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Field relations, geochemistry, origin and emplacement of the Baltoro granite, Central Karakoram

  • M. P. Searle (a1), M. B. Crawford (a2) and A. J. Rex

Abstract

The Miocene Baltoro granite forms a massive plutonic unit within the Karakoram batholith, and is composed of comagmatic monzogranites and leucogranites with a mineralogy consisting of quartz-K-feldspar-plagioclase-biotite ± muscovite ± garnet, with accessory sphene, zircon, monazite and opaques. Geochemically the Baltoro granites are mildly peraluminous, and show a calc-alkaline trend on trace-element normalised diagrams with high LIL/HFS element ratios and negative Nb, P and Ti anomalies. REE are strongly fractionated with little or no Eu anomaly. Leucogranites are depleted in most elements compared to monzogranites with notable exceptions being Rb, K and the HREEs. Initial 87Sr/86Sr ratios are 0·7072-0·7128, considerably lower than High Himalayan leucogranites (0·74-0·79), and are indicative of a lower continental crust source. The probable petrogenesis of the Baltoro granite involves dehydration melting of a biotite-rich pelite to produce a voluminous, hot, water-undersaturated magma which could then separate from its source and intrude through an already thickened and still hot crust. Fractional crystallisation of the monzogranites produced the leucogranites and a pegmatite dyke swarm. A suite of lamprophyre dykes including amphibolerich vogesites and biotite-rich minettes intrude the country rock, dominantly to the north, around the Baltoro granite. These calc-alkaline shoshonitic lamprophyres are volatile-rich mantle-derived melts intruded around the same time as the granite, indicating simultaneous melting of the mantle and lower crust beneath the Karakoram during the Miocene, approximately 30 Ma after the India-Asia collision which initially caused the crustal thickening. Intrusion of mantle melts provided heat to promote crustal melting and may have selectively contaminated the granite magma.

The Baltoro granite intrudes sillimanite gneisses with melt pods along the southern margin indicating temperatures above 700°C at the time of intrusion. Locally, internal fabrics and numerous aligned xenoliths along the southern margin in the Biafo glacier region indicate steep, southward-directed thrusting during emplacement. Along the northern contact, the Baltoro granite intrudes anchimetamorphic to greenschist facies metasedimentary rocks with an andalusite-bearing contact aureole. Northward-directed culmination collapse normal faulting during Miocene emplacement is inferred, in order to explain the P-T differences either side of the pluton. This also provided an extensional stress regime in the upper crust to accommodate the rising magma.

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Allègre, C. J. & Othman, B. D. 1980. Nd-Sr isotopic relationships in granitoid rocks and continental crust development: a chemical approach to orogenesis. NATURE 286, 335–42.
Asif Khan, M., Qasim Jan, M., Windley, B. F., Tarney, J. & Thirlwall, M. F. 1989. The Chilas mafic-ultramafic igneous complex; The root of the Kohistan island arc in the Himalaya of northern Pakistan. GEOL SOC AM, SPEC PAP 232, 7594.
Burchfiel, C. & Royden, L. 1985. N-S extension under the convergent Himalayan regime. GEOLOGY 13, 679–82.
Burchfiel, B. C., Molnar, P., Ziyun, Z., K'uangyi, L., Shuji, W., Minmin, H. & Sutter, J. 1989. Geology of the Ulugh Muztagh area, northern Tibet. EARTH PLANET SCI LETT 94, 5770.
Burg, J. P., Brunel, M., Gapais, D., Chen, G. M. & Liu, G. H. 1984. Deformation of leucogranites of the crystalline Main Central thrust sheet in southern Tibet (China). J STRUCT GEOL 6, 535–42.
Coulon, C., Maluski, H., Bollinger, C. & Wang, S. 1986. Mesozoic and Cenozoic volcanic rocks of central and southern Tibet. 39Ar-40 Ar dating, petrological characteristics and geodynamical significance. EARTH PLANET SCI LETT 79, 281302.
Clemens, J. D. & Vielzeuf, D. 1987. Constraints on melting and magma production in the crust. EARTH PLANET SCI LETT 86, 287306.
Cuney, M., LeFort, P. & Zhixiang, W. 1984. Uranium and thorium geochemistry and mineralogy in the Manaslu leucogranite (Nepal Himalaya). In Geology of Granites and Their Metallogenic Relations, 853–73. Proc. International symposium Nanjing University, China.
