Journal of the Geological Society, London, Vol. 147, 1990, pp. 603-606, 3 figs 2 tables. Printed in Northern Ireland

SHORT PAPER evidence of Precambrian inheritance (Parrish & Tirrull989). Earlier pre-collision within theKarakoram Age of crystallization and cooling of the include the unit (Fig. 1) composed in the Baltoro of and hornblende-rich foliated granodiorites, which gave three K-Ar hornblende ages spanning 82-75 f 3 Ma M.P. SEARLE', R. R. PARRISH', (Searle et al. 1989), and the Hushegneiss, SE of the Baltoro R.TIRRUL** & D.C. REX3 area, which has a U-Pb zircon age of 145 f 5 Ma and two 'Department of Earth Sciences, Oxford University, 40Ar-39Arages of 203 f 0.6 Ma and 204 f 1.4 Ma (Searle et Parks Road, Oxford OX1 3PR al. 1989). Further west,hornblende-bearing granodiorites 'Geological Survey of Canada, 601 Booth Street, from the Hunza plutonic unit gave a U-Pb age of 95 f 4 Ottawa, Canada KlA OE8 (LeFort et al. 1983) and similar granites at the Darkot Pass Department of Earth Sciences, Leeds University, gave a Rb-Sr isochron age of 111 f 6 (Debon et al. 1987). Leeds, LS2 9JT These pre-collision granites of the Karakoram batholith all have calc-alkaline geochemical affinities and have been interpretedas Andean-type granitesalong thesouthern continental margin of the Asian plate, related to the Themountains of K2 (8611 m)and (8047111) in the northward of Tethyan oceanic crust (LeFort et Baltoro (northernPakistan) are composedof Karakoram al. 1983; Debon et al. 1987; Rex et al. 1988). -hornblendeand biotite-hornblende-K-feldspar ortho- and Fphiholite-facies paragneisses,intruded by - K2 gneiss. The K2 gneiss in northernmost is not biotite-muscovite-tourmaline leucograniticveins. A U-Pb zircon part of the Karakoram batholith sensu stricto but outcrops age of 115-1u) Ma was obtained on an orthogneiss from the south along a wide belt NE of the batholith, separated bylow- face of K2. 40Ar-J9Aranalysis on hornblende yields a plateau age of grademetasediments. These metasedimentswere termed 90.6 f 1.8 Ma, consistent with a mid- phase of - the Doksamsequence and the Biange gneiss by Desio & tism, concomitant with early subduction-related components of the Zanettin (1970), and are metamorphosedequivalents of Karakoram batholith (Muztagh Tower unit, Hunza plutonic unit). the Carboniferous black shales and Permo-Triassic carbo- We interpret the K2 gneiss as representing a culmination of mid- nates exposed along the Range. crustal along a discontinuousbut wide zone north of the rocks The gneiss includes both orthogneisses and para- Karakoram batholith. K2 gneisses which appear to bein -bounded relationship Recent mapping around the upper Baltoro, Godwin-Austin with the adjacentsediments. The paragneisses include and Savoia glaciers in the central Karakoram has identified clinopyroxene-hornblende bearing psammites and garnet- very large-scale culminations of mid-crustal gneisses (K2 diopside bearingmarbles in the amphibolite facies, and gneiss) surrounded by upper crustalsedimentary rocks of were subdivided by Desio & Zanettin (1970) who des- theGasherbrum sedimentaryseries (Fig. 1).The regional cribed them as the Falchan gneiss. We include these rocks geology of the area has been described by with the orthogneisses as the K2 gneiss because theyap- Desio & Zanettin (1970) andmore recentlySearle et al. pear to be interbanded on the west flank of Broad Peak (1986, 1989), and Rex et al. (1988). This paper describes the and the SW spur of K2. The orthogneisses include K2 gneiss in thenorthern Karakoram terrane and we plagioclase-hornblende gneiss and biotite-hornb1ende-K- interpret its crystallization and cooling history from a study feldspar gneiss, which are stongly foliated and intruded by of regional structural constraints, U-Pb and 40Ar-39Ar,K-Ar late leucogranitic dykes containing garnet, muscovite, geochronology. biotite, tourmaline, plagioclase, K-feldspar and quartz. K- The northern Karakoram terrane lies north of the main Ar ages from separatesfrom the dykesrange from Karakoram axial batholith which stretches at least 500 km 70-58Ma (Searle et al. 1989). The leucogranitedykes from the Hindu Kush across northern Pakistan post-date an early deformation event in the K2 gneiss but into the Siachen glacier areanorth of .The are clearly offset by later shear zones. Gasherbrum sedimentaryseries includes Carboniferous Southeast of K2-Broad Peak, intrusions of quartz black shales overlain by thick Permain-Jurassic/Lower diorite porphyry have been mapped on the south flank of Cretaceous carbonates with minor sandstones, conglomer- BroadPeak andaround Gasherbrum IV and 111, which ates and shales. Ash-flow tuffs have also been recognized have an identical geochemical signature tothe K2 ortho- associated with the Lower Cretaceous Savoia Fm. gneiss and we presume that the two are related. along theeast bank of the Godwin-Austin U-Pb geochronology. A 4 kg sample of K2 orthogneiss glacier. Along the northern margin of the Baltoro plutonic from the SSE ridge of K2 was collected approximately unit near (Fig. l), the Carboniferous black shales 200 m NE of K2 base camp on the upper Godwin-Austin have been thermally metamorphosed to andalusite-bearing glacier. The sample is a strongly foliatedhornblende- hornfels along a narrow contact metamorphic aureole (Mitre biotite granodioriticorthogneiss with zircon, allanite, and sequence). apatiteas the main accessory minerals. U-Pbdating of In the Baltoro glacier region the Karakoram batholith is zircon was carried out using the methods outlined by Par- dominantlycomposed of theBaltoro plutonicunit with rish et al. (1987): data are given in Table 1 and plotted on compositions ranging from monzogranite to garnet two-mica a concordia diagram (Fig. 2). Some of the zircon fractions , associated with numerous pegmatite-aplite were abraded (Krogh, 1982) to removeany potential sec- dykes. AU-Pb zircon age of 21 f 0.5 Madates the ondary Pb loss, although this failed to improve the con- crystallization age of this unit,and the zircons show cordance of the data. Five analyses of high quality zircon plotin a small * Deceased. region bounded between 117 and 123 Ma on the concordia 603

