Incipient India-Eurasia collision and plutonism: the Lower Cenozoic Batura granites (Hunza Karakorum, North Pakistan)

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Abstract
Plutonism of the Karakorum, north of the India-Eurasia suture zone, comprises two major intrusive stages: the oldest, mid-Cretaceous, predates the collision; the youngest, Miocene, is post-suturing. In between, Lower Cenozoic plutonism, roughly syn-suturing and represented by the Batura complex, is made up of a few small and usually undeformed plutons. The complex has a subalkaline (i.e. intermediate between alkaline and calc-alkaline) and ferriferous association, dominated by light-coloured metaluminous and slightly peraluminous granites and adamellites with biotite ± amphibole ± titanite. Whole rock Rb-Sr isochron ages of 63 Ma (this study) and 43 Ma ( Debon et al. 1987b ) have been obtained for two plutons, with rather low initial 87 Sr/ 86 Sr ratios (0.7050, 0.7056). A comparison with granitoids of similar age from Kohistan suggests that the complex relates to the subduction of the Tethys ocean, and originates from a mantle source with but small crustal contribution, through a two-stage melting process. The apparently limited extent of Lower Cenozoic plutonism in the Karakorum contrasts with its wider development in the Transhimalaya and Kohistan. This contrast could be partly accounted for by changes in tectonic and magmatic suduction-related processes after the initial contact between India and Eurasia was established, in the NW Himalaya.