Debon, F. & LeFort, P. 1982. A chemical mineralogical classification of common plutonic rocks and associations. TRANS R SOC EDINBURGH EARTH SCI 73, 135–49.
Debon, F., Zimmerman, J. L. & Betrand, J. M. 1986. Le granite du Baltoro (batholite axial due Karakoram, nord Pakistan): une intrusion subalcaline d'age Miocène Superieur. C. R. ACAD SCI 303, 463–8.
Deng, W. 1978. A preliminary study on the petrology and petrochemistry of the Quaternary volcanic rocks of northern Tibet autonomous region (in Chinese). ACTA GEOL SINICA 52, 148–62.
Deniel, C., Vidal, Ph., Fernandez, A., LeFort, P. and Peucat, J. J. 1987. Isotopic study of the Manaslu granite (Himalaya, Nepal): inferences on the age and source of Himalayan leucogranites. CONTRIB MINERAL PETROL 96, 7892.
De Paolo, D. J. 1981. A neodymium and strontium isotopic study of the Mesozoic calc-alkaline granite batholiths of the Sierra Nevada and Peninsula Ranges, California: J GEOPHYS RES 86, 10470–88.
Desio, A. 1964. Geological tentative map of the western Karakoram. Institute of Geology, University of Milan.
Desio, A. & Zanettin, B. 1970. Geology of the Baltoro Basin. Leiden: E. J. Brill.
England, P. C. & Thompson, A. 1986. Some thermal and tectonic models for crustal melting in continental collision zones. In Coward, M. P. & Ries, A. (eds) Collision Tectonics, 8394. Geological Society London, Special Publication 19.
Gromet, L. P. & Silver, L. T. 1983. R.E.E. distributions among minerals in a granodiorite and their petrogenic implications. GEOCHIM COSMOCHIM ACTA 47, 925–39.
Harris, N. B. W., Pearce, J. A. & Tindle, A. G. 1986. Geochemical characteristics of collision zone magmatism. In Coward, M. P. & Ries, A. (eds), Collision Tectonics, 6781. Geological Society, London Special Publication 19.
Hawkesworth, C. J. & Norry, M. J. 1983. Continental Basalts and Mantle Xenoliths. Cheshire: Shiva Publications.
Henderson, P. 1984. General geochemical properties and abundances of rare-earth elements. In Henderson, P. (ed.) Rare-earth Element Geochemistry, 129. Barking: Elsevier.
Herren, E. 1987. Zanskar Shear Zone: Northeast-southwest extension within the Higher Himalayas (Ladakh, India). GEOLOGY 15, 409–13.
Hodges, K. V., Hubbard, M. S. & Silverberg, D. S. 1988a. Metamorphic constraints on the thermal evolution of the central Himalayan orogen. PHILOS TRANS SOC LONDON A326, 257–80.
Hodges, K. V., LeFort, P. & Pêcher, A. 1988b. Possible thermal buffering by crustal anatexis in collisional orogens: Thermobarometric evidence from the Nepalese Himalaya. GEOLOGY 16, 707–10.
Huppart, H. E. & Sparks, R. S. J. 1988. The generation of granitic magmas by intrusion of basalt into continental crust. J PETROL 29, 599624.
Jacobssen, S. B. & Wasserburg, G. J. 1980. Sm-Nd isotopic evolution of chondrites. EARTH PLANET SCI LETT 50, 149–55.
Jaupart, C. & Provost, A. 1985. Heat focussing, granite genesis and inverted metamorphic gradients in continental collision zones. EARTH PLANET SCI LETT 73, 385–97.
LeFort, P. 1975. Himalayas: The collided range, present knowledge of the continental arc. AM J SCI 155A, 144.
LeFort, P. 1981. Manaslu leucogranite: A collisional signature of the Himalaya, a model for its genesis and emplacement. J GEOPHYS RES 86, 10, 545–68.
LeFort, P. 1986. Metamorphism and magmatism of the Himalaya. In Coward, M. P. & Ries, A. C. (eds) Collision Tectonics, 159–72. Geological Society of London Special Publication 19.
