Rb-Sr dating of the Tirich Mir pluton, NW Pakistan: Pre-140 Ma accretion of the Karakoram terrane to the Asian margin

The Karakoram and East Hindu Kush-Wakhan blocks are parts of the Mega Lhasa composite terrane (Zanchi et al., 2000). The identity of their components and the timing of their juxtaposition are still poorly known due to access difficulties. Zanchi et al. (1997, 2000) first reported the existence of a NE-SW trending left-lateral strike-slip fault zone, the Tirich Mir Fault Zone separating the East Hindu Kush from the Karakoram. The fault system is characterized by a sandwich of mantle peridotites, deformed gabbros, amphibolites and gneisses named Tirich Mir Boundary Zone (TBZ). The presence of ultramatic bodies is a strong argument to interpret the TBZ as the northwestern boundary of the Karakoram terrane. In any case the timing of the accretion of the Karakoram to the East Hindu Kush and to the Pamir belts has never been directly constrained. The Tirich Mir pluton (TMP) is a little-deformed large NE-SW trending granodioritic to granitic body, preserving intrusive margins with the exception of its southern part which is strongly deformed or even mylonitic. The preservation is generally poor and alteration (hydrothermal and/or meteoric) almost ubiquitous. It had been visited by Desio et al. (1964), but its structural importance had been overlooked. Zanchi et al. (2000) observed that the TMP intrudes the TBZ, cutting the mantle ultramafics and clearly post-dating peak metamorphism and associated deformation. This implies that dating the TMP puts a stringent lower limit for the age of the East Hindu Kush-Karakoram accretion. Desio et al. (1964) attributed to the TMP a Rb-Sr age of 115 Ma. We analysed two TMP samples, ZP-61 (Tirich Gol), and ZP-22 (Dir Gol). Muscovite ZP-61 has a lower 87Sr/86Sr ratio than its whole rock. This points to a hydrothermal perturbation of the whole rock system and makes the sample unsuitable for dating of the magmatic event. That the whole-rock systems are perturbed is attested by an apparent Rb-Sr WR age of 15 ± 2 Ma. It is likely that Desio et al.'s (1964) interpretation is incorrect because the influence of hydrothermalism on isotope systematics was unknown at that time. In sample ZP-22, muscovite defines a two-point age of 141.7 ± 1.9 Ma (95% c.1.). Altered biotite, giving a two-point age of 103 ± 2 Ma, testifies that the granite was affected by younger tectonic events, resulting in chloritization and mylonitization of the southern boundary as also suggested by Hildebrand et al. (2000). The accretion between Karakoram and East Hindu Kush thus occurred before 140 Ma (Tithonian), pre-dating the intrusion of the TMP. Our work has further shown the East Hindu Kush plutonic belt was active in Late Jurassic, as also suggested by the K-Ar 144 Ma age of the Shushar pluton north-east of the TMP (Gaetani et al., 1996).