The Chilas Stratiform Complex: Field and Mineralogical Aspects
Abstract
The Chilas Complex in northern Pakistan stretches for about 300 km from Astor to western Dir and attains o width of 40 km in the middle. It was probably emplaced during the early stages of development of the Cretaceous Kohistan island arc. Field observations, supported by petrography rand mineral chemistry, suggest that the complex is composed of two groups of rocks. Group A consists predominantly of noritic rocks with subordinate pyroxenites, anorthosites and hypersthene-quartz diorites. These cover most of the complex, are generally strongly foliated and display some layering. The mineral composition of these rocks varies as follows: orthopyroxene En75 to En51; clinopyroxene Mg= 43 to 27, Fe= 9 to 22, Ca= 42 to 30; plagioclase An62 to An43.
Group R rocks occur in the form of lenses (upto 5 km2) and smaller bodies showing complex contact relations, ranging from concordant to discordant, with their host (group A) rocks. These comprise dunite, peridotites, pyroxenites, troctolites, norite, anorthosite, and pyroxene pegmatites; of these the former two are the most abundant. The principal outcrops of these rocks are found in Chilas area but isolated, small bodies occur as far to the west as Swat. The range in the mineral composition of these rocks is: olivine Fo88 to Fo73; orthopyroxene En90 to En62; clinopyroxene Mg= 46 to 37, Fe= 5 to 17, Ca= 49 to 45; plagioclase An93 to An77. Characteristic features of this group of rock are the development of excellent layering and corona structure.
The group A association may have evolved from a high-alumina basalt (calc-alkaline) magma. It is not clear whether the group B rocks represent magmatic cumulates of this magma or younger intrusions of a more basic (picritic) magma. Both groups were probably metamorphosed under pyroxene granulite facies, to be followed by amphibolite dykes and hornblende- plagioclase pegmatites.
References
Andrews-Speed, C.D. & Brookfield, M.E., 1982. Middle Paleozoic to Cenozoic geology and tectonic evolution of the northern Himalaya. Tectonophysics 82, 253-275.
Arif, M., Rehman, F. & Ahmad, A., 1984. Petrography of the Khwaza Khela area, Swat. M.Sc. Thesis, Univ, Peshawar.
Banaras, M. & Ghani, A., 1983. Petrography of the Tora Tigga complex, Munda area, Dir district. MSc. Thesis, Univ. Peshawar.
Bard, J.P., 1983a. Metamorphism of an obducted island arc: example of the Kohistan sequence (Pakistan) in the Himalayan collided range. Earth Planet. Sci. Letters 65, 133-44.
Bard, J.P., 1983b. Metamorphic evolution of an obducted island arc: example of the Kohistan sequence (Pakistan) in the Himalayan collided range. Geol. Bull. Univ. Peshawar 16, 105-184.
Bard, J.P., Maliski, H., Matte, P. & Proust, F., 1980. The Kohistan sequence: crust and mantle of an obducted island arc. Geol. Bull. Univ. Peshawar (Spec. Issue) 13, 87-93.
Cameron, EN., 1970. Composition of certain coexisting phases in the eastern part of the Bushveld Complex. Geol. Soc. S. Africa. Spec. Publ. 1, 46-58.
Chaudhry, M.N. & Chaudhry, A.G., 1974. Geology of Khagram area, Dir district. Geol. Bull. Panjab Univ. 11, 21-43.
Chaudhry, M.N., Kausar, A.B., and Lodhi, S.A.K., 1974. Geology of Timurgara-La1 Qila area, Dir district, N.W.F.P. Geol. Bull. Punjab Univ. 11, 53-73.
Coward, U.P., Jan, M.Q., Rex, D., Tarney, J., Thirlwall, M. & Windley, B.F., 1982. Tectonic frame-work of the Himalaya of N. Pakistan. J. Geol. Soc. London 139, 299-303.
