1

Metamorphic Rocks

Metamorphism

•Literally translates to “change of form

”

•In geology it refers to solid-state changes

in m

ineral assemblages of a rock, and/or

the texture of these m

inerals

•Due to changes in tem

pera

ture

and/or

pre

ssure

Sources of Heat for Metamorphism

•Heat from Earth’s interior

•Geotherm

al gradient is the

increase in temperature with

depth

–Typical continental geotherm

al

gradient is 25-30°C

/km

–Volcanically active areas have

geotherm

al gradients of 30-

50°C

/km

–Oceanic trenches have

geotherm

al gradients as low as

5-10°C

/km

Sources of Heat for Metamorphism

•Heat from m

agma

•Emplacement of magma chambers will add heat to the

immediately surrounding rock

–Gabbroic m

agma ~1300°C

–Granitic m

agma ~700°C

2

Pressure Associated with M

etamorphism

•Lithostaticpressure:the confining pressure created by

the m

aterial that sits above a particular location.

Lithostaticpressure is equal in all directions and

compresses the volume of rock.

–Basalt: 3 g/cm

3(3000 kg/m

3)

–Granite: 2.7 g/cm

3(2700 kg/m

3)

–The lithostaticpressure at a 10 km

depth is≈3 kbar= 0.3 G

Pa

Pressure Associated with M

etamorphism

•Directed pressure:pressure is imposed in a particular

direction due to a regional stress field.

•Directed pressure affects

the shape and arrangement

of the

minerals

•Directed pressure varies w

ith tectonic environment

–Compressionalenvironments: Horz

> VertPressure

–Extensional environments: Vert> Horz

Pressure

Types of Metamorphism

•C

onta

ct M

eta

morp

his

m

–Therm

al variation controls processes

•R

egio

nal M

eta

morp

his

m

–OrogenicMetamorphism

•Combination of temperature and

directed pressure

–Burial Metamorphism

•Combination of temperature and

lithostaticpressure

•Fault-Z

one M

eta

morp

his

m

–Directed pressure controls

processes (GEOL 41.1)

Contact Metamorphism

•Occurs adjacent to

igneous intrusions

•Temperature contrast

between m

agma

chamber and host rock

•Most evident in low-

pressure (near-surface)

environments

3

Regional Metamorphism

•Increase in temperature

is accompanied by an

increase in pressure

•Usually there is directed

pressure, so rock

deform

ation increases

with m

etamorphic grade

Development of Foliations

Recrystallization

Pressure Solution

Remobilization

Rotation

Metamorphic Zones

and Facies

Excerp

ted F

rom

Gille

n (1982) Metamorphic Geology. An

Introduction to Tectonic and Metamorphic Processes.

Mineral Zones of G.W

. Barrow (1893)

•Barrow noted that pelitic

rocks of the Scottish

Highlands had distinct

mineral zones (Gt, Ky, Sil)

•He concluded that this was

the result of increasing

metamorphic grade (T)

•Tilley (1925) added the low-

grade Biotite and Chlorite

zones

•Bt, G

t, St, Ky, and Silare

Index M

inera

lsin

metapeliticrocks

4

Index M

ineral Isograds

•The line that defines

the first appearance of

an index m

ineral

corresponds to a line

of equal metamorphic

grade

•Introduction of the

concept of an isogra

d

Eskola

and M

etamorphic Facies

•Eskola

(1914, 1915) noted

that metapelitic

rocks in

southern Finland (Orijärvi)

contained the assemblage

Bt-Ms w

hereas near Oslo,

rocks contained the

compositionally equivalent

mineral assemblage Kf-Cd

•If rocks are the same

composition, then the

mineralogical difference

must be due to a difference

in physical conditions

2 K

Mg 3A

lSi 3O

10(O

H) 2

+ 6

KA

l 2A

lSi 3O

10(O

H) 2

+ 1

5 S

iO2

↔

3 M

g 2Al 4S

i 5O18

+ 8

KA

lSi 3O

8+

8 H

2O

Eskola

and M

etamorphic Facies

•Eskola

(1915) introduced the concept of

meta

morp

hic

facie

s:

–“In any rock or metamorphic form

ation w

hich has

arrived at a chemical equilibrium through

metamorphism at constant temperature and

pressure conditions, the m

ineral composition is

controlle

d only by the chemical composition.”

•A m

eta

morp

hic

facie

sis a set of repeatedly

associated m

etamorphic m

ineral assemblages

•If you find a specified m

ineral assemblage, then you

can assign a m

etamorphic facies to the area, and

thereby assign a range of pressure and temperature

conditions.

Eskola

and M

etamorphic Facies

•In 1920, Eskola

introduced five m

etamorphic facies that were

defined by m

ineral assemblages in m

etabasites:

–Greenschist

–Amphibolite

–Hornfels

–Sanidinite

–Eclogite

•In 1939, Eskola

added an additional 3 m

etamorphic facies:

–Granulite

–Epidote-amphibolite

–Glaucophane-schist (now called Blueschist)

•In 1959 and 1960, Coombs added two additional metamorphic

facies:

–Zeolite

–Prehnite-Pumpellyite(now called Subgreenschist)

5

Progressive M

etamorphism of Shales

Sla

teP

hyllite

Schis

tG

neis

s

Incre

asin

gTem

pera

ture

Non-Foliated M

etamorphic Rocks