Timing of normal faulting along the Indus Suture in Pakistan Himalaya and a case of major Pa-231/U-235 initial disequilibrium in zircon

We report age data by TIMS U-Pb, LA-ICP-MS and Ar-39-Ar-40 techniques for m ain magmatic events in the Lower Swat region of Pakistan, in order to const rain the tectonic evolution of the northwestern Himalaya. The pre-Himalayan history of the Indian continent is documented by single-zircon U-Pb result s from the peraluminous Choga granite gneiss, which yielded a 468 +/-5 Ma l ower concordia intercept interpreted to approximate the time of magmatic em placement. The presence of a well-defined 870 +/-7 Ma inherited component ( upper intercept) suggests a plutonic, or volcanic protolith residing at une xposed levels of the Indian crust. Zircon data for the Swat granite gneiss from the northern part of the Loe Sar dome give an emplacement age of 267+6 /-3 Ma, which is at variance with earlier correlations favoring an early Pa leozoic origin. Subsequent to metamorphic overprint by the Himalayan orogen y, the Swat granite was intruded by late kinematic alkali-granite dykes. Si ngle zircons from one of these dykes show reproducible Pb-206/U-238 data gi ving a precise mean age of 29.26 +/-0.12 Ma, whereas the Pb-207/U-235 ages scatter between 34 and 81 Ma, pointing to huge and variable enrichment in P b-207. The unsupported Pb-207 can be explained by incorporation of Pa-231, an intermediate long-lived daughter nuclide in the U-235 decay chain, in ex cess of the secular equilibrium ratio. LA-ICP-MS measurements confirm the p resence of unsupported Pb-207, but do not show any correlation between the latter and other selected trace element concentrations in these zircons. A concordant Ar-Ar muscovite age of 28.4 +/-1.1 Ma obtained for the same dyke postdates regional mica 'cooling' ages and indicates a lack of younger reg ional events capable of resetting the K-Ar system in muscovite. Because the dyke is pre- to synkinematic relative to normal faulting and related north -vergent folds, its emplacement age provides a maximum age for this event o r reflects already ongoing local extension. A lower limit of 15 Ma has prev iously been established by apatite fission track analysis. The 29-15 Ma age bracket for normal faulting is coeval with extension along the South Tibet an Detachment System (STDS) on the north side of the High Himalaya. This su ggests that the Indus Suture in Pakistan has acted as a western continuatio n of the STDS and that related faulting was roughly contemporaneous over mo st of the Himalaya. (C) 2001 Elsevier Science BY. All rights reserved.