How%20is%20the%20petrography%20affected%20by%20shear%20zones%20in%20the%20Kohistan%20complex?

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How is the petrography affected by shear zones in
the Kohistan complex?
Preliminary report
GEOLOGICAL INSTITUTE
ZURICH

• Gerold Zeilinger, Laurent Arbaret,
• Jean-Pierre Burg, Nawaz Chaudhry,
• Hamid Dawood Shahid Hussain

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Overview

• Petrological analysis of shear zones to document
  the dynamics of intra arc deformation in the
  bottom parts of the Kohistan Complex
• Granulitic gabbro (upper level of Jijal-Complex)
• Sarangar gabbro (lower level of Kamila shear zone)

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Granulitic gabbro at site 13/01/01
S
N
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Sarangar gabbro at site 16/01/01
W
E
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13/01/01 P1
Granulitic gabbro (13/01/01)
13/01/01 P2
3 cm
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Samples

• Granulitic gabbro (13/01/01)
• Plagioclase, clinopyroxene, garnet, amphibole,
  quartz and rutile.
• mylonitic zone
• Decrease in amount of pyroxene and increase of
  amphibole and quartz. Garnet is porphyroclastic
  in the very fine grained matrix (0.3mm), partly
  rotated and fractured with new grown amphibole
  inside these fractures.
• undeformed part
• Granoblastic texture. Pyroxene is hypidioblastic
  with lobate to symplectitic contacts with
  plagioclase. Garnets are poikiloblastic,
  intergrown with plagioclase, quartz and
  amphibole.

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Sarangar gabbro (16/01/01)
3 cm
16/01/01 P1
16/01/01 P3
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Samples

• Sarangar gabbro (16/01/01)
• Amphibole, clino- and orthopyroxene, plagioclase,
  garnet, epidote, ilmenite and quartz.
• mylonitic zone
• Amphiboles underline the fabric in the shear zone
  and are cut by small quartz veins. Poikiloblastic
  garnet occurs infrequently. Porphyroclasts (2
  mm) are plagioclase and quartz.
• undeformed part
• Orthopyroxene is idioblastic close to the
  mylonitic zone whereas farther away it rims the
  clinopyroxene. This rim shows symplectitic
  intergrowth with a second rim comprised of
  greenish hornblende.

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Method

• Electron Microprobe analysis
• Cameca SX50 (University of Bern)
• Geobarometry with Al (tot) in hornblende
• Hollister et al. 1987
• Hammarstrom and Zen 1986
• Schmidt 1992
• Geothermometry
• hornblende - plagioclase
• Holland and Blundy 1994
• garnet - clinopyroxene
• P Eckert et al. 1991, Perkins et al. 1981
• T Krogh 1988, Ellis and Green 1979, Pattison and
  Newton 1989

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Plagioclase
Orthoclase
KAlSi3O8
100
10
90
20
80
30
70
40
60
50
50
60
40
70
30
80
20
90
(
10
(
(
(
100
(
(
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NaAlSi3O8
CaAl2Si2O8
100
90
80
70
60
50
40
30
20
10
14
Clinopyroxene
Ca2Si2O6(Wo)
40
60
50
50
Diopside
Hedenbergite
"
(
(
"
(
(
(
"
"
"
(
(
"
(
(
(
(
(
(
(
60
(
(
40
70
(
Augite
30
80



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Pigeonite
90
10
100

Clinoenstatite
Clinoferrosilite
Fe2Si2O6(Fs)
Mg2Si2O6(En)
100
90
80
70
60
50
40
30
20
10
Ortho- pyroxene
Ca2Si2O6(Wo)
Ferrosilite
Enstatite
Mg2Si2O6(En)
Fe2Si2O6(Fs)
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Pa
Ed
1.0
Edenite
Paragasite
Amphibole
(Na K) A
0.5
Tschermakite
Tremolite
Hornblende
Hb
Ts
0.0
Si
Si
Si
Si
Si
Si
5.5
6.0
6.5
7.0
7.5
8.0
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Garnet
Almandine Spessartine
100
10
90
20
4
80
1 eclogites 2 granulites 3 amphibolites 4 epidote
amphiboles (after Dobretsov et al., 1972)
30
70






3
40


























60
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40
70
30
80
1
20











90
2
10
100
100
90
80
70
60
50
40
30
20
10
Pyrope
Grossular Andradite
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Results

• Plagioclase
• Andesine in both mylonitic zones
• Labradorite in the Sarangar gabbro
• Oligoclase to Andesine in the Granulitic gabbro
• Pyroxene
• Diopside in the Granulitic gabbro
• Enstatite - Ferrosilite in the Sarangar gabbro

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Results

• Amphibole
• Granulitic gabbro Hornblende (undef. zone) to
  Tschermakite/Paragasite (myl. zone)
• Sarangar gabbro Hornblende to Tschermakite
  (undef. zone) and Tschermakite (myl. zone)
• Garnet
• Almandine-Pyrope in Granulitic gabbro
• Almandine rich garnet in mylonitic zone and
  Grossular in Sarangar gabbro

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Granulitic gabbro
mylonitic zone
undeformed part
1.6
1.6
gt-cpx barometer
1.4
1.4
1.2
1.2
1.0
1.0
r
a
b
k

P in GPa
0.8
0.8
n
i

P
0.6
0.6
0.4
0.4
0.2
0.2
0.0
0.0
0.50
1.00
1.50
2.00
2.50
3.00
3.50
0.50
1.00
1.50
2.00
2.50
3.00
3.50
Al (tot) in hornblende
Al (tot) in hornblende
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Sarangar gabbro
mylonitic zone
undeformed part
1.6
1.6
1.4
1.4
1.2
1.2
1.0
1.0
r
a
b
k
P in GPa

0.8
0.8
n
i

P
0.6
0.6
0.4
0.4
0.2
0.2
0.0
0.0
0.50
1.00
1.50
2.00
2.50
3.00
3.50
0.50
1.00
1.50
2.00
2.50
3.00
3.50
Al (tot) in hornblende
Al (tot) in hornblende
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Results

• Hornblende - plagioclase thermometer
• Granulitic gabbro mylonitic zone 700 - 750 C
• Sarangar gabbro mylonitic zone 740 - 770 C
• Sarangar gabbro undeformed part 750 - 850 C
• Garnet - clinopyroxene barothermometer
• Granulitic gabbro undeformed part 1.2 - 1.55 GPa,
  750-930 C

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Results

• Al (tot) - hornblende barometer
• Granulitic gabbro mylonitic zone 0.9 - 1.2 GPa
• Granulitic gabbro undeformed part ? 1.25 - 1.4
  GPa (0.6 , 0.3)
• Sarangar gabbro mylonitic zone 1.05 - 1.25 GPa
• Sarangar gabbro undeformed part 1.2 -1.6 GPa
  (0.9 , 0.2)

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Interpretation

• Shearing under lower amphibolite facies in both,
  the Granulitic and Sarangar gabbro.

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P in GPa
Temperature in C
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Acknowledgements

• Supported by the Swiss National Science
  Foundation (grant 20-49372.96)
• Special thanks to Dr. Edwin Gnos (University of
  Bern) for his support during microprobe analysis
  and Prof. Jacques Martignole for helpful comments

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and thanks to our Pakistani guides