Journal of the Geological Society, L ondon, Vol. 154, 1997, pp. 935–946, 8 figs, 3 tables. Printed in Great Britain Geochemical and isotopic constraints on subduction polarity, magma sources, and palaeogeography of the Kohistan intra-oceanic arc, northern Pakistan Himalaya M. ASIF KHAN 1 , ROBERT J. STERN 2 , ROBERT F. GRIBBLE 2 & BRIAN F. WINDLEY 3 1 Centre for Excellence in Geology, Peshawar University, Peshawar, Pakistan 2 Center for Lithospheric Studies, University of Texas at Dallas, Box 830688, Richardson, TX 75083-0688, USA ( e-mail: rjstern@utdallas.edu) 3 Department of Geology, University of Leicester, Leicester LE1 7RH, UK Abstract: Geochemical and isotopic data are presented for 18 representative samples from the intra- oceanic phase of the Kohistan arc. A restricted range of initial 87 Sr/ 86 Sr (0.7036–0.7066) and Nd (+2.8 to + 7.4) along with measured 206 Pb/ 204 Pb (18.0–18.6) are consistent with formation of the arc complex in an intra-oceanic setting. The isotopic data demonstrate the involvement of enriched, DUPAL-type mantle, suggesting that the Kohistan arc formed at or south of the present equator. Subduction polarity inferred from geochemical and isotopic data indicate that the Chalt Volcanics and Kamila Amphibolites represent a forearc and backarc basin sequence, respectively. These inferences are most simply resolved with a tectonic model whereby the intra-oceanic Kohistan arc evolved over a south-dipping subduction zone, implying that Kohistan and India moved northwards on the same plate, although separated, during much of Cretaceous time. Collision of Kohistan with the Karakorum caused a new, north-dipping subduction zone to form on the south side of Kohistan, leading to collision with India in early Tertiary time. Keywords: Himalayas, Pakistan, island arcs, isotopes. The Kohistan terrane of northern Pakistan is a superb example of juvenile crust that formed by magmatic additions at an intra-oceanic convergent margin (Hamilton 1994). It is particularly important to tectonicists because its formation and accretion history constrains the Cretaceous palaeo- geography of the Neotethys, and to igneous geochemists because it provides a complete and well-exposed section of primitive arc crust. Almost no isotopic data has been pre- viously reported for the Kohistan intraoceanic arc terrane, and several important aspects about its tectonic significance remain controversial, particularly how wide was the oceanic seaway that separated it from Asia, and what was Kohistan’s sub- duction polarity? We report here new major and trace element and isotopic compositions for igneous rocks that formed during the evolution of Kohistan as an intra-oceanic arc, and use these data to (1) confirm that Kohistan represents an intra-oceanic arc; (2) demonstrate that, prior to collision with the Karakorum terrane, the Kohistan arc faced north over a south-dipping subduction zone; and (3) infer that the Kohistan intra-oceanic arc originated near the present location of the DUPAL isotopic anomaly, at near equatorial latitudes. Geological setting The Kohistan terrane is located in northern Pakistan and is bounded to the north by the Shyok (or Northern) Suture and to the south by the Indus Suture (Fig. 1). Sedimentary sequences indicate that formation of the intra-oceanic Kohistan arc began in Early Cretaceous time (Yasin Group carbonates of Aptian–Albian age; Pudsey 1986), consistent with Ar/Ar and Rb/Sr ages indicating that the bulk of the igneous infrastructure formed between 110 and 90 M a (Schärer et al. 1984; Petterson & Windley 1985; Treloar et al. 1989). The inference that Kohistan originated as an intra- oceanic arc results from its mafic bulk composition, presence of pillowed lavas and marine sediments, low-K nature of associated felsic rocks, absence of continental crust or detritus, and separation of the inferred intra-oceanic arc of Kohistan from continental crust to the north and south by unequivocal suture zones. Kohistan and Ladakh comprise correlative parts of this Cretaceous intraoceanic arc separated by the late Cenozoic Nanga Parbat–Haramosh uplift. The intraoceanic phase of Kohistan lasted until sometime between 102 and 85 Ma, when Kohistan collided with the Karakorum (Asia) across the Shyok Suture (Treloar et al. 1996). From this time until collision with India about 50 Ma ago, Kohistan existed as an Andean-type margin. For a current overview of the geology of Kohistan see Treloar et al. (1996). This report focuses on the intra-oceanic phase of Kohistan. The intra-oceanic arc crust of Kohistan consists of five principal units, extending from the Indus Suture in the south to the Shyok Suture in the north (Fig. 1): (1) basal ultra- mafic–mafic cumulates (Jijal Ultramafics); (2) Kamila Amphibolite; (3) Chilas Complex of mafic to intermediate plutonic rocks; (4) early bimodal suite of intrusive rocks (Stage 1 of Petterson & Windley 1985) and Gilgit gneisses; and (5) Chalt Volcanics (Khan et al. 1993). The early bimodal suite is a small part of the region shown as ‘Kohistan batholith’ in Fig. 1. The bulk of the Kohistan batholith formed after collision of Kohistan and Karakorum, during the Andean- margin phase of Kohistan, and is of no concern to this report. We report geochemical and isotopic data on Kamila amphibolites, Chilas Complex intrusive rocks, and Chalt Volcanics. The Kamila Amphibolite belt is a composite mass dominated by amphibolite-facies meta-plutonic and meta- volcanic rocks. Hornblende Ar–Ar cooling ages of about 935