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SavolaFm dykes L

Khalkhal Fm. m Tulls. Porphyrys

Quartz diorites m Aghll Fm IGasherbrum m sedimentary Doksam sequence serles UrdokFm I*'\'21 Muztagh Tower unlt Shaksgam Fm

Baltoro Fm JMU/ Masherbrumcomplex Fig. 1. Geological map of the Baltoro galcier area includingK2, Broad Peak and the Gasherbrum Range. Geochronological data are taken from Parrish & Tirrul (1989) Searle et al. (1989) and the present work. diagram,but do notdefine either aline or aquasi-linear faceted, and typical of igneous zircons, Although one array. Their '"'Pb/'"Pb ages rangefrom 124 to180Ma. could argue for extreme Pb loss and a much older age, this Thistype of zircon data is not amenableto rigorouses- is not consistent with the rock's history, which is not charac- timation of age uncertainty in itsage interpretation.The terized by excessively high temperature metamorphism. scatter is related to small amounts of older zircon in- heritance in most or all analyses, with the age of inherited 40Ar-39Argeochronology. Full experimentaldetails of the zircons being variable andindeterminate. Nevertheless, irradiationand argon analytical methods used at Leeds the visible evidence of cores in some grains and the rela- University are found in Parsons et al. (1988). Hornblendes tively tight cluster of dataare mostconsistent with this from samples of K2 orthogneiss and Muztagh Tower gneiss being an igneous rock 115-120 Ma old.The zircon mor- have been analysed and results are shown in Fig. 3. phologies areeuhedral tosubhedral, elongate, simply The low temperature steps of the Ar-Ar analysis of the

