marble flows at

relatively low temperatures.

Marble

Quartzite

i C i i


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Metamorphic Compositions

• Occasionally, for simplicity, geologists will simply refer to the

composition of a metamorphic rock

– Mafic (or Basic) Metamorphic Rock – lots of mafic minerals

– Calcareous Metamorphic Rock – Calcite-bearing protoliths (limestone)

– Quartzo-Feldspathic (i.e. felsic) Metamorphic Rocks – form from

protoliths than contain a lot of feldspar and quartz (e.g. granite, diorite)

• Occasionally geologists will simply refer to metamorphic rocks by

their protolith

– Metasedimentary rock

– Metaigneous rock – Pelitic Metamorphic Rock

Sedimentary protolith

Metaconglomerate is a metasedimentary rock

M t hi G d


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Metamorphic Grade• Not all metamorphism occurs under the same conditions, so geologists classify the

metamorphic grade , or specific set of conditions under which certainmetamorphic rocks form

• Metamorphic F acies – groups of metamorphic minerals that form under similartemperature and pressure conditions.

• Low-Grade – rocks that form under low temperatures (200-320o C)

• I ntermediate-Grade – rocks that form under temperatures (320-600o C)

• High-Grade – rocks that form above ~600o C.


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Prograde & Retrograde Metamorphism

• Prograde Metamorphism – Metamorphism that occurs whiletemp and pressure progressively increase. They form minerals

that are stable at higher temp and pressure. Neocrystallizationcommonly releases water in the host rock, so high grade rockstend to be drier (little no OH-) than low grade rocks. So, schistloses its schistosity at high grades and may form gneiss. – Biotite: K(Mg,Fe)3AlSi3O10(F,OH)2

– Muscovite: KAl2(AlSi3O10)(F,OH)2

– Chlorite: (Fe, Mg, Al)6(Si, Al)4O10(OH)8 • Chlorite is common in retrograded rocks

• Retrograde Metamorphism – Metamorphism that occurs when

temp and pressure decreases. For metamorphic reactions to occurin these conditions, water must be added to the rock(hydrothermal fluids). Without water, high grade rocks cannot beretrograded. This is why very old (billions of years) high graderocks are exposed at the surface of the Earth in certain places.


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Metamorphic Grade: Graphical View

Sh l Di i Hi h G d


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Shale: Diagenesis to High-Grade

Metamorphism

• A single protolith (shale shown below) canform a variety of metamorphic rocksdepending on the grade of metamorphismincurred after burial. Certain mineralassemblages reflect the grade ofmetamorphism

M t hi F i


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Metamorphic Facies• A given P-T

horizon has a

characteristic setof minerals that

form. Which ones

form depend on

protolith

composition

• If you know the

P-T conditions

and the protolith

composition, youcan predict the

mineralogy of the

resultant

metamorphic

rock

P T P th


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P-T Paths• The metamorphic rocks that we see are now exposed at the surface of the

Earth, so we can describe the life of a metamorphic rock with a P-T path .

• The life cycle of a rock can be plotted on a pressure temperature plot.These plots outline the P-T (pressure, temperature) history of a rock

I d Z f Si il P T C diti


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Isograds – Zones of Similar P-T Conditions

• Since we can determine

the P-T conditions of a

metamorphic rock based

on its metamorphic

facies, we can map out

regions of similarmetamorphic grade.

• These regions are

separated by lines called

isograds , which are lines

that delineate the firstappearance of a mineral

from a new metamorphic

facies

M t hi E i t


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Metamorphic Environments• The geothermal gradient varies throughout various tectonic

environments, so it stands to reason that metamorphic processes

will vary depending on tectonic environment.• In general, heat flow is HIGH in:

– Near magma bodies

– Rifts (rising magma)

– Young mountain belts (faults bring up warm rocks)• Heat Flow is LOW in:

– Stable continents (called cratons).

