Multiple sources of magmatism: granitoids from southeast Kohistan, NW Himalayas, Pakistan
Abstract
The Kohistan island arc terrane in the northwestern Himalayas of N. Pakistan is sandwiched between the Indian and Karakoram plates. The base of the arc is occupied by a major stratiform ultramafic-gabbroic complex (the Sapat-Babusar complex), which overrides the crust of the Indian plate along the Indus suture (i.e., the Main Mantle Thrust; MMT). It was intruded into- the base of a thick pile of metavolcanics (the Kamila belt), which comprise a tectonic collage of MORB-type tholeiitic basalts, island-arc tholeiites and calc-alkaline andesites. The Chilas complex, comprising ultramafic and gabbronorite rocks, is also intrusive into the Kamila belt, it is emplaced onto the top rather than the base of the Kamila belt. A sizeable proportion of granitoid rocks are present in the south-eastern part of Kohistan, which intruded the Kamila amphibolites. These are predominantly dioritic in composition, but include gabbros, granodiorites, granites and trondhjemites. The granitoids occur in two types: (l) large sheet-like lenticular masses, and (2) minor intrusives in the form of veins, sills or dykes. Three large sheets like bodies are mapped. All these bodies are composite, comprising gabbros, diorite/tonalite, granodiorite and granite. The. minor intrusions of granitic and trondhjemitic composition are abundantly present in the form of veins, sills and dykes; and are characterized by variation in distribution. Strong shearing transformed the rocks into blastomylonite gneisses. The mineral assemblage consists of quartz, plagioclase, amphibole, epidote, chlorite, biotite, muscovite, sphene, magnetite and apatite. The granitoids show a wide variation in SiO2 content. Spidergrams show three distinctive patterns;(i) flat patterns showing small enrichment in HFSE with lack of Nb anomaly, (ii) enrich in LILE sloping towards right with Nb anomaly and showing coherent trends with increasing LILE component from diorite through granodiorite to granite and (iii) the highly spiked patterns sloping towards right with more depletion in Nb, P, Zr, Ti and Y. The HFSE enriched granitoids are comparable with the host metavolcanics of MORB type Kamila amphibolite belt. This supports the petrogenetic link between this group of granitoids and the host Kamila amphibolites. The HFSE depleted granitoids include rock types gabbro, diorite, tonalite, granodiorite and granite, which show enrichments in LILE relative to HFSE, and coherent behavior. Majority of the rocks have typical subduction-related chemistry, with characteristics such as low TiO2 and high A12O3 contents, high LIL/HFS element ratios, variable interalkali ratios and distinct Nb depletion anomalies, all consistent with magma derivation from a metasomatized mantle wedge above a subduction zone. The trondhjemites, characterized by enriched HFSE, are in intimate association with Kamila amphibolite belt and the HFSE depleted trondhjemites with highly spiked patterns with very low incompatible trace element abundances are considered (l) to be unrelated with gabbro-diorite-tonalite-granodiorites-granite series, and (2) to have resulted from partial melting of Kamila amphibolites. The melt producedfor these trondhjemites whether due to the process of subduction or due to the intrusion of 90-80 Ma Chilas Complex. and crustal shortening accompanying the Jal shear zone of 80 Ma is not clear.
References
Collision-related granitoid magmatism and crustal structure of the Hunza Karakoram, North Pakistan. In: Himalayan Tectonics (P.J. Treloar & M.P. Searle eds.). Geol. Soc. Sp. Pub., 74, 53-68.
Crawford, M.B. & Windley, B.F., 1990. Leucogranites of the Himalaya/Karakoram: implications for magmatic evolution within collisional belts and the study of collision-related leucogranite petrogenesis. J. Geotherm. Res., 44, 1-19.
Crawford, M.B., 1988. Leucogranites of the NW Himalaya: crust-mantle interaction beneath the Karakoram and the magmatic evolution of collisional belts. Unpublished PhD thesis, UniVersity of Leicester.
Davies, R.G., 1965. The nature of the Upper Swat Hornblendic Group. Geol. Bull. Univ. Punjab, 2, 51-52.
De La Roche, H., Leterrier, J., Grande C.P. & Marchal, M., 1980. A classification of volcanic and plutonic rocks using RlR2 diagrams and major element analyses its relationships and current nomenclature. Chem. Geo., 29, 183-210.
Debon, F. & Le Fort, P., 1983. A chemical mineralogical classification of common pllutonic rocks and associations. Earth sci., 73, 135-149.
Deniel, C., Vidal, P., Fernandez, A., Le Fort, P. & Pecaut, J.J., 1987. Isotopic study of the Manaslu granite (Himalayas, Nepal): inferences on the age and source of Himalayan leucogranites. Cont. Mineral. Petrol., 96, 78-92.
Drummond, M.S. & Defant, M.J., 1990. A model for trondhjemite-tonalite-dacite genesis and crustal growth via slab melting: Archean to modern comparisons. J. Geophy. Res., 95, 503-521.
Evenson, N.M., Hamilton, P. J. & O'Nions, R K., 1978. Rare-earth abundances in chondritic meteorites. Geochim. Cosmochim. Acta., 42, 1 199-1212.
George, M.T., Nigel, B.W., Harris & R.W. H. Butler., 1993. The tectonic implications of contrasting granite magmatism between the Kohistan island arc and the Nanga Parbat-Haramosh Massif, Pakistan Himalaya. In: Himalayan Tectonics (P.J. Treloar & M.P. Searle eds.), Geol. Soc. sp. Pub. 74, 173-191.
Ghazanfar, M., Chaudhry, M.N. & Hussain, M.S., 1991. Geology and petrotectonics of southeast Kohistan, northwest Himalaya, Pakistan. Kashmir. J. Geol., 8 & 9, 67-97.
Harris, N.B.W. & Inger, S. 1992. Trace Element modeling of Pelite-derived Granites. Cont. Mineral. Petrol., 110, 46-56.
Hayden, H.H., 1916. Notes on the geology of the Chitral, Gilgit and Pamirs. Rec. Geol. Sur. India, 45, 271-335.
Honegger, K., Dietrich, V., Frank, W. Gansser, A., Thoni, M. & Trommsdorff, V., 1982. Magmatism and metamorphism in the Ladakh Himalayas the Indus-Tsang-Po Suture Zone, Earth Planet. Sci. Lett., 60, 253-292.
Inger, S & Harris, N., 1993. Geochemical constraints on leucogranite magmatism in the Langtang Valley, Nepal Himalaya. J, Petrol., 34, 345-368.
Ivanac, J.F., Traves, D.M. and King, D., 1956. The geology of the northwest portion of the Gilgit Agency. Rec. Geol. Sur. Pakistan, 3, 1-27.
Jan, M.Q. & Khan, M.A., 1983. Geochemistry of tonalites and quartz diorites of the Kohistan-Ladakh (Transhimalayan) granitic belt in Swat, 110
N. Pakistan. In: Granites of Himalayan, Karakorum and Hindu Kush (F.A. Sham ed). Institute of Geology, Punj. Univ., Lahore, 355-376.
Jan, M.Q. & Howie, R.A., 1981. The mineralogy and geochemistry of the metamorphosed basic and ultrabasic rocks of the Jijal complex, Kohistan, NW Pakistan. J. Petrol., 22, 85-126.
Jan, M.Q. & Mian, I, 1971. Preliminary geology and petrography of Swat Kohistan. Geol. Bull. Peshawar, 6, 1-32.
Jan, M.Q., 1970. Petrography of the upper part of Kohistan and southwestern Gilgit Agency along the Indus and Kandia rivers. Geol. Bull. Peshawar, 5, 27-48
Khan, M.A., 1997. Geology of southeast Kohistan, NW Himalayas, Pakistan. Unpublished PhD Thesis, Peshawar University, Pakistan,
Le Bas, M.J., Le Maitre, R.W., Streckeisen, A. & Zanettin, B., 1986, A chemical classification of volcanic rocks based on total alkali-silica diagram. J. Petrol., 27, 745-750.
Le Maitre, R. W., Bateman, P., Dudek, A., Keller, J., Lameyre, Le Bas, M.J., Sabine, P.A., Schmid, R., Sorensen, H., Streckeisen, A., Woolley, A.R. & Zanettin, B., 1989. A classification of igneous rocks and glossary of terms. Blackwell, Oxford.
Martin, H., 1987. Petrogenesis of Archean Trondhjemites, Tonalites, and Granodiorites from Eastern Finland: Major and Trace Element Geochemistry. J. Petrol., 23, 921-953.
O'Connor, J.T., 1965. A classification for quartz-rich igneous rocks based on feldspar ratios. U.S. Geol. Sur., Prof. Pap., 525B, B9-B4.
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, 956-983.
Petterson, M.G. & Windley, B.F., 1985. Rb-Sr dating of the Kohistan arc batholith in the Trans Himalaya of N. Pakistan and tectonic implications. Earth Planet. Sci. Lett., 74, 54-75.
Petterson, M.G. & Windley, B.F., 1991. Changing source regions of magmas and crustal growth in the Trans-Himalayas: Evidence from the Chalt volcanics and Kohistan batholith, Kohistan, N. Pakistan. Earth Planet. Sci. Lett. 102, 326-346.
Saunder, A.D., Norry, M.I. & Tarney, J., 1988. origin of MORB and chemically depleted Mantle Reservoirs: trace Element Constraints. J. Petrol. Sp. Lithosphere Issue, 29, 415-445.
Saunder, A.D., Tarney, J., Marsh, N.G., Wood, D.A. & Panayiotou, A., 1980. Ophiolites as ocean crust or marginal basin crust; a geochemical approach. In: Ophiolites; Pro., Int. ophiolite sym., 193-204.
Searle, M.P., Rex, A.J., Tirrul, R., Rex, D.C. & Barnicoat, A., 1989. Metamorphic, magmatic and tectonic evolution of the Central Karakoram in the Biafo-BaltoroHushe regions of N. Pakistan. In: Tectonics of Western Himalayas (L.L. Malinconico & R.J. Lillee eds.), Geol. Soc. Am., sp. Pap., 232, 47-73.
Searle, M.P. & Khan, M.A., 1996. Geological map of North Pakistan and adjacent areas of northern Ladakh and, western Tibet.
Searle, M.P., Crawford, M.B. & Rex, A.J., 1992. Field relations, geochemistry, origin and emplacement of the Boltoro granite, central Karakoram. Trans. Royal Soc. Edinburgh, 83, 519-538.
Sun, S.S. & Mcdonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: Magmatism in the Ocean Basins (A.D Saunders & M.J. Norry eds.) Geol. Soc., Lond., sp. Pub., 42, 313-345.
Tahirkheli, R.A.K. & Jan, M.Q., 1979. Geology of Kohistan, Karakoram Himalaya, northern Pakistan. Geol. Bull. Peshawar, 11, 187p.
