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How is the petrography affected by shear zones in the Kohistan complex? Gerold Zeilinger, Laurent Arbaret, Jean-Pierre Burg, Nawaz Chaudhry, Hamid Dawood.

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Presentation on theme: "How is the petrography affected by shear zones in the Kohistan complex? Gerold Zeilinger, Laurent Arbaret, Jean-Pierre Burg, Nawaz Chaudhry, Hamid Dawood."— Presentation transcript:

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2 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

3 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

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6 Granulitic gabbro at site 13/01/01 S N

7 Sarangar gabbro at site 16/01/01 WE

8 Granulitic gabbro (13/01/01) 13/01/01 P2 13/01/01 P1 3 cm

9 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.

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

11 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.

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

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14 % % %% Orthoclase " " """"" """" " " ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( (( ( % % %% % % % %% %% Plagioclase AlbiteAnorthite OligoclaseAndesineLabradoriteBytownite CaAl 2 Si 2 O 8 NaAlSi 3 O 8 KAlSi 3 O 8 Legend Granulitic gabbro undeformed Granulitic gabbro mylonitic zone Sarangar gabbro undeformed Sarangar gabbro mylonitic zone

15 Clinoenstatite Clinoferrosilite ( ( ( ( ( ( ( ( ((( ( ( ( ((( " " " " " " Diopside Hedenbergite Pigeonite Augite Fe 2 Si 2 O 6 (Fs) Mg 2 Si 2 O 6 (En) Ca 2 Si 2 O 6 (Wo) Clinopyroxene Enstatite Ferrosilite Ca 2 Si 2 O 6 (Wo) Fe 2 Si 2 O 6 (Fs) Mg 2 Si 2 O 6 (En) Ortho- pyroxene Legend Granulitic gabbro undeformed Granulitic gabbro mylonitic zone Sarangar gabbro undeformed Sarangar gabbro mylonitic zone

16 Amphibole Edenite Paragasite Tremolite Hornblende Tschermakite Ed Pa Si HbTs Si (Na + K) A Legend Granulitic gabbro undeformed Granulitic gabbro mylonitic zone Sarangar gabbro undeformed Sarangar gabbro mylonitic zone

17 Almandine + Spessartine Grossular + Andradite Pyrope % % % % % % % % % % % % % % % % % % % % % % % % % % % % ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ((( ( (( ( (( " " " " " " " " " " " " " "" " " " " " " " "" "" " " " " "" " " "" " " " "" """ Garnet Legend Granulitic gabbro undeformed Granulitic gabbro mylonitic zone Sarangar gabbro undeformed Sarangar gabbro mylonitic zone eclogites 2 granulites 3 amphibolites 4 epidote amphiboles (after Dobretsov et al., 1972)

18 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

19 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

20 P i n k b a r Schmidt (± 0.6 kbar) Hollister (± 1.0 kbar) Hammarston et Zen (± 3.0 kbar) P in GPa mylonitic zone Al (tot) in hornblende gt-cpx barometer undeformed part Granulitic gabbro Al (tot) in hornblende

21 Al (tot) in hornblende P i n k b a r Schmidt (± 0.6 kbar) Hollister (± 1.0 kbar) Hammarston et Zen (± 3.0 kbar) P in GPa mylonitic zone undeformed part Sarangar gabbro Al (tot) in hornblende

22 Results Hornblende - plagioclase thermometer: –Granulitic gabbro mylonitic zone °C –Sarangar gabbro mylonitic zone °C –Sarangar gabbro undeformed part °C Garnet - clinopyroxene barothermometer: –Granulitic gabbro undeformed part GPa, °C

23 Results Al (tot) - hornblende barometer: –Granulitic gabbro mylonitic zone GPa –Granulitic gabbro undeformed part? GPa (~0.6, ~0.3) –Sarangar gabbro mylonitic zone GPa –Sarangar gabbro undeformed part GPa (~0.9, ~0.2)

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

25 Temperature in °C Legend Granulitic gabbro undeformed mylonitic Granulitic gabbro undeformed mylonitic P in GPa

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

27 …and thanks to our Pakistani guides


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