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Lecture 18: Metamorphism of characteristic rock suites
The Earth has a huge range of bulk-rock compositional material that can be subjected
to metamorphism, from granites, to pelites, to (ultra)mafics, to limestones, to salt, to
ore deposits, to coal, etc etc etc...
However, there are a few dominant bulk rock compositions, directly related to how
plate tectonics works:
As a result, we can focus on just a few characteristics rock types, which cover probably
>90% of metamorphic rocks outcropping:
Pelites, Mafics, Ultramafics + Calc-silicates, and Granites
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1
Source material for meta-mafics
Mafic rocks make up a very important class of metamorphic rocks. Their source
material is dominantly basalt, in all its complexity
Metamorphic reactions for mafic rocks are
best illustrated in an AFM diagram:
A: Al2O3 + Fe2O3 - K2O - Na2O
C: CaO
F: FeO + MnO + MgO
Sp
low-Al basalt
high-Al basalt
Gt
tholeiite
alk. basalt
OL
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2
Source material for meta-mafics
A basaltic bulk composition can give rise to a wide diversity of meta minerals
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3
Metamorphism of the oceanic crust
after Ernst 1976
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4
Mafic rock metamorphism: MORB
chl, qtz, cc,
zeolite
H2O CO2
ol, px, pl
glass
NOAA image gallery
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5
Initial metamorphism on the ocean floor
A
Zeolite facies metamorphism
+qtz, H2O
clay
plag
zeo
C
cpx
preh
F
chl
opx
cc
dol
forsterite + plagioclase + fluid = chlorite + quartz + fluid
2.5 Mg2SiO4 + CaAl2Si2O8 + 5H2O = Mg5Al2Si3O10(OH)8 + 1.5SiO2 + Ca2+(aq) + 2OH-(aq)
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Progressive metamorphism of mafic rocks
clay
+qtz, H2O
zeo
preh
chl
cc
dol
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7
prl
Pump-Act
+qtz, H2O
Preh-Pump and Pump-Act metamorphic
facies
facies
+qtz, H2O
clay
Zeolite
facies
ep
stp
pump
chl
zeo
cc
preh
chl
+qtz, H2O
cc
dol
act
prl
Preh-Pump
facies
dol
ep
pump
stp
preh
chl
cc
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dol
8
prl
prl
Pump-Act
+qtz, H2O
+qtz,metamorphic
H2O
Preh-Pump
Preh-Pump
and
Pump-Act
facies
facies
ep
facies
stp
pump
ep
stp
pump
preh
chl
cc
dol
chl
cc
act
dol
ms, kya
+qtz, H2O
Greenschist
facies
ep
chl
gt
cc
act
tlc
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9
Greenschist to amphibolite facies
+qtz, H2O
ms, kya
+qtz, H2O
Greenschist
facies
Amphibolite
facies
ep
pl
chl
gt
cc
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ms, sil
act
tlc
grs
cc
gt
cpx
amp
ath
10
Amphibolite to granulite facies
+qtz, H2O
+qtz, H2O
sil, kya
Granulite
facies
crd,
spr
pl
ms, sil
Amphibolite
facies
gt
cc
pl
grs
cc
cpx
opx
gt
cpx
amp
ath
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11
Medium-pressure vs. high-pressure metamorphism
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12
ms
Blueschist
Subduction path: zeolite +qtz,
to blueschist
facies
H2O
facies
law
ep
gln
arag
+qtz, H2O
clay
Zeolite
facies
zeo
preh
chl
cc
dol
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13
Subduction path: blueschist to eclogite facies
+qtz, H2O
+qtz, H2O
ms
Blueschist
facies
grs
arag
law
ep
kya
Eclogite
facies
gt
cpx
opx
gln
arag
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14
High pressure vs. medium pressure metamorphism
4
5
3
2
1
http://www.geol.ucsb.edu/faculty/hacker/geo102C/
lectures/part13.html
1. Chl + Ab + Qtz = Gln + Pg
2. Cc = Arag
3. Ab = Jd + Qtz
4. Gln + Pg = Gt + Omp
Ile de Groix, France
5. Pl + Amp = Omp + Gt
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15
UHP metamorphism: coesite and diamond
0
200
400
Temperature (oC)
1200
1400
Kokchetav
50
gr
ap
h
dia ite
mo
nd
elting
elting
nite m
qtz
coe
m
basalt
bsent
fluid-a
Dora Maira
Scourian,
Scotland
nt gra
Pressure (kbar)
20
Napier,
Antarctica
bse
fluid-a
10
1600
0
25
50
75
depth (km)
ated granite meltin
g
satur
fluid-
0
Sample from the Erzgebirge in Germany
175
200
60
conditions not
found on Earth
70
Wednesday, March 25, 15
Su-Lu
225
Kokchetav Massif - Schertl & Sobolev 2013
16