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Table 1. U-Pb analytical data for zircon, K2 gneiss

U Pb t 2o"Pbl msPb t mPh/206pb Sample Size pm* wt. (mg) (ppm) (wm) 2MPb5 (%) mPh/z38U v mPb/z35UmPb/mPb n age, (Ma)**

1 +149n 1 15910.3926 27.79 10980 5.0 0.01838 (0.09) 0.1234(0.11)0.04869(0.04) 133.1f2.0 2 +149a 0.1759 1678 29.47 11490 4.1 0.018@3(0.11) 0.1251 (0.13) 0.04856 (0.04) 126.8f2.0 3 +149a 0.1635 1227 21.78 7253 6.20.01848 (0.07) 0.1236(0.10) 0.04851 (0.05) 124.2 k 2.4 4 -149+105n 0.171 1506 26.07 10930 4.2 0.01839 (0.07) 0.1241(0.10) 0.04893 (0.04) 144.2 k 2.0 5 -149+ lO5a 0.2543 1405 24.82 12010 5.0 0.01859 (0.06) 0.1274(0.09) 0.04968 (0.04) 180.0 f 1.8 *a, abraded; n, needles; analysed zircons were generally clear and had rare faint evidence of cores. t Weighing error = 0.002 mg. t Radiogenic Pb. 0 Measured ratio, corrected for spike and fractionation; fractionation was 0.12 f 0.036%/amu. 11 Total common Pb in analysis corrected for fractionation andspike. Corrected for blankPb and U, common Pb(Stacey-Kramers 116Ma Pb: M6Pb/Z04Pb= 18.53 f 0.54;m7pb/~b= 15.62 f 0.8); errors in parentheses are 1 standard error of the mean in percent. Pb blanks were about 20 pg. '*Corrected for blank and common Pb, errors are 2 standard errors of the mean.

steps there is evidence thatthe rock was subjected to a

,lg. ,lg. secondary thermal eventafter cooling below itshorn- I 12OJ I blende blocking temperature.The youngest stepon the Ar-Ar plot shows that this evenoccurred ator after 18 Ma. The position of the Muztagh Tower gneiss along thenorthern margin of the Miocene Baltoro is consistent with heating during the intrusion of the latter.

Tectonic implications. The K2 gneiss cannot represent basementrocks of the Asianplate as previously thought l 0 120 0 nr 0 128 because of itsearly Cretaceous (115-120Ma) zircon age,

207Pb/ 235U interpreted as the age of crystallization. We suggest that Fig. 2. Concordia diagram for zircons from the K2 gneiss sample the orthogneiss was derivedfrom a granodiorite forming no. H-2. Error ellipses reflect two standard error uncertainties; the part of composite,a calc-alkaline batholithalong the interpreted age of crystallization is 115-120 Ma. southern margin of the Karakoram (Asian) plate. Zircons from the K2 sample contain inherited zircon components, hornblende R27 from the K2 orthogneiss shows some ex- probably derived initially fromPrecambrian continental cess radiogenicargon. The high temperature steps crust forming the basement of the Karakoram plate. The (990 "C-1340°C) yield 67.6% of the 39Ar and define a hornblende-bearinggranodiorites, tonalites and diorites of plateau age, as defined by Lanphere & Dalrymple (1974), the K2 gneiss together with theHushe gneiss, Muztagh of 90.6-e 1.8Ma. We interpret thisas thetime at which Tower gneiss and Hunza plutonic unit, all clearly pre-date the K2 gneiss cooled through 500°C. continent-continent collision along the Shyok suture (late For hornblende R41 from the Muztagh Tower gneiss, Cretaceous) andtheIndus suture (middle Eocene, the higher temperature steps (960 "C-1080 "C, yielding c50 Ma). They are presumablyrelated to melting at the 85.7% of the 39Ar) give aplateau age of 76.4f 1.6Ma. base of the crust in a region of high heat flow above the We consider this plateau age to be the cooling age for the northwarddirected subduction zone of Tethyan oceanic Muztagh Tower gneiss. However, in the low temperature lithosphere.

