Multi-geochronology analyses of Pamirs river detritus: Insights into Pamir-Tibet connections

The Pamir and Tibet are the product of tectonic processes related to multiple collisions in the Paleozoic-Mesozoic and Cenozoic, resulting in the accretion of different terranes. The correlation of these terranes is the subject of debate. Whereas two present models (Schwab et al., 2004; Robinson et al., 2007) suggest the northward displacement of Pamir with respect to Tibet, the former correlates the Northern and Central Pamir to the Songpan-Ganze and Qiangtang terranes of Tibet, and calls for a large magnitude (~250km) of offset along the Karakorum fault, whereas the latter correlates the Northern and Central Pamir to the Qaidam and Songpan-Ganze terranes, and calls for a smaller magnitude (~150km) of offset along the Karakorum fault. In order to evaluate these models, a combined study utilizing detrital U-Pb and ɛHf analyses was conducted on zircons from 12 modern river samples; seven along the Eastern Chinese Pamir and five from the Western Tajikistan Pamir (Lukens et al., 2010). A total of approximately 700 and 250 new zircons were analyzed for U-Pb and hafnium respectively by MC-LA-ICPMS at the Laserchron lab at the University of Arizona in order to determine the crystallization ages of the source areas and its crustal evolution. Our compiled U-Pb data from the Pamir (this study; Lukens et al., 2010) together with previously published geochronologic data from Tibet (Weislogel et al., 2006; Gehrels et al., 2010) indicate relatively similar zircon U-Pb ages between the Northern Pamir and Songpan-Ganze and the Central Pamir and the Qiangtang (~200-250 Ma and ~500-1000 Ma respectively), favoring models with large magnitude offset along the Karakoram fault. However, significant differences exist within the Pamir. Detrital U-Pb ages in the western Pamir have a large component of grains that are younger than 200 Ma. The Eastern Pamir samples, which mostly drain the Northern Pamir, yielded a majority of zircons with ɛHf(t) from -5 to 5 with a model Hf age of about ~1150 Ma, indicating that these intermediate-natured zircons were derived almost exclusively from a single source. The Western Pamir samples, which drain the Northern Central and Southern Pamir, instead yielded zircons that are more juvenile with approximately 55 out of the 130 grains analyzed with ɛHf(t) from -10 to -90. Model Hf ages cluster around ~950 Ma, ~1450 Ma and ~3400 Ma suggesting mixing of sources and recycling of the Pre-Cambrian crust. The exhumational histories vary as well, 668 detrital 40Ar/39Ar white mica ages from the Eastern Pamir, analyzed at the USGS laboratory in Denver, are mainly clustered around 70-120 Ma and 160-210 Ma with no Miocene component. Published data from rivers in the Western Pamir show a greater age spread with a distinct population at 15-20 Ma. The broader age spectra in the Western Pamir is consistent with the fact that western rivers are draining a larger area which includes the Northern, Central and Southern Pamir whereas the eastern rivers are mainly draining the Northern Pamir. Apatite fission track data from the same samples will aid us in resolving the erosional and tectonic history of this region.