Rock Magnetism of the Kohistan Island Arc, Pakistan
Abstract
In Northern Pakistan a section of island arc crust is exposed in Kohistan, northwest Himalaya. To better understand how magnetism and magnetic mineralogy depend on metamorphism, we have studied, by means of field and laboratory investigations, crystalline rocks of the Kohistan island arc. Our results are based on susceptibility and sampling traverses over 500 km in length, and rock magnetic and petrographic study of collected samples. Measurements and sampling concentrated on two profiles through Kohistan, one following the Swat River, the other following the Karakoram Highway. Of the lithologies sampled along these two profiles, the main sequence of interest consists of rocks of the Kohistan island arc. With the exception of volumetrically-minor serpentinized ultramafic rocks, the magnetic lithologies are pyroxene granulite, and gabbro-norite of the Chilas complex. Amphibolites, garnet granulites, metasupra-crustals and Kohistan arc diorites and granites are only weakly magnetized. The same is true of Karakoram batholith granitoids. As far as prograde metamorphism of moderate to high-grade mafic rocks in the section is concerned, magnetic pyroxene granulites are strongly magnetized, relative to amphibolite facies equivalents and garnet granulites. Considering all lithologies, the calculated Koenigsberger's ratios are generally less than 2 (110 of 127 samples), commonly less than 1 (89 samples), and rarely in excess of 10 (3 samples), which indicates that the magnetization of these rocks is rarely dominated by remanence. The directions of natural remanent magnetization (NRM) are scattered, with a mean declination not far from zero, and a positive mean inclination. Petrographic and electron microprobe study of mineral assemblages and textures related to retrogressive metamorphism of pyroxene and garnet granulites shows that amphibolite facies overprinting along planar zones was an Fe-Ti oxide-consuming event. Lamellae of magnetite in magnetite-ilmenite intergrowths were replaced by silicates during retrogressive metamorphism of the pyroxene granulites. Regional-scale magnetic structure in this part of the crust is defined by magnetic contrasts between the Chilas complex and the surrounding Kamila amphibolites and Kohistan granites and diorites.
References
Bard, J.P., 1983. Metamorphism of an obducted island arc: example of the Kohistan sequence (Pakistan) in the Himalayan Collided Range. Earth Planet. Sci. Lett. 65, 133-144.
Bohlen, S.R., Wall, V.J. and Boettcher, A.L., 1983. Geobarometry in granulites. In: Kinetics and equilibrium in mineral reactions. (S.K. Saxena, ed.). Advances in Physical Geo-chemistry, 3, 141-171.
Coward, M.P., Butler, R.W.H., Asif Khan, M., and Knipe, R.J., 1987. The tectonic history of Kohistan and its implications for Himalaya structure. Geol. Soc. London, 144, 377-391.
Coward, M.P., Jan, M.Q., Rex, D., Tarney, J., Thirwall, M., and Windley, B.F., 1982. Structural evolution of a crustal section in Western Himalayan. Nature, 295, 22-24.
Frost, B.R. and Chacko, T. The granulite uncertainty principle: Limitations on thermobarometry in granulites. J. Geol., in press.
Frost, B.R. and Shive, P.N., 1986. Magnetic mineralogy of the lower continental crust. J. Geo-phys. Res., 91, 6513-6521.
Hamidullah, S. and Jan, M.Q., 1986. Preliminary pertrochemica1 study of the Chilas complex, Kohistan Island Arc, Northern Pakistan. Geol. Bull. Univ. Peshawar, 19, 157-182.
Ivanac, J.F., Traves, D.M. and King, D., 1956. The geology of the north west region of Gilgit Agency. Rec. Geol. Sum. Pak., 8, 3-26.
Jan, M.Q., 1979a. Petrography of the Jijal Complex, Kohistan. In: Geology of Kohistan, Karakoram Himalaya, Northern Pakistan. (R.A.K. Tahirkheli, and M.Q, Jan, eds.) Special Issue Geol. Bull. Univ. Peshawar, 11, 31-50.
Jan, M.Q., 1979b. Petrography of the amphibolites of Swat and Kohistan. In: Geology of Kohistan, Karakoram Himalaya, Northern Pakistan. (R.A.K. Tahirkheli, and M.Q, Jan, eds.) Special Issue Geol. Bull. Univ. Peshawar, 11, 51-64.
