A MICROSTRUCTURAL AND ARGON LASERPROBE STUDY OF SHEAR ZONE DEVELOPMENT AT THE WESTERN MARGIN OF THE NANGA PARBAT-HARAMOSH MASSIF, WESTERN HIMALAYA

A sample of banded amphibolite from the western margin of the Nanga Pa rbat-Haramosh Massif as Sassi has been studied using microstructural a nd Ar-40/Ar-39 laserprobe techniques to investigate the relationship b etween deformation and argon isotope variations in a natural system. A mphibolite-grade deformation occurred during south-directed overthrust ing of the Kohistan are over India along the Main Mantle Thrust and wa s overprinted by extensional reactivation of the earlier fabric and th e formation of biotite-rich shear zones. Subsequent deformation along discrete fine-grained fault zones was characterised by the formation o f scapolite, chlorite and K-feldspar, early plastic deformation and la ter cataclasis. Different minerals developed during this history show a wide range in apparent Ar-40/Ar-39 ages. Biotite, chlorite and scapo lite exhibit much lower concentrations of excess argon, indicating the ir equilibration in a fluid relatively poor in excess argon. A 'true' age of ca. 8 Ma from biotite represents a minimum age for deformation associated with formation of the Nanga Parbat Syntaxis and also preclu des Pliocene metamorphism in this area of the syntaxis. Both high- and low-closure temperature minerals (amphiboles and feldspars) record ap parent ages which are associated with the incorporation of excess argo n within the mineral lattice. Although differential thermal resetting of minerals at different closure temperatures is important, variations in the inherited Ar-40/Ar-36 ratio throughout the sample is dominated by deformation and fluid infiltration. Consequently it appears that w ithin deforming metamorphic rocks, areas with significantly different argon isotope compositions may be present and need not be homogenised by diffusion.