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

Gerold Zeilinger, Laurent Arbaret,

Jean-Pierre Burg, Nawaz Chaudhry,

Hamid Dawood & Shahid Hussain

Preliminary report

• 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)

Overview

Granulitic gabbro at site 13/01/01

S N

Sarangar gabbro at site 16/01/01

W E

Granulitic gabbro (13/01/01)

13/01/01 P213/01/01 P2

13/01/01 P113/01/01 P1

3 cm

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.

Sarangar gabbro (16/01/01)

16/01/01 P116/01/01 P1 16/01/01 16/01/01 P3P3

3 cm

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.

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

100

90

80

70

60

50

40

30

20

10

100

90

80

70

60

50

40

30

20

10

100 90 80 70 60 50 40 30 20 10

Orthoclase

Plagioclase

Albite AnorthiteOligoclase Andesine Labradorite Bytownite

CaAl2Si2O8NaAlSi3O8

KAlSi3O8

LegendGranulitic gabbro undeformedGranulitic gabbro mylonitic zoneSarangar gabbro undeformedSarangar gabbro mylonitic zone

60

50

40

30

20

10

100

90

80

70

60

50

40

100 90 80 70 60 50 40 30 20 10

Clinoenstatite Clinoferrosilite

Diopside Hedenbergite

Pigeonite

Augite

Fe2Si2O6(Fs)Mg2Si2O6(En)

Ca2Si2O6(Wo)Clinopyroxene

Enstatite Ferrosilite

20

10

100

90

80

100 90 80 70 60 50 40 30 20 10

Ca2Si2O6(Wo)

Fe2Si2O6(Fs)Mg2Si2O6(En)

Ortho-pyroxene

LegendGranulitic gabbro undeformedGranulitic gabbro mylonitic zone

Sarangar gabbro undeformedSarangar gabbro mylonitic zone

Amphibole

5.56.06.57.07.58.0

0.0

0.5

1.0

Edenite Paragasite

Tremolite Hornblende Tschermakite

Ed Pa

Si

Hb Ts

Si Si Si Si Si

(Na

+ K

) A

LegendGranulitic gabbro undeformedGranulitic gabbro mylonitic zone

Sarangar gabbro undeformedSarangar gabbro mylonitic zone

100

90

80

70

60

50

40

30

20

10

100

90

80

70

60

50

40

30

20

10

100 90 80 70 60 50 40 30 20 10

Almandine + Spessartine

Grossular + Andradite Pyrope

Garnet LegendGranulitic gabbro undeformedGranulitic gabbro mylonitic zone

Sarangar gabbro undeformedSarangar gabbro mylonitic zone

1

2

2

3

4

1 eclogites2 granulites3 amphibolites4 epidote amphiboles(after Dobretsov et al., 1972)

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

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

0.50 1.00 1.50 2.00 2.50 3.00 3.500.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

P in

kba

r

Schmidt. 1992 (± 0.6 kbar)

Hollister. 1987 (± 1.0 kbar)

Hammarston et Zen. 1986 (± 3.0 kbar)

P in

GP

amylonitic zone

0.50 1.00 1.50 2.00 2.50 3.00 3.50Al (tot) in hornblende

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

gt-cpx barometer

undeformed partGranulitic gabbro

Al (tot) in hornblende

0.50 1.00 1.50 2.00 2.50 3.00 3.50Al (tot) in hornblende

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

P in

kba

r

Schmidt. 1992 (± 0.6 kbar)

Hollister. 1987 (± 1.0 kbar)

Hammarston et Zen. 1986 (± 3.0 kbar)

P in

GP

amylonitic zone undeformed partSarangar gabbro

0.50 1.00 1.50 2.00 2.50 3.00 3.50Al (tot) in hornblende

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

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

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)

Interpretation

• Shearing under lower amphibolite facies in both,

the Granulitic and Sarangar gabbro.

Temperature in °C

LegendGranulitic gabbroundeformedmylonitic

Granulitic gabbroundeformedmylonitic

Granulit

e

BS

GS

EAA

PP

Eclogite

And

Gar

net i

n

Alb

ite

Jade

ite Q

uart

z

Sil

Ky

Sil

300 400 500 600 700 800 900 1000 1100 12000.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

solid

us

P in

GP

a

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

…and thanks to our Pakistani guides