Jan, M.Q., 1979c. Petrography of the pyroxene granulites from northern Swat and Kohistan. In: Geology of Kohistan, Karakoram Himalaya, Northern Pakistan. (R.A.K. Tahirkheli, and M.Q. Jan, eds.). Special Issue Geol. Bull. Univ. Peshawar, 11, 65-87.
Jan, M.Q., 1983, Further data on orth- and clinopyroxenes from the pyroxene granulites of Swat-Kohistan, northern Pakistan. Geol. Bull. Univ. Peshawar, 16, 55-64.
Jan, M.Q., 1988. Geochemistry of amphibolites from the southern part of the Kohistan arc, N. Pakistan. Min, Mag., 52, 147-159.
Jan, M.Q. and Howie, R.A., 1980. Ortho-and clinopyroxenes from the pyroxene granulites of Swat Kohistan, northern Pakistan. Min. Mag., 43, 715-726.
Jan, M.Q. and Howie, R.A., 1981. The mineralogy and geochemistry of the metamorphosed basic and ultrabasic rocks of the Jijal complex, Kohistan, NW Pakistan. J. Petr., 22, 85-126.
Khan, M.A., Jan, M.Q., Windley, B.F., Tarney, J. and Thirlwall, M.F., 1989. The Chilas mafic-ultramafic igneous complex; the root of the Kohistan Island Arc in the Himalaya of northern Pakistan. GSA special paper, 232.
Larsen, H.C., 1981. A high-pressure granulite facies complex in north-west Payers Land, East Greenland fold belt. Bull. Geol. Soc. Denmark, 29, 161-174.
Malinconico, L.L., 1982, Structure of the Himalayan suture zone of Pakistan interpreted from gravity and magnetic data. Unpub. Ph.D. thesis, Dartmouth College, Hanover, New Hampshire.
Newton, R.C. and Perkins, D., 1981. Ancient granulite terrains - "eight kbar metamorphism." EOS, 62, 420.
O'Hara, M.J., 1977. Thermal history of excavation of Archaean gneisses from the base of the continental crust. Geol. Soc. London, 134, 185-200.
Petterson, M.G. and Windley B.F., 1986. Petrological and geochemical evolution of the Kohistan Arc Batholith. Geol. Bull. Univ. Peshawar, 19, 121-149.
Pudsey, C.J. and Maguire, P.K.H., 1986. Magnetic profiles across the Northern Suture, Kohistan, NW Pakistan. Geol. Bull. Univ. Peshawar, 19, 47-60.
Schlinger, C.M., 1985. The magnetization of the lower crust and the interpretation of regional magnetic anomalies: The example from Lofoten and Vesteralin, Norway. J. Geophys. Res., 90, 11484-1 1504.
Schilnger, C.M. and Veblen, D.R., 1989. Magnetism and transmission electron microscopy of Fe-Ti oxides and pyroxenes in a granulite from Lofoten, Norway. J. Geophys. Res., in press.
Tahirkheli, R.A.K., 1979a. Geology of Kohistan and adjoining Eurasian and Indo-Pakistan continents, Pakistan. In: Geology of Kohistan, Karakoram Himalaya, Northern Pakistan. (R.A.K. Tahirkheli, and M.Q. Jan. eds.). Special Issue Geol. Bull. Univ. Peshawar, 11, 1-30.
Tahirkheli, R.A.K., 1979b. Geotectonic evolution of Kohistan. In: Geology of Kohistan, Karakoram Himalaya, Northern Pakistan. (R.A.K. Tahirkheli and M.Q. Jan. eds.). Special Issue Geol. Bull. Univ. Peshawar, 11, 113-130.
Wasilewski, P. and Warner, R.D., 1988. Magnetic petrology of deep crustal rocks - Ivrea Zone. Italy, Earth Planet. Sci. Lett. 87, 347-361.
Wasilewski, P. and Fountain, D.M., 1982. The Ivrea zone as a model for the distribution of magnetization in continental crust. Geophys. Res. Lett, 9, 333-336.
Wasilewski, P. and Mayhew, M.A., 1982. Crustal xenolith magnetic properties and long wavelength anomaly source requirements. Geophys. Res. Lett., 9, 329-332.
Williams, M.C., Shive, P.N., Fountain, D.M. and Frost, B.R., 1986. Magnetic properties of exposed deep crustal rocks from the Superior province of Manitoba. Earth Planet. Sci. Lett., 76, 176-184.
