erth2404 l6 metamorphic upload

7/28/2019 ERTH2404 L6 Metamorphic Upload

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ERTH2404

Lecture 4: Metamorphic Rocks

Dr. Jason Mah

Photo:W.Carter,CarletonU.

Shattercones,Sudbury,ON


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Reading assignment

Please read Kehews book to complement the

material presented in this lecture:

Chap. 6;

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Review: Sedimentary rocks

Detrital and Chemical

Detrital rocks classified by particle size

Chemical can be inorganic or biochemical

Sedimentary rocks tell us about the

environment of which they are formed in

Sedimentary structures include bedding,

ripple marks, mud cracks, and fossils

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Lecture objectives

To learn the different types of metamorphism

To understand the relationship between

metamorphic rocks and other rock types

To be able to characterize and identify

metamorphic rocks

To be aware of how the characteristics ofmetamorphic rocks might impact engineering

work

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Remember the rock cycle

Where do

metamorphic

rocks fit in?

Formed from

any type of

pre-existing

rock

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Metamorphism

Metamorphism: transformation of one rock

into another through the action of

Heat

Pressure

Chemically active fluids (metamorphic agents)

Rocks retain their solid form

But they can flow in a plastic-manner

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Metamorphism

Metamorphism may cause many changes

to rock

Formation of larger grains

Reorientation of mineral grains

Texture

banded appearance, sheen

Increase in density

Transformation of existing minerals into minerals

stable in the new T-P conditions

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Metamorphism

Metamorphism occurs in the Earths crust over

hundreds of Ma

In the core of mountain belts

Within stable continental interiors (shields)

often in association with igneous intrusions

They become exposed at the surface when

overlying rocks have been removed by erosion Exposed outcrops are commonly of Precambrian age

due to the long time needed for erosion to occur

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Metamorphism

Metamorphic grade

Certain minerals and textures are diagnostic of the

degree of metamorphism experienced by rocks

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Low-grade

Low TLow P

Mineralogy and

texture changes High-grade

High THigh P


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Metamorphism: Temperature

Most important factor for metamorphism

Drives chemical changes

Recrystallization of existing minerals to new

minerals (Bowens reaction series)

Impacts:

Texture, crystal structure, crystal size

Source:

Magma intrusion, geothermal gradient

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Metamorphism: Temperature

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Geothermal gradient Bowens Reaction Series


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Metamorphism: Pressure

Approximate increase of 1 kbar per depth of 4.4

km

Confining pressure: pressure applied in all

directions

Causes decrease in rock size and an increase in density

Does not fold or deform rock

Differential stress:pressure applied from specificdirection

Creates deformed strata

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Metamorphism: Fluids

Water acts as catalyst during metamorphism

Conduit for ion exchange in growing of crystals

At low to medium T, water is removed from

minerals

At high T, water is driven from the rock

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Types Metamorphism

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Scale Type of metamorphismMetamorphic

agents

Change in

chemical

composition

Regional

Sedimentary rocks only: BurialContact Heat

Dynamic P

Impact P

Hydrothermal Fluids Yes

Regional

Local

Heat and P

No

Regional: large scale, plate tectonics

Local: smaller scale, faults, igneous bodies


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Types Metamorphism

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Contact

(Low P, High T)

Regional

(High P, High T)

Regional

(High P, Low T)


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Regional Metamorphism

Continental crust exposed to extreme

T and pressure at convergent plate boundaries

Enormous thickness of rocks involved

(9-12 km)

Greatest quantity of metamorphic rocks produced

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Crustal

thickening,

continental

collision

Leads to

foliation,

alignment ofminerals

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18Ref.: ERTH2404 Lab manual


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19Ref.: ERTH2404 Lab manual


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Low Grade: rock harder, more compact

(interlocking crystals), minor increase in grain

size

Medium Grade: grain size increases, new

mica crystals form

High grade: grains recrystallize with preferred

orientation (foliation)

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At subduction zones ofthe lithospheric plate

Low T, High P

Cold, slow to heat byconduction, butplunges quickly to greatdepth

Sediments, basalts formhigh-P minerals

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Burial Metamorphism

Subset of regional metamorphism

Associated with thick sedimentary strata

Metamorphism due to burial beneathother rocks

Certain mineral become unstable due to increased T and P,and change to new minerals

Required depth varies from one location to anotherdepending on the geothermal gradient

Clay becomes mica when buried several kms deep

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Continental geotherm

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Ref.: Kehew, A.E. 1995. Geology for Engineers & Environmental

Scientists. 2nd Edition. Fig. 5.2. Shown with permission.