Coward, U.P., Windley, B.F., Broughton, R., Luff, I.W., Patterson, M., Pudsay, C., Rex, D. & Khan, MA., in press. Collision tectonics in the N.W. Himalayas. J. Geol. Soc. London.
Desio, A., 1974. Geological reconnaissance in the middle Indus valley between Chilas and Besham QiIa (Pakistan). Bull. Soc. Geol. Ital. 93, 345-368.
Honegger, K., Dietrich, V., Frank, W., Gansser, A., Thoni, M. & Trommsdorff, V., 1982. Magmatism and metamorphism in the Ladakh Himalayas (the Indus-Tsangpo suture zone). Earth Planet. Sci. Lett. 60, 253-292.
Habib, M., Pervez, J. & Rehman, S., 1984. Petrography of the Thor and Thorly mafic-ultramafic association, Chilas complex, northern Pakistan. Unpubl. MSc. Thesis, Univ. Peshawar.
Ivanac, JT., Traves, D.M. and King, D., 1956. The geology of the N.W. portion of the Gilgit Agency. kc., Geol. Sum. Pakistan 8, 1-27.
Jackson, E.D., 1967. U1tramafic cumulates in the Stillwater, Great Dyke and Bushveld intrusions. In Ultramafic and Related Rocks, (P.J. Wyllie, ed.), Wiley Interscience, 20-38.
Jan, M.Q., 1970. Petrography of the upper part of Kohistan and southwestern Gilgit Agency along the Indus and Kandia rivers. Geol. Bull. Univ. Peshawar, 5, 27-48.
Jan, M.Q., 1979. Petrography of pyroxene granulites from northern Swat and Kohistan. Geol. Bull, Univ. Peshawar (Special Issue) 11, 65-87.
Jan, M.Q., 1980. Petrology of the obducted mafic and ultramafic metamorphites from the southern part of the Kohistan island arc sequence. Geol. Bull. Univ. Peshawar (Special Issue) 13, 95-108.
Jan, M.Q., 1982. Chemical changes accompanying the granulites to amphibolite transition in Swat, NW Pakistan. In Contemporary Geoscientific Researches in Himalaya, 2 (A.K. Sinha, ed.), B. Singh-M.P. Singh, Dehradun, India, 49-52.
Jan, M.Q., 1983. Further data an ortho- and chopyroxenes from the pyroxene granulites of Swat-Kohistan, northern Pakistan. Geol. Bull. Univ. Peshawar 16, 55-64.
Jan, M.Q., & Asif, M., 1981. A speculative tectonic model for the evolution of NW Himalaya and Karakoram. Geol. Bull. Univ. Peshawar 14, 199-201.
Jan, M.Q., & Asif, M., 1983. Geochemistry of tonalites and (quartz) diorites of the Kohistan-Ladakh (Transhimalayan) granitic belt in Swat, NW Pakistan. In "Granites of Himalaya, Karakorarn and Hindukush" (F.A. Shams, ed.) Inst. of Geol. Punjab Univ. Lahore, Pakistan.
Jan, M.Q., Banaras, M., Ghani, A. P: Asif, M., 1983. The Tora Tigga ultramafic complex, southern Dir district. Geol. Bull. Univ. Peshawar 16, 11-29.
Jan, M.Q., & Wowie, R.A., 1980. Ortho- and clinopyroxenes from pyroxene granulites of Swat Kohistan, Northern Pakistan. Min. Mag. 43, 715-728.
Jan, M.Q., & Wowie, R.A., 1981a. The mineralogy and geochemistry of the metamorphosed basic and ultrabasic rocks of Jijal Complex, Kohistan, NW Pakistan, Jour. Petrology, 22, 85-126.
Jan, M.Q., & Wowie, R.A., 1982. Hornblendic amphiboles from basic and intermediate rocks of Swat Kohistan, Northwest Pakistan. Am. Mineral. 67, 1155-1178.
Jan, M.Q., & Kempe, D.R.C., 1973. The petrology of the basic and intermediate rocks of Upper Swat, NW Pakistan. Gal. Mag. 110, 285-300.
