U-PB ZIRCON DATING AND 40AR/39AR THERMOCHRONOLOGY OF THE AMDO GNEISS, CENTRAL TIBET

The Amdo gneiss comprises one of the few exposures of metamorphosed crystalline basement in Tibet. It is located ~270 km northeast of Lhasa and associated with ophiolitic fragments of the Late Jurassic-Early Cretaceous Bangong suture between the Lhasa and Qiangtang terranes. Previous studies yielded U-Pb zircon and sphene ages of ~530 Ma and ~170 Ma for the gneiss, respectively (Xu et al., 1985). These data were interpreted to record a high-grade metamorphic event during the Cambrian, coeval with ductile deformation of the gneiss, and a low-grade metamorphic event during the Late Jurassic Lhasa-Qiangtang collision and subsequent exhumation of the gneiss. Presented here are LA-ICP-MS U-Pb zircon and ⁴⁰Ar/³⁹Ar thermochronologic data from the Amdo gneiss. Two samples of orthogneiss yielded concordant zircon crystallization ages of 500-520 Ma, similar to previous results. However, a third sample of orthogneiss provided an older crystallization age of ~845 Ma, and suggests that a component of Tibetan basement is Neo-Proterozoic in age and unrelated to Pan-African events. Analyses of the tips of zircon crystals from the older sample yielded younger discordant ages which define a discordia line with a Mesozoic lower intercept age. Th/U ratios of older concordant zircon analyses are characteristic of typical igneous zircon (0.1-1.0) whereas those of the younger discordant analyses provide a trend toward lower values characteristic of metamorphic zircon. These results suggest that high-grade metamorphism of the Amdo gneiss is likely Mesozoic as opposed to Cambrian in age. Biotite separated from the orthogneiss samples yielded flat ⁴⁰Ar/³⁹Ar age spectra with plateau ages of ~165 Ma, consistent with rapid cooling during Late Jurassic exhumation of the gneiss. The timing of this exhumation postdates obduction of the nearby Donqiao ophiolite (~175-185 Ma; Zhou et al., 1997), and could mark the initiation of the Lhasa-Qiangtang collision in this area. Future studies of the Amdo gneiss which also integrate structural mapping, thermobarometry, and low-temperature thermochronology have unique potential to elucidate the Mesozoic-Cenozoic deformation history of the Bangong suture as well as the age and composition of Tibetan basement.