40C

K2 gneiss gneiss R27 Hnb. R41Hnb. Run 912 Run 913 300 IntegratedAge 300 Integrated Age 104.420.4 Ma. 89.7 t0.4 Ma.

h I d 200 vI a, 6 2 76 4*_ 16 - 100 100 - - - c3 Q. 3. Age spectra for the K2 gneiss sample no. R27 and 0 0 0 I I I I foliated biotite-hornblende 0 40 20 60 loo granodioriteMuztagh of the Cum % 39 Ar Cum % 39 Ar Towersampleunit, R.41.

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Table 2. 40Ar-39Ardata for samples of Muztagh Tower granodiorite (R41) K2 gneks (R27)

Volume Volume VolumeVolumeVolume "K 37ca 3sc1 %"Ar Error TemperatureT X lnPsCc X X lo-'kc C~/K "A~RAD/"K Atmospheric AgeMa 20 %"A,

R41 Hornblende O.wBwg, I volue 0.002890run number 913 750 0.533 0249 0.006 0.93 49.305 54.5 240.3 2.11 4.5 860 0.318 0.380 0.001 2.38 3.471 72.0 18.0 3.45 2.7 925 0.402 2.015 0.005 9.98 8.880 39.7 45.7 1 .K5 3.4 Y60 I 914 11.597 0.031 12.06 12.901 13.0 66.0 1 .W 16.2 ID00 2.426 12.586 0.043 10.32 15.403 4.5 78.6 0.64 20.6 1M5 2.459 11.582 0.044 9.37 15.988 3.5 81.5 0.77 20.9 lOB0 1.466 7.599 0.025 10.32 15 201 5.6 77.6 0.70 12.4 1210 1.845 8.936 0.031 964 20.225 4.5 1M.5 1 .M 15.6 1330 0.428 1.590 0.008 7.40 37.721 9.9 186.7 4.89 3.6

R27 hornblende O.o6271g, I value O.WZ8Wrun number 912 750 0.~0s 0.439 0.m 1.73 45.404 63.1 222 4 3.46 37 850 n3w 0.593 0.001 3.94 16.984 37 8 86.5 425 2.2 920 0.862 3.877 0.013 8.95 30.526 12.3 152.5 1 24 6.4 955 2.741 11.009 0.035 7.99 22.640 6.2 114 3 044 20.2 990 3.W 15.539 0049 7.79 18 4fi6 3.9 93.8 0.42 29.3 1025 2.128 8.088 n.ms 7.56 16 728 2.5 85.2 0.32 15.7 1075 1.329 5.120 0.015 7.55 17.606 5.2 89.5 I .74 10.0 1215 1.439 5.518 0.016 7.63 17.920 6.7 91 1 100 10.6 1340 0.268 1.224 0.~4 9.10 17.666 27.2 89.8 6 40 2.0 Gas volumes are corrected to standard temperature and pressure (STP) 40Ar RAD, radiogenic argon 40; SCC, standard cubic centimetres.

Following early Cretaceous intrusion, the K2 tonalitic- References granodioritic body was buried to depths of around 20 km, DEBON.F., LEFORT,P,, DAUTEL,D., SONET,J. & ZIMMERMAN,J. L. 1987. metamorphosed in the upper amphibolite facies and highly Granites of westernKarakoram and northern Kohistan (Pakistan): a deformed. It was then intruded by garnet, two-mica leuco- compositemid-Cretaceous to upperCenozoic magmatism. Lithos, 20, granitic dykes and finally exhumed along major breakback 19-40. DESIO,A. & ZANETTIN,B. 1970. Geology of the Baltoro Basin (Brill-Leiden, thrustfaults during post-Miocene time.The K2 orthog- Holland), 308 pp. neiss culmination is now surrounded by low-grade to un- KROGH,T. E. 1982. Improved accuracy of U-Pb zircon dating by selection of metamorphosedsedimentary rocks. Contactstheon moreconcordant fractions using a highgradient magnetic separation northeast () and southeast (BroadPeak) technique. Geochimica et Cosmochimica Acta, 46, 631-635. appearto benormal faults downthrowing the sediments LANPHERE,M. A. & DALRYMPLE,G. B. 1974. 4UAr/"Ar age spectra of some undisturbed terrestrial samples. Geochimica et Cosmochimica Acta, 38, against the K2 gneiss. Small-scale structures within the K2 715-738. gneiss atthe Gilkey-Puchozmemorial outcrop above the LEFORT,P,, MICHARD,A., SONET,J. & ZIMMERMAN,J. L. 1983. Petrography, site of K2 Base Camp, suggest that culmination of the K2 geochemistry and geochronology of some samples from the Karakoram gneiss may have been relatedto steep NNE-dipping con- axial batholith (Northern Pakistan): In: SHAMS, F.A. (ed.) Granites of the , Karakoram and Hindu Kush. Institute of Geology, tractionfaults with a SSW sense of vergence. The Savoia Punjab University, Lahore, 377-387. glacier divides K2 orthogneissfrom low-grade sediments, PARRISH,R.R. & TIRRUL,R. 1989. U-Pbage of theBaltoro granite, and we speculate that a majorthrust faultoccurs here northwestHimalaya and implications for monazite U-Pb Systematics. which may have beenresponsible forthe recent uplift of Geology, 17, 1076-1079. -, RODDICK,J. C., LOVERIDGE, W. D. & SULLIVAN,R. W. 1987. K2 itself. Roughestimates of P-T conditions either side Uranium-leadanalytical techniques at the GeochronologyLaboratory, of the Savoia glacier along the SW flank of K2 (Fig. 1) GeologicalSurvey of Canada. In: Radiogenic age and isotope studies. indicate minimum offsets of 10-15 km (down to the SW) Geological Survey of Canada Paper 87-2, 3-7. on this fault. Uplift of the K2-Broak Peak-Gasherbrum PARSONS,I., REX.D. C., GUISE,P. & HALLIDAY.A. N. 1988. Argon loss by Range was extremely rapid, recent and probably ongoing, alkali feldspars. Geochimica et Cosmochimica Acta, 52, 1097-1112. REX.A. J., SEARLE.M. P,, TIRRUL. R.,CRAWFORD. M. B., PRIOR,D. J., REX. producing five peaks over8000m altitude (K2,Broad D. C. & BARNICOAT.A. 1988. The geochemical and tectonic evolution of Peak N and S, Gasherbrum I1 andHidden Peak) and the central Karakoram, N. Pakistan. In: SHACKLETON.R., DEWEY,J. & numerous peaks above 7000 m. WINDLEY.B. F. (eds) Geology and Tectonics of the Himalaya and Tibet. PhilosophicalTransactions of theRoyal Society of London, A326, 229-255. SEARLE.M. P,, REX.A. J., TIRRUL. R.,REX. D. C. & BARNICOAT,A. 1989. Metamorphic, magmatic and tectonic evolutionof the central Karakoram This work was supported by NERC (UK) Grants GR3/4242 to B. in the Biafo-Baltoro- regions of N. Pakistan In: MALINCONICO,L. F. Windley, GT5/F89/GS6 to M. P. S., GR/5932 to D. C. R. and L. & LILLIE.R. S. (eds) Geology and Tectonics of the , Geological Society of America Special Publication 232, 47-73. by theGeological Survey of Canada.We thank T. Rexand B. --, , WINDLEY. F.,B. TIRRUL, R.,ST. ONGE,M. & HOFFMAN.P. 1986. Windley for helpful discussions and the University of Peshawar for A geological transect across the Baltoro Karakoram. Geological Bulletin logistical support. University of Peshawar, 19, 1-12.

Received 22 December 1989; revised typescript accepted 6 February 1990.

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