Jan, M.Q., & Mian, I., 1971. Preliminary geology and petrography of Swat Kohistan. Geol. Bull. Univ. Peshawar 6, 1-32.
Jan, M.Q., Parvez, M.K., & Khattak, M.U.K., 1984. Coronites from the Chilas and Jijal-Patan complexes of Kohistan, N. Pakistan. (This volume).
Khattak, M.U.K. & Parvez, M.K., 1982. A petrographic account of the east-central part of the Chilas complex, N. Pakistan. MSc. Thesis, Univ. Peshawar.
Klootwijk, C., Sharma, M.L., Gergan, J., Tirkey, B., Shah, S.K. & Agarwal, V., 1979. The extent of greater India, 11, Palaeomagnetic data from the Ladakh intrusive at Kargil, northwestern Himalaya. Earth Planet., Lett, 44, 47-64.
Powell, C.McA. 1979. A speculative tectonic history of Pakistan and surroundings: Some constraints from the Indian ocean. In 'Geodynamics of Pakistan'. (A. Farah and KA, DeJong, eds.) Geol. Surv. Pakistan, Quetta, 5-24.
Rai, H. & Pande, L.C., 1983. Study of norite from the Kargil igneous complex, Indus suture zone, Ladakh, India. In Contemporary Geoscientific Researches in Himalaya, 2, (AK. Sinha, ed.), 41-47.
Sharma, K.K. & Gupta, K.R., 1983. Calc-alkaline island arc volcanism in Indus-Tsangpo suture zone. 'In 'Geology of Indus suture zone of Ladakh'. (V.C. Thakur & KX. Sharma, eds.). Wadia Inst. Himal. Geol. Dehradun, India, 71-18.
Shams, F.A., 1975. The petrology of the Thak valley igneous complex, Gilgit Agency, Northern Pakistan. Accad. Nazionale Dei Lincei, Series VIII, V. UX, 453-464.
Tahirkheli, R.A.K., Jan, M.Q. (eds.), 1979. Geology of Kohistan, Karakoram Himalaya, northern Pakistan. Geol. Bull. Univ. Peshawar 11.
Tahirkheli, R.A.K., Mattauer, M., Proust, F. & Tapponnier, P., 1979. The India-Eurasia suture zone in Northern Pakistan: Synthesis and interpretation of data on plate scale. In 'Geodynamics of Pakistan'. (A. Farah and K.A De Jong, eds.). Geol. Sum. Pakistan, 125-130.
Thakur, V.C. & Misra, D.K., 1984. Tectonic frame-work of Indus and Shyok suture zones in eastern Ladakh, Northwest Himalayas. Tectonophysics (in press).
Jackson. E.D. & Thayer, T.P., 1972. Some criteria for distinguishing stratiform, concentric and alpine peridotite-gabbro complexes. Proc. 24th Int. Geol. Cong. 2, 28-96.
Weiblen, P.V. & Morey, G.B., 1980. A summary of the stratigraphy, petrology, and structure of the Duluth Complex. Amer. Jour. Sci. 280 A, 88-133.
Windley, B.Y., Coward, M.P. & Jan, M.Q., In Press. The geology and tectonic evolution of the Karakoram-Kohistan range of the Himalaya of N. Pakistan. Symp. Vol. Academia Sinica, Beijing, China.
Zeitler, P.X., Johnson, NM., Naeser, C.W. and Tahirkheli, R.A.K., 1982. Fission track evidence for Quaternary uplift of the Nanga Parbat region, Pakistan. Nature 298, 255-257.
Casnedi, K. & Ebblin, C., 1977. Geological notes on the area behvren Astor and Skardu (Kashmir). Accad. Naz. Lancei, Ser. VUI, 62, 662468.
Virdi, N.S., 1981. Presence of parallel metamorphic belts in the northwestern Himalaya -Discussion. Tectonophysics 72, 141-146.
