Age and isotopic constraints on magmatism along the Karakoram-Kohistan Suture Zone, NW Pakistan: evidence for subduction and continued convergence after India-Asia collision

Abstract

Detailed geological mapping in the Drosh-Shishi area in southern Chitral (NW Pakistan) was combined with high-precision U-Pb dating on zircons to constrain the timing of magmatism and associated deformation/metamorphic events related to the Kohistan-Karakoram convergence and collision. Our new ages indicate that the Mesozoic to Tertiary magmatic history of this region is influenced by long-lived melt generation above an active subduction zone. Dated intrusive rocks range in age from 130 to 39 million years, indicating that subduction-related magmatism continued after the Kohistan-Karakoram and the India-Asia collisions. Initial hafnium isotope ratios were measured on the dated zircons to constrain the type of melt source of the dated plutons. The data reveal the different nature of partly coeval magmatism in these units, i.e. continental arc magmatism in the Karakoram (ca. 130-104 Ma) and arc magmatism on the Kohistan side (112-39 Ma). Intrusions within the suture zone can be clearly traced to be Karakoram-derived on the basis of initial Hf isotopic compositions. Granite dykes crosscutting the Kohistan units have sampled an underlying, old continental basement of Gondwana affinity. The geochronological evidence presented in this paper is consistent with Cretaceous subduction beneath the Karakoram Terrane. The related calc-alkaline magmatism seems to have stopped at about 100 Ma. Granite dykes on the Kohistan side show that the magmatic and tectonic history of the KarakoramKohistan Suture Zone continued to the Eocene. This long tectono-metamorphic and magmatic activity in the arc plates was likely due to complex and few million year long interplays between subduction and thrusting events in the forearc, within-arc and back-arc regions between two active subduction zones.

Construction of the granitoid crust of an island arc part I: geochronological and geochemical constraints from the plutonic Kohistan (NW Pakistan)

“…Accordingly, the modal amount of amphibole diminishes during partial melting with increasing (Cox et al 1979;Wilson 1989). The results of this study are compared to the other granitoid samples from the (northern) Kohistan Batholith (Petterson and Windley 1985;Petterson and Windley 1991;Heuberger et al 2007) and to granitoid samples from the Southern Plutonic Complex Garrido et al 2006). Kohistan granitoids are compared to oceanic arc granitoids from the Aleutian (Perfit et al 1980) and New Britain (Whalen 1985) degree of partial melting i.e.…”

“…olivine, pyroxene, and plagioclase). In such a liquid line of decent the high SiO 2 content (52-54 wt%) of early fractionating plagioclase and Schaltegger et al 2002;Yamamoto et al 2005;Heuberger et al 2007). Available ages document a continuous magmatic history within the Kohistan arc from at least *120 to 38 Ma.…”

TTG-type plutonic rocks formed in a modern arc batholith by hydrous fractionation in the lower arc crust

“…At Drang, in the southeast of the batholith, pressures are higher with a maximum of 0.77-0.94 GPa. Pressures calculated on samples collected by Heuberger et al (2007) near the western limit of the batholith yield *0.6 GPa, with isobars parallel to the Karakoram-Kohistan suture. About 20 km to the SE, samples from the batholith in the hanging wall of the Dir-Kalam fault intruded at 0.33-0.37 GPa, but pressures are much higher in the footwall of this fault, eastward near Kalam and in the south-central part (0.54-0.57 GPa).…”

Microscale strain partitioning? Differential quartz crystallographic fabric development in Phyllite, Hindu Kush, Northwestern Pakistan

“…The geology of the area is dominated by Paleozoic protoliths, mainly low-grade metasedimentary rocks that locally reach sillimanite grade (Gaetani et al, 1996;Zanchi et al, 2000;Hildebrand et al, 2001;Zanchi and Gaetani, 2011;Faisal et al, 2014). These metasedimentary rocks are intruded by a series of plutonic bodies that range in age from Paleozoic (Kafiristan -483 ± 21 Ma; Debon et al, 1987), through Mesozoic (Tirich Mir -114 to 121 Ma, Desio, 1964;Hildebrand et al, 2000;Heuberger et al, 2007), to Cenozoic (Garam Chasma -24 Ma;Hildebrand et al, 1998). The region records a protracted deformational history with the earliest records indicating Late Triassic deformation and metamorphism and recent events culminating in the Early Miocene (Faisal et al, 2014).…”