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Local Metamorphism: Contact

Magma invades a hostrock leading to anincrease intemperature Increase in temperature

leads to re-crystallization

Aureole: zone of

alteration, forms in therock surrounding theigneous intrusion

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Local Metamorphism: Contact

Aureole

Extent depends on the size of the intrusion and

the host rock

Sills, dikes cmm wide aureole

Batholiths km wide aureole

Typically fine-grained, dense and tough

No directional alignment of minerals becausepressure is not a major factor

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Local Metamorphism: Dynamic

Pressure is the only metamorphic agent

Dynamic metamorphism is associated with

localized events along fault zones

Rocks are broken and pulverized as rock blocks alongfault zone grind past each other

Fault breccia: loose rock consisting of broken and

crushed rock fragments

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Local Metamorphism: Impact

Occurs when large space bodies strike the

Earths surface

Creates shattered and melted rocks adjacent to

the impact crater

Shatter cone: a conical fragment of host rock that

is formed from the high pressure of meteorite

impact and has striations radiating from the apex 2-30 GPa of pressure required

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Blast shatter cone

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Photo: W. Carter, Carleton U.

Rousell et al. 2003

http://dx.doi.org/10.1016/S0012-8252(02)00091-0
http://dx.doi.org/10.1016/S0012-8252(02)00091-0http://dx.doi.org/10.1016/S0012-8252(02)00091-0http://dx.doi.org/10.1016/S0012-8252(02)00091-0http://dx.doi.org/10.1016/S0012-8252(02)00091-0http://dx.doi.org/10.1016/S0012-8252(02)00091-0http://dx.doi.org/10.1016/S0012-8252(02)00091-0http://dx.doi.org/10.1016/S0012-8252(02)00091-0


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Shocked quartz

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Chicxulub crater, Mexico

dia > 180km

Dislocation of crystal orientations due to

shock wave (~58 Gpa)


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Local Metamorphism: Hydrothermal

A form ofmetasomatism: change in thecomposition of a rock or mineral by theaddition or replacement of chemicals

Chemical alteration caused by hot, ion-rich fluids(hydrothermal solutions) that circulate throughcracks

Hydrothermal solutions are expelled from cooling

plutons Invade country rock

Add ions to rock to form new minerals

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Local Metamorphism: Hydrothermal

May generate metallic ore deposits

Copper, gold, molybdenum, tin, iron

Example:

Hydrothermal metamorphism is wide spread

along mid-ocean ridges

Fe, Mg silicates talc, serpentine

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Protolith

Protolith: original rock from which a givenmetamorphic rock was formed

Most metamorphic rocks have the same overallchemical composition as their protolith

Original mineral makeup determines, to large extent,the degree of which each metamorphic agent willcause change

Example: quartz sandstone is the protolith ofquartzite

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Protolith

Different metamorphic minerals can be

produced from the same original rock under

different temperature and pressure conditions

Example: shale slate schist gneiss

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Metamorphic textures

Three metamorphic textures:

Foliated textures

Pressure is the key metamorphic agent

Mostly associated with regional metamorphism Non-foliated texture

Temperature is the key metamorphic agent

Mostly associated with contact metamorphism

Porphyroblastic texture Large crystals growing in a finer matrix

Example: garnet

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Foliation

Foliation: parallel orientation of mineral grains

within a metamorphic rock

Foliation give metamorphic rocks a banded

appearance

Watch out! Foliation resembles the bedding

of sedimentary rocks

Generated by different mechanisms

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Foliation

Foliation can develop in different ways:

1. Rotation of platy minerals under pressure

2. Recrystallization of minerals under pressure

3. Flattening of rounded grains in the direction of

maximum stress

New mineral orientation is perpendicular tothe direction of maximum stress

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Foliation

Rotation of platy minerals

Flattening of rounded grains

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Direction of

maximum stress


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Foliation

Different expressions of foliation

Parallel alignment

Compositional banding

Zones of weakness where rocks can easily split

into thin sheets

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Foliated textures

Slaty cleavage

Closely spaced planar surfaces along which rocks

split

Schistose

Platy mineral grains grow larger when rock is

subjected to higher T and pressure Rock exhibit a scaly appearance

Platy minerals visible to unaided eye

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Foliated textures

Gneissic texture

During higher grades of metamorphism,

ion migration results in segregation of minerals

Distinct banded appearance due to segregation ofdark and light silicate minerals

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Foliated textures

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Metamorphic grade


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Non-Foliated textures

Develop in environments where deformationis minimal

Mostly associated with contact metamorphism

Develop from rocks composed primarily ofone mineral, exhibiting equi-dimensionalcrystals

Examples: Limestone marble

Quartz sandstone quartzite

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Porphyroblasts

Large mineral grains (porphyroblasts)

surrounded by a fine-grained matrix of other

minerals

Grows within the solid state

Examples: garnet, staurolite, andalusite

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Porphyroblasts

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Classification

Metamorphic rocks are classified according to:

Texture

Protolith

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Metamorphism of Shale

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Ref.: ERTH2404 Lab manual


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Slate

Generated from low-grade metamorphism of shale

Very fine grained (minute mica flakes)

Color depends on mineral constituent

Black: organic material

Red: iron oxide Green: chlorite minerals

Excellent cleavage

Tiles

Original bedding may or not be preserved Slaty cleavage may develop at an angle to original bedding

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Slate

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Source: Earth Science Australia

(http://earthsci.org)


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Slate

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Slaty cleavageShale


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Phyllite

Lies between slate and schist

Characteristic rock sheen

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Schist

Generated from intermediate-grademetamorphism of shale Often related to mountain building

Medium to coarse grained rock readily split into

thin scales Platy mica minerals dominate

Often contains accessory minerals unique tometamorphic rocks (e.g. garnet)

Term schist describes the texture To indicate composition, mineral names are used

(e.g. mica schist)

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Schist

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Schist

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Garnet schist

Naming convention applies porphyroblast(s) first If there are more than one, name most abundant first

Muscovite schist


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Gneiss

Generated from high grade metamorphism of

shale or granite

Medium to coarse grained rock with elongated

granular minerals (quartz, feldspar,

hornblende) and a lesser amount of platy

minerals (mica)

Banded appearance

Break in irregular fashion

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Gneiss

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Gneiss

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Marble

Common non-folitated

metamorphic rock

Protolith is limestone or

dolostone

Composed essentially of

calcite or dolomite

Decorative stone

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Quartzite

Common non-folitatedmetamorphic rock

Protolith is quartz

sandstone Quartz grains are fused

together

Very hard rock

Rock splits through quartzgrains, not in betweenoriginal grains

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Engineering considerations

Non-foliated metamorphic rocks have similar

properties as intrusive igneous rocks

Great strength

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Foliated metamorphic rocks are susceptible to

failure along specific planes

Orientation of foliation planes is a critical factor in

slope stability Avoid transferring load (from bridges, dams,

building foundations) onto foliated rock mass in a

direction parallel to the foliation

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Landslides associated with schist

Generally controlled by orientation of schistosity

Presence of slick minerals (e.g. talc, clay, graphite)

may increase sliding potential

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Failure of St. Francis Dam, CA

Built in 1926 to regulate the Los Angeles

Aqueduct

12 March 1928, the dam suffered from

catastrophic failure

Releasing 45 Billion liters

> 600 people killed from the resultant flood

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Geological context

Poorly consolidated conglomerate of gravels,

sands and muds held together by clay and

cemented by gypsum Schist with steeply dipping foliation planes

Load of the dam parallel to foliation

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Ref.: Kehew, A.E. 1995. Geology for Engineers & Environmental

Scientists. 2nd Edition. Fig. 5.16. Shown with permission.


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Water seeping from reservoir weakened rocks

beneath the dam

Fault zone acts as a conduit for water

Degradation of weak bounds between mica grainsalong foliation planes in the schist

Water dissolved gypsum cement in the

conglomerate

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Next: Plate Tectonics!!

erth2404 l6 metamorphic upload

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