B i l M hi


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Burial Metamorphism• As sediments are buried in a sedimentary basin…

– P increases because of the weight of the overburden. – T increases because of the geothermal gradient.

• Requires burial below diagenetic effects.

– This is ~ 8 – 15 km depending on the geothermal gradient.

C t t M t hi


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Contact Metamorphism• Heat flows from hotter to

colder materials, so when

a hot igneous intrusion(magma) comes into

contact with cold country

rock, it creates a

metamorphic aureole /

contact aureole or baked

zone.

• This results in contact

metamorphism, whereby

rocks undergo

metamorphic reactions

due to heating (little or no

pressure change)

• Contact metamorphism

typically produces

hornfels, a nonfoliated

metamorphic rock.

D i M t hi


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Dynamic Metamorphism

• Although at shallow

depths faults break rockand slide past each other,in the asthenosphere,rocks don’t break, ratherthey flow past each other.

• Rock in the deep portionsof faults undergoesdynamic metamorphismand creates a fine-grainedmetamorphic rock called amylonite

• Mylonites are thus foundat all plate boundaries atdepth.


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Fault Breccia

• At shallow depths, faults break rocks into angular

pieces forming a rock called fault breccia


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Mylonite

• An exposure where a

gneiss once met aductile shear zone.

• Same composition, but

grain size is greatly

reduced.

• Grain size change,

causes the color to

change.

Gneiss Mylonite

Regional Metamorphism


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Regional Metamorphism• Mountain belts commonly produce a range of metamorphic rocks.

• When subduction eats up all available oceanic crust, collisional

orogens (mountain building events) happen.

• Mountains get eroded and expose the once deep metamorphic rocks

(e.g. most of the High Country)

R i l M t hi


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Regional Metamorphism

• Regional metamorphism creates foliated rocks.

• This type of metamorphism is, by far, the most

important in terms of the amount of rock altered.

– Collisional belts are often…

• 1000’s of km long.

• 100’s of km wide.

H d th l M t hi


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Hydrothermal Metamorphism

• Alteration by hot, chemically aggressive water.

• A dominant process near mid-ocean ridge magma. – Cold ocean water seeps into fractured crust.

– Heated by magma, this water then reacts with mafic rock.

– The hot water rises and is ejected via black smokers.

S bd ti M t hi


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Subduction Metamorphism

• Subduction creates the unique blueschist facies.

• Trenches and accretionary prisms have… – Low temperature (low geothermal gradient)

– High pressures

• High P & Low T favor

glaucophane, a blueamphibole mineral.

Blueschist from Shell Beach CA


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• Meteor impacts can generate a large amount of heat from the

conversion of kinetic energy to heat

• This heat may melt or even vaporize rock

• Causes quartz to change phase to coesite or stishovite

(polymorphs)

• sometimes referred to as shocked quartz.

• Meteor Crater, AZ has abundant shocked quartz

• Chicxulub Crater also has shocked quartz!

Exhumation


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Exhumation• How do metamorphic rocks return to the surface?

• Exhumation is due to... – Uplift – Compression squeezes deep rocks upward.

• Faults bring up deep rocks

– Erosional unroofing – Weathering and erosion removes vast

amounts of rock.San Gabriel Mountains: Los Angeles, CA

Deep rocks brought up by the Sierra Madre Fault.

Wh C Y Fi d M t hi R k T d ?


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Where Can You Find Metamorphic Rocks Today?

• Shield – Older portions of the continental crust where large

amounts of metamorphic rock crop out at the surface of the Earth.

The Rock Cycle


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The Rock Cycle

• Once a rock is formed it may be changed into a new type

of rock by various processes… • The Rock Cycle is a mass transport cycle that outlines

the progressive transformation of Earth materials from

one rock type to another.

• The Earth is affected by various cycles

– Temporal Cycle – time dependant, such as lunar cycles,

seasons, etc…

– Mass-Transfer Cycle – involving the transport of materials

Weathering Metamorphism

Granite Arkose Gneiss


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