Granites of western Karakorum and northern Kohistan (Pakistan): A composite mid-cretaceous to upper cenozoic magmatism

The petrography, the chemical-mineralogical typology and the ages of six plutonic units, four from the Karakorum axial batholith (Darkot Pass, Ghamu Bar, Batura and Hunza units) and two of northern Kohistan (Gindai and Nomal plutons) are defined and compared based on field data and analyses of up to 78 samples (major elements, REE, Rb-S.sbnd;Sr and KAr isotopes). The Karakorum axial batholith is a composite body. Three major intrusive stages occurred: (1) around Mid-Cretaceous times (ca. 110-95 Ma) with the emplacement of subalkaline, i.e. monzonitic (Darkot Pass) and calc-alkaline (Hunza, ?Ghamu Bar) units; (2) during Palaeogene, maybe up to 43 Ma (Batura subalkaline unit). A strong tectonommetamorphic event, recorded in the gneissification of the Cretaceous intrusives, occurred between these two stages; it may be of Palaeocene age. P-T estimates of the highest metamorphic grade rocks of the Hunza unit have yielded values of 580–640°C and 5 ± 0.5 kbar; and (3) during Upper Miocene (ca. 9 Ma; Baltoro subalkaline unit; Debon et al., 1986c). In addition, a conspicuous network of aplo-pegmatitic dykes emplaced into the Hunza area, possibly from the Eocene up to the Upper Cenozoic with a maximum during the Middle Miocene (ca. 15 Ma). Most of these major magmatic stages are met again among the acidic intrusives and dykes of northern Kohistan: the first one as blastomylonitic tholeiitic plutons (Nomal; ca. 102 Ma; Petterson and Windley, 1985), the second one as subalkaline plutons (Gindai; ca. 59 Ma), and a third one as leucocratic dykes, of Oligocene age (ca. 30 Ma; Petterson and Windley, 1985).These data may be related to the geodynamic evolution of the NW part of the India-Eurasia suture zone, thus allowing better constraints on the major steps of this evolution. The partly synchronous closures, by N-dipping subduction, of the two Tethys branches which are assumed to have encircled the Kohistan arc in Upper Mesozoic times, may have generated both the Karakorum and the Kohistan intrusives of Cretaceous and Palaeogene ages. The northern branch very likely closed before the southern one. At the time of the second intrusive stage (Palaeogene): (a) Kohistan and Karakorum had already collided, were welded and had suffered the same major tectonometamorphic event; (b) subduction of the southern Tethys floor beneath the welded Kohistan-Karakorum was still active; (c) however, collision between India and Kohistan-Karakorum may have already begun, particularly at the level of the Nanga Parbat promontory. Finally, it is emphasized that the intrusive processes continued in the Karakorum long after the collision (e.g., Baltoro granite).