Mineralogical constraints of the Shergarh Sar amphibolites, Allai Kohistan, northern Pakistan
Abstract
Shergarh Sar amphibolites, a part of the southern amphibolite belt, are widely exposed in the area around Bana in Allai Kohistan. Epidote-amphibolite is the main unit with scattered patches of garnet-bearing variety. Amphibole, epidote and in some, garnet are the main phases with minor amount of quartz, plagioclase, clinopyroxene, opaque oxides and rutile. Amphiboles range in composition from tchermakitic-hornblende to actinolite with magnesio-homblende as an intermediate phase. This transition is marked by decrease in WAI Ti, alkalies and Fe2+/Mg ratio and increase in Si. Epidote occurs as large crystals (both zoned and unzoned) with low Ps contents (9-15) and as small granular aggregates with high Ps contents (17-27). Garnet occurs sporadically in the form of porphyroblasts and is found in association with leucocretic veins. It is chemically unzoned and is predominantly made up of almandine-grossular-pyrope solid solution. Plagioclase ranges from, An27 to pure albite and Clinopyroxene is chemically homogeneous diopside.
Petrographic observations and thermobarometry that the rocks were initially equilibrated under amphibolite facies with overprinting of greenschist facies assemblage. This retrogression may have been facilitated by shearing during the obduction (uplift-cooling) of the rocks along the Indus suture following their Eocene collision with the Indian plate.
References
Bard, J. P., 1983. Metamorphic evolution of an ducted island arc: Example of the Kohistan sequence (Pakistan) in the Himalayan collided range. Geol. Bull. Univ. Peshawar., 16, 105-184.
Brown, E. H., 1977. The crossite content of Ca amphibole as a guide to pressure of metamorphism. Jour. Petrol., 18, 53-72.
Blundy, J. D. & Holland, J. B., 1990. Calcic amphibole equilibria and a new amphibole-plagioclase geothermometry. Contrib. Mineral. Petrol., 104, 208-224.
Buddington, A E., 1966. The occurrence of garnet in the granulite facies terrane of the Andirondack Highlands. Jour. Petrol., 7, 331-335.
Cooper, A. F., 1972. Progressive metamorphism of metabasic rocks from the Haast schist group of southern New Zealand. Jour. Petrol., 13, 457-492.
Dobretsov, L N., Khlestov, V. V., Reverdatto, V. V. & Sobolev, V. S., 1972. The facies of metamorphism (transl. by D. A. Brown). Australian National University Press, Canberra.
De Waard, D, 1967. The occurrence of garnet in the granulite-facies terrane of the Andirondack Highlands and elsewhere, an amplication and a reply. Jour. Petrol., 8, 210-232.
Doolan, B.L, 1975. Evaluation of the quality of amphibole analyses and an estimate of "ideal hornblende". Trans. Amer. Geoph. Union., 56, 461.
Engel, A. E. J. & Engel, C. G., 1962. Progressive metamorphism of amphibolites, north-west Adirodack mountains, New York. In: Buddington Volume. Geol. Soc. Amer. Bull., 37- 82.
Fleet, M. E. & Barnet, R., 1978. partitioning in calciferous amphiboles from the Frood mine, Subury, Ontario. Canad. Mineral., 16, 527-532,
Hamidullah, S., Bangash, I. H. & Reuber, L., 1990. Petrology of the southern amphibolite belt rocks from Mahak and surrounding area, Kohistan arc, North Pakistan. Geol. Bull. Univ. Peshawar, 23, 27-43.
Harnidullah, S., Zahid, M. & Majid, M., 1991. Mineralogy and mineral chemistry of the amphibolite belt and Main Mantle Thrust rocks from Gantar area, Allai Kohistan, North Pakistan. Geol. Bull. Univ. Peshawar, 24, 133-146.
Holdaway, M. J., 1966. Hydrothermal stability of Clinozoisite plus quartz. Amer. Jour. sci., 267, 643667.
Holdaways, M. J., 1972. Thermal stability of Al-Fe epidote as a function of and Fe contents. Contrib. Mineral. Petrol., 37, 307-340.
Jan, M. Q., 1979. Petrology of the obducted mafic and ultramafic metamorphites from the southern part of the Kohistan island arc sequence. Geol. Bull. Univ. Peshawar., 13, 95-107.
Jan, M. Q., 1988. Geochemistry of amphibolites from the southern part of the Kohistan arc, N. Pakistan. Mineral. Mag., 52, 147-159.
Jan, M. Q & Howie, R. A., 1987. Some garnets, epidotes, biotitic micas, and feldspars from the southern part of Kohistan, Acta. Mineralogica Pakistanica, 3, 5-25.
Knopf, E. B., 1931. Retrogressive metamorphism and phyllonitization. Amer. Jour. Sci., 21, 1-27.
Laird, J. & Albee, A. L., 1981. Pressure temperature and time indicators in mafic schist: their application to reconstructing the polymetamorphic history of Vermont. Amer. Jour. Sci., 281, 127-175.
Leake, B. E., 1963. Origin of amphibolites from northern Adirondacks, New York. Geol. Soc. Amer. Bull., 74, 1193-1202.
Leake, B.E., 1971. On aluminous and edenitic hornblendes. Mineral. Mag., 38, 389-407.
Leake, B E, 1978. Nomenclature of amphibole. Canad. Mineral., 16, 501-520.
Liou, J. G., Kuniyoshi, S., & Ito, K., 1974. Experimental studies of the phase relations between greenschist and amphibolite in a basaltic system. Amer. Jour. Sci., 274, 613- 632.
Manna, S. S. & Sen, S. K., 1974. Origin of garnet in the basic granulites around Saltora W. Bengal, India. Contrib. Mineral. Petrol., 44, 195-218.
Miyashiro, A., 1973. Metamorphism and metamorphic belts. George Allen and Unwin, London.
Patriat P. & Achache, J., 1984. India-Eurasia collision chronology and its implication for crustal shortening and driving mechanism of plates. Nature, 311, 615-621.
Perchuk, L. L., 1967. Temperature dependence of the coefficient of distribution of calcium between coexisting amphibole and plagioclase. Geochemistry, Vol.?? 203-205.
Plyusnina, L.P., 1982. Geothermometry and geobarometry of plagioclase-hornblende bearing assemblages: Contrib. Mineral. Petrol., 80, 146.
Powell, R., 1985. Regression diagnostics and robust regression in geothermometer / geobarometer calibration: the garnet-clinopyroxene geothermometer revisited. Jour. Met. Geol., 3, 231-243.
Raase, P., 1974. Al and Ti contents of hornblende, indicators of pressure and temperature Of regional metamorphism. Contrib. Mineral. Petrol., 45, 231-236 & 157-176.
Shah, M. T., 1986. Petrochemistry of the rocks from Shergarh Sar area, Allai Kohistan, Northern Pakistan. Unpubl. M.Phil Thesis, Univ. Peshawar.
Shah, M. T. & Majid, M., Hamidullah, S. & Shervais, J. W., 1992. Petrochemical characteristics of aphibolites from Shergarh sar area, Allai Kohistan, northern Pakistan Kashmir Jour. Geol., 10, 123-139.
Spear, F. S., 1980. NaSi=CaAl exchange equilibrium between plagioclase and amphibole. Contrib. Mineral. Petrol., 72, 33-41.
Spear, F. S., 1981. An experimental study of hornblende stability and compositional variability in amphibolite. Amer. Jour. Sci., 281, 697-734.
Spry, A., 1969. Metamorphic textures. Pergamon press, New York.
Takasu, A., 1984. Prograde and retrograde eclogites in the Sambagawa metamorphic belt, Besshi district, Japan. Jour. Petrol., 25, 619-643.
Treloar, P.J. & eight others., 1989. K-Ar and Ar-Ar geochronology of the Himalayan collision in NW Pakistan. Constraints on the timing of suturing, deformation, metamorphism and uplift. Tectonics., 8, 881-909.
Treloar, P.J. & seven others., 1990. The evolution of the Komila shear zone, Kohistan, Pakistan. In: Exposed Cross-Sections of the continental crust (MH. Salisbury, & D.M. Fountain, eds.). Kluwer Acad. Press, Amsterdam., 175-214.
Zahid, M., 1991. Petrology and geochemistry of the melange zone and southern amphibolite belt rocks from Gantar area, Alai Kohistan, Hazara, N. Pakistan. Unpubl. M.Phil. Thesis, Univ. Peshawar.
