Reaction Isograds and P-T Estimates in Metasediments on the Edge of the Karakorum Plate, Hunza, N. Pakistan
Abstract
The Huma area of N. Pakistan contains a metasedimentary sequence on the southern margin of the Karakorum Plate. The sequence is largely pelitic with carbonate layers, cut by amphibolite and granite sheets, and is bounded to the N.E. by the Karakorum Batholith, and to the S.W. by the Northern Suture at Chalt. The grade of metamorphism increases from S.W. to N.E., and this is marked by variation in mineral chemistry, and changes in mineral assemblages at a series of reaction isograds, which separate zones in which stable mineral pairs are: garnet-chlorite, chloritoid-biotite, staurolite-biotite, kynite-biotite and sillimanite-biotite from S.W, to N.E. respectively. A reaction isograd marked by the paragenesis forsterite-diopside-calcite-dolomite occurs in the siliceous dolomites, and the northeasterly dip of the isograds indicates that the zones are inverted. At Huzza, maximum metamorphic conditions given by politic rocks are 669oC and 5.5 kb, with XCO2, Fl = 0.66. The inversion of the metamorphic zones was associated with major southward thrusting during collision of the Karakorum Plate with the Kohistan Arc.
References
Broughton, R.D., 1981. Aspects of the geochemistry of some igneous and metamorphic rocks from the Hunza area, Karakorum, Pakistan. M.Sc. Thesis, Univ. Birmingham (unpubl.).
Coward, M.P., Jan, M.Q., Rex, D., Tarney, J., Thirlwall, M. & Windley, B.F., 1982a. Structural evolution of a crustal section in the Western Himalaya. Nature, London. 295, 22-24.
Coward, M.P., Jan, M.Q., Rex, D., Tarney, J., Thirlwall, M. & Windley, B.F., 1982b. Geo-tectonic framework of the Himalaya of N. Pakistan. J. Geol. Soc. London, 139, 299-308.
Desio, A., 1974. Karakorum Mountains. In: A.M. Spencer (ed.), Mesozoic – Cenozoic Orogenic belts, data for orogenic belts. Geol. Soc. London Spec. Publ. 4, 199-211.
Ferry, J.M. & Spear, F.S., 1978. Experimental calibration of the partitioning of Fe and Mg between biotite and garnet. Contrib. Mineral. Petrol. 66, 113-117.
Graham, C.M., 1974. Metabasite amphiboles of the Scottish Dalradian. Contrib. Mineral. Petrol. 47, 165-185.
Hariya, Y. & Kennedy, G.C., 1968. Equilibrium study of anorthite under high pressure and high temperature. Am. J. Sci. 266, 193-203.
Hays, J.F., 1966. Lime - alumina - silica. Carnegie Inst. Washington Ybk. 65, 234-239.
Henson, B.J. & Essene, E.J., 1971. Stability of pyrope-quartz in the system MgO - A12O3-SiO2. Contrib. Mineral. Petrol. 30, 72-83.
Holdaway, M.J., 1971. Stability of andalusite and the aluminum silicate phase diagram. Am. J. Sci. 271: 97-131.
Hoschek, G., 1969. The stability of staurolite and chloritoid and their significance in metamorphism of pelitic rocks. Contrib. Mineral. Petrol. 22, 208-232.
Lal, R.K., Mukerji, S. & Ackermand, D., 1981. Deformation and metamorphism at Takdah, Darjeeling (Eastern Himalaya). In: P.S. Saklani (ed.) Metamorphic tectonites of the Himalaya, Today and Tomorrow, New Delhi: 231-278.
Newton, R.C., 1978. Experimental and thermodynamic evidence for the operation of high pressures in Archaen metamorphism. In: B.F. Windley and S.M. Naqvi (eds.) Archaen geochemistry. Elsevier, Amsterdam, 221-240.
Okrusch, M., Bunch, T.E. & Bank, H., 1976. Paragenesis and petrogenesis of a corundum-bearing marble at Hunza (Kashmir). Mineral. Deposita (Berl.) 11, 278-297.
Orville, P.M., 1972. Plagioclase cation exchange equilibria with aqueous chloride solution: results at 700oC and 2000 bars in the presence of quartz. Am. J. Sci. 272, 234-272.
Powell, C.McA., & Vernon, R.H., 1979. Growth and rotation history of garnet porphyroblasts with inclusion spirals in a Karakorum schist. Tectanophysics 54: 2543.
Powell, R., 1978. Equilibrium thesmodynamics in geology. An introduction. Harper Row, London.
Rice, J.M., 1977. Contact metamorphism of impure dolomite limestone in the Boulder aureole, Montana. Contrib. Mineral. Petrol. 59: 237-259.
Richardson, S.W., Gilbert, M.C. $t Bell, P.M., 1969. Experimental determination of kyanite-andalusite, and andalusite-sillimanite equilibria: the aluminium silicate triple point. Am. 3. Sci. 267: 259-272.
Robie, R.A., Bethke, P.M. & Beardsley, K.M., 1 961. Selected X-ray crystallographic data, molar volumes and densities of minerals and related substances. U.S. Surv, Bull. 1248.
Robie, R.A., & Waldbaum, D.R., 1968. Thermodynamic properties of minerals and related substances at 298.15 K (25.0") and one atmosphere (1.013 bars) pressure and at higher temperatures. Bull. US. Geol. Surv. 1259.
Schmidt, R. & Wood, B.J., 1976. Phase relationships in granulitic metapelites from the Ivrea-Verbano Zone (Northern Italy). Contrib. Mineral. Petrol. 54: 255-279.
St. Onge, M.R., (1 981). 'Normal' and 'inverted' metamorphic isograds and their relation to syntectonic Proterozoic batholiths in the Wopmay orogen, Northwest Territories, Canada. Tectonophys. 76, 295-3 16.
Tahirkheli, R.A.K. & Jan, M.Q., 1979. (Eds.) Geology of Kohistan, Karakorum Himalaya, northern Pakistan. Geol. Bull. Univ. Peshawar. Spec. issue 11, 1-187.
Thompson, A.B., 1976. Mineral reactions in pelitic rocks: II. Calculation of some P-T-X (Fe-Mg) phase relations. Am. J. Sci. 276, 425-454.
substances at 298.15 K (25.0oC) and one atmosphere (1.013 bars) pressure and at Winkler, H.G.F., 1974. Petrogenesis of Metamorphic Rocks. Springer-Verlag, New York.
Wones, D.R. & Dodge, F.C.W., 1966. On the stability of phlogopite. Abs. Ann. Mtg. Amer. Geophys. Union 242.
Yakowitz, H., Myklebust, R.L. & Heinrich, K.F.J., 1973. ZAF program FRAME. NBS Tech. Note 796: 46.
Yardley, B.W.D., Leake, B.E. & Farrow, C.M., 1980. The metamorphism of Fe-rich pelites from Connemara, Ireland. J. Petrol. 21, 365-399.