LeFort, P., Cuney, M., Deniel, C., France-Lanord, C., Sheppard, S. M. F., Upreti, B. N. & Vidal, P. 1987. Generation of the Himalayan leucogranites. Tectonophysics 134, 3957.
Marsh, N. G., Saunders, A. D., Tarney, T. & Dick, H. J. B. 1979. Geochemistry of basalts from the Shikoku and Daito basins, DSDP Leg 58. In Vries Klein, G. de & Kobayashi, K. et al. (eds) Initial Reports of the Deep Sea Drilling Project 49, 657–91. Washington DC.
McKenna, L. W. & Walker, J. D. 1990. Geochemistry of crustally derived leucocratic igneous rocks from the Ulugh Muztagh area, northern Tibet and their implications for the formation of the Tibetan Plateau. J GEOPHYS RES 95, 21, 483502.
Miller, C. F. 1985. Are strongly peraluminous magmas derived from pelitic sedimentary sources? J GEOL 93, 673–89.
Molnar, P. 1984. Structure and tectonics of the Himalaya: constraints and implications of geophysical data. ANNU REV EARTH PLANET SCI 12, 489518.
Norin, E. 1946. Geological Explorations in Western Tibet. InReports from the Scientific expedition to the northwestern provinces of China under the leadership of Dr Sven Hedin”, Publ 29 (III) Geology 7, Trycheri Aktiebolaget, Thule, 205p., Stockholm.
Parrish, R. R. & Tirrul, R. 1989. U-Pb age of the Baltoro granite, northwest Himalaya, and implications for monazite U-Pb systematics. Geology 17, 1076–9.
Pearce, J. A. & Houjon, Mei 1988. Volcanic rocks of the 1985 Geotraverse. Lhasa to Golmud. PHILOS TRANS R SOC LONDON A327, 169201.
Pearce, J. A., Harris, N. B. W. & Tindle, A. G. 1984. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. J PETROL 25, 956–83.
Pinet, C. & Jaupart, C. 1987. A thermal model for the distribution in space and time of the Himalayan granites. EARTH PLANET SCI LETT 84, 8799.
Pitcher, W. 1983. Granite type and tectonic environment. In: Hsu, K. (ed.) Mountain Building Processes, 1940, London: Academic Press
Pognante, U. 1990. Shoshonitic and ultrapotassic post-collisional dykes from northern Karakoram (Sinkiang, China). LITHOS 26, 305–16.
Potts, P. J., Williams, Thorpe O., Isaacs, M. & Wright, D. W. 1985. High precision instrumental neutron-activation analysis of geological samples employing simultaneous counting with both planar and coaxial detectors. CHEM GEOL 48, 145–55.
Rex, A. J., Searle, M. P., Tirrul, R., Crawford, M. B., Prior, D. J., Rex, D. C. & Barnicoat, A. 1988. The geochemical and tectonic evolution of the central Karakoram, North Pakistan. PHILOS TRANS SOC LONDON A326, 229–55.
Rogers, N. W., Hawkesworth, C. J., Parker, R. J. & Marsh, J. S. 1985. The geochemistry of potassic lavas from Vulsini, central Italy, and implications for mantle enrichment processes beneath the Roman region. CONTRIB MINERAL PETROL 90, 244–57.
Saunders, A. D., Rogers, G., Marriner, F. G., Terrel, D. J. & Verma, S. P. 1987. Geochemistry of Cenozoic volcanic rocks Baja California, Mexico: implications for the petrogenesis of post-subduction magmas. J VOLCANOL GEOTHERM RES 32, 223–46.
Scaillet, B., France-Lanord, C. and LeFort, P. 1990. Badrinath-Gangotri plutons (Garhwal, India): petrological and geochemical evidence for fractionation processes in a High Himalayan leucogranite. J VOLCANOL GEOTHERM RES 44, 163–88.
Schärer, U. 1984. The effect of initial 230Th disequilibration on U-Pb ages: the Makalu case. EARTH PLANET SCI LETT 67, 191204.
Schärer, U., Xu, R. H. & Allegre, C. J. 1986. U-(Th)-Pb systematics and ages of Himalayan leucogranites, south Tibet. EARTH PLANET SCI LETT 77, 3548.
Schärer, U., Copeland, P., Harrison, T. M. & Searle, M. P. 1990. Age, cooling history and origin of post-collisional leucogranites in the Karakoram batholith; a multi-system isotope study N. Pakistan; J GEOL 98, 233–51.
Searle, M. P. 1986. Structural evolution and sequence of thrusting in the High Himalayan, Tibetan-Tethys and Indus suture zones of Zanskar and Ladakh, western Himalaya. J STRUCT GEOL 8, 923–36.
Searle, M. P. 1991. Geology and Tectonics of the Karakoram Mountains. Chichester: John Wiley and Sons. (With Geological map of the Central Karakoram, scale 1:250,000.)
Searle, M. P. & Fryer, B. J. 1986. Garnet, tourmaline and muscovite-bearing leucogranites, gneisses and migmatites of the Higher Himalaya from Zanskar, Kulu, Lahoul and Kashmir. In Coward, M. P. & Ries, A. C. (eds) Collision Tectonics, 185201. Geological Society of London Special Publication 19.
Searle, M. P. & Rex, A. J. 1989. Thermal Model for the Zanskar Himalaya, J METAMORPH GEOL 7, 127–34.
Searle, M. P. & Tirrul, R. 1991. Structural and thermal evolution of the Karakoram crust. J GEOL SOC LONDON 148, 6582.
Searle, M. P., Rex, A. J., Tirrul, R., Windley, B. F., St Onge, M. & Hoffman, P. 1986. A geological profile across the Baltoro Karakoram Range, N. Pakistan. UNIV PESHAWAR GEOL BULL 19, 112.
Searle, M. P., Windley, B. F., Coward, M. P., Cooper, D. J. W., Rex, A. J., Rex, D.,Tingdong, Li, Xuchang, Xiao,Jan, M. Q., Thakur, V. C. & Kumar, S. 1987. The closing of Tethys and the tectonics of the Himalayas. BULL GEOL SOC AM 98, 678701.
Searle, M. P., Cooper, D. J. W. & Rex, A. J. 1988. Collision tectonics of the Ladakh-Zanskar Himalaya. PHILOS TRANS R SOC LONDON A326, 117–50.
Searle, M. P., Rex, A. J., Tirrul, R., Rex, D. C., Barnicoat, A. & Windley, B. F. 1989. Metamorphic, magmatic and tectonic evolution of the Central Karakoram in the Biafo-Baltoro-Hushe regions of N. Pakistan. GEOL SOC AM, SPEC PAP 232, 4773.
Searle, M. P., Parrish, R. R., Tirrul, R. & Rex, D. C. 1990. Age of crystallisation and cooling of the K2 gneiss in the Baltoro Karakoram. J GEOL SOC LONDON 147, 603–6.
Srimal, N. 1986. India-Asia collision: implications from the geology of the eastern Karakoram. GEOLOGY 14, 523–7.
Tarney, J., Saunders, A. D., Weaver, S. D., Donellan, N. C. B. & Hendry, G. L. 1979. Minor element chemistry of Leg 49, North Atlantic Ocean. In Luyendyk, B. P., Cann, J. R. et al. (eds) Initial Reports of the Deep Sea Drilling Project 49, 657–91 Washington DC.
Taylor, S. R. & McLennan, S. M. 1981. The composition and evolution of the continental crust: rare-earth evidence from sedimentary rocks. PHILOS TRANS SOC LONDON A301, 381.
Taylor, S. R. & McLennan, S. M. 1985. The Continental Crust: It's Composition and Evolution. Oxford: Blackwell Scientific Publishers.
Vidal, P., Cocherie, A. & LeFort, P. 1982. Geochemical investigations of the origin of the Manaslu leucogranite (Himalaya, Nepal). GEOCHIM COSMOCHIM ACTA 46, 2279–92.
Vielzeuf, D. & Holloway, J. R. 1988. Experimental determinations of the fluid-absent melting relations in the pelitic system: consequences for crustal differentiation. CONTRIB MINERAL PETROL 19, 111–31.
White, A. J. R. & Chappell, B. W. 1977. Ultrametamorphism and granitoid genesis. TECTONOPHYSICS 43, 722.
Wyllie, P. J., Huang, C. R., Stern, C. R. & Maaloe, S. 1976. Granitic magmas: possible and impossible sources, water contents and crystallization sequences. CAN J EARTH SCI 13, 1007–19.

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