G. Zeilinger1, L. Arbaret1, J.-P. Burg1, N. Chaudhry2, H. Dawood3 & S. Hussain3

1 Geologisches Institut, ETH-Zentrum, Sonneggstr. 5, CH-8092 Zürich, Switzerland 
2 Institute of Geology, Punjab University, Quaid-e-Azam Campus, Lahore 54590, Pakistan
3 Museum of Natural History, Garden Avenue, Shakaraparian, Islamabad 44000, Pakistan

In NW Pakistan the Kohistan complex separates the Indian and Asian plates. It was formed as an Island Arc during Mesozoic times (Bard, 1983; Coward et al., 1986). Accretion to Asia and subsequent thrusting over the northern margin of the Indian plate along the northward dipping Indus Suture (also called Main Mantle Thrust = MMT) (Tahirkheli et al., 1979) constitute the collisional history (Coward et al., 1987). Earlier history involves arc related deformation like splitting the arc in a volcanic and remnant arc (Burg et al., 1998) and SW-thrusting expressed by anastomosing shear zones in the southern, i.e. lower part of the arc probably imposed by subduction of the Tethys oceanic crust.
Our area of interest is comprised of two lithologies: The granulitic gabbro forms the upper part of the Jijal complex and is described as a calc-alkaline magma emplaced during the arc activity (Jan and Howie, 1981). The granulitic gabbro is intrusive in, and intruded by hornblendites. P-T conditions point to granulite facies metamorphism (T>750°C and P>1.8 GPa, Ringuette et al., 1998) and Sm-Nd cooling ages are 91 Ma and 96 Ma (Yamamoto and Nakamura, 1996; Anczkiewicz and Vance, 1997).
The Sarangar gabbro (NE of Patan) crystallised at 800°C and 0.8-1.1 GPa as deduced from preserved plagioclase-clinopyroxene assemblages (Yoshino et al., 1998). Ages are not available but structural relationship indicates that the Sarangar gabbro is younger and intrusive into the granulitic gabbro (see companion abstract Arbaret et al., this conference). Regional amphibolite facies metamorphism affected these rocks and ended probably 83 Ma ago (Ar-Ar age of amphibole, Treloar et al., 1990). All these sites show remarkable strain localisation and provide sections from undeformed, weakly deformed with well preserved magmatic fabric, to mylonitic high shear strain zones within tens of centimetres. Bard (1983) and Treloar et al. (1990) estimate the metamorphic conditions for the shear zones in the Sarangar gabbro at 550-650°C and 0.9-1.0 GPa.
Many petrographical and geochemical studies on these rocks have been published and are partly cited above. Unfortunately they do not describe in details the relationship between petrography and deformation. Our particular aim is to obtain a better understanding of the interaction of petrography and ductile deformation. Therefore petrographic analysis across anastomosing shear zones was carried out at these distinct sites, in both the granulitic and Sarangar gabbros.
The composition in the undeformed rock of the granulitic gabbro is plagioclase, pyroxene, garnet, amphibole, quartz and rutile. The mid to fine grained heteroblastic rock shows a granoblastic texture. The pyroxene is hypidioblastic with lobate to symplectitic contacts to plagioclase. The intensity of the symplectitic feature is getting stronger closer to the shear zone. The garnets are poikiloblastic intergrown with plagioclase, quartz and amphibole. At the scale of hand-specimen a magmatic foliation is visible but there is no evidence, even under the microscope, for deformation structures. The magmatic foliation is bent close to the shear zone into the well defined mylonitic foliation within the shear zone. Beside a strong decrease in grain size, the decrease in amount of pyroxene and the increase of amphibole and quartz are the most striking phenomenon. The garnet is porphyroclastic in the very fine grained matrix (~0.3mm), partly rotated and fractured with new grown amphibole inside these fractures. Recrystallised quartz in the pressure shadows of garnets as well as the amphibole crystals oriented parallel to the foliation point to a higher shear strain. The change in the mineral assemblage points to a sharp increase of hydration from the undeformed to the mylonitic part of the garnet granulite.
Dominant composition in the Sarangar gabbro is amphibole (mainly hornblende), pyroxene, plagioclase, epidote and quartz. In a 13 cm long section from the undeformed gabbro to the centre of one reference shear zone the main changes are as follow:
The hypidioblastic magmatic pyroxene is successively replaced by metamorphic pyroxene, amphibole and quartz. Close to the shear zone the metamorphic pyroxene is idioblastic whereas further away it rims the magmatic pyroxene. This first rim shows symplectitic intergrowth with the second rim comprised of greenish amphibole. In the shear zone, no magmatic pyroxene was observed. The plagioclase shows also symplectitic structures within the vicinity of the shear zone. The amphiboles underline the fabric in the shear zone and are cut by small quartz veins. Poikiloblastic garnet occurs infrequently. The macroscopic porphyroclasts (~2 mm) are plagioclase, quartz and pyroxene. Hydration towards the shear zone is remarkable.
Amphibolitic facies developed new minerals preferably in the shear zones and progressively (in the range of a few centimetres) into the undeformed gabbro. The hydration in the shear zone is strong and is propagating successively from the high shear strain zones to the nearly undeformed parts.
Our observation points to a high fluid flux in developing shear zones that were weak features (reduced grain size) with the hydration being concentrated in these thin zones. This process further reduced the hydrated rock strength in which in turn could be the site of increased fluid flow and subsequent amphibolitisation in the shear zone and their vicinity. The biggest amount of amphibolised gabbro is therefore limited to the shear zones protecting the granulitic undeformed parts by channelling the fluid flux. The quantity of amphibolised gabbro is small relatively to the undeformed gabbro.

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