Abstract:This study uses apatite fission track (FT) analysis to constrain the exhumation history of bedrock samples collected from the Altai Mountains in northern Xinjiang, China. Samples were collected as transects across the main structures related to Palaeozoic crustal accretion events. FT results and modeling identify three stages in sample cooling history spanning the Mesozoic and Tertiary. Stage one records rapid cooling to the low temperature part of the fission track partial annealing zone circa 70±10°C. Stage two, records a period of relative stability with little if any cooling taking place between ~ 75 Ma and ~25-20 Ma suggesting the Altai region had been reduced to an area of low relief. Support for this can be found in the adjacent Junngar Basin that received little if any sediment during this interval. Final stage cooling took place in the Miocene at an accelerated rate bringing the sampled rocks to the Earth's surface. This last stage, linked to the far field effects of the Himalayan collision, most likely generated the surface uplift and relief that define the present-day Altai Mountains.

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Apatite fission track dating of five samples from Cenozoic volcanic strata in the Nimu District in the southern Gangdese Terrane exhibits single population grain ages with a single mean age and associated central ages ranging from 6.8 Ϯ 0.6 Ma to 9.7 Ϯ 1.2 Ma. Mean track lengths are between 12.9 Ϯ 1.7 mm and 14.2 Ϯ 2.3 mm with a single peak characteristic of a single thermal event. The newly documented ages coincide well with the age of high sedimentation rates in the North Tibet Basin that resulted from a 9-5 Ma compressional event. Track length modeling allows three stages to be identified in the sample cooling. The first stage (12-8 Ma) records a period of relative stability with little, if any, cooling at temperatures of 120-110°C suggesting this region had low relief. The second stage (8-2 Ma) reflects rapid cooling with temperatures decreasing from~110°C to surface temperatures of~15°C. This stage can be related to far-field effects of the Himalayan collision, which probably generated the surface uplift and relief that defines the presentday Gangdese Mountains. The mean uplift rate of this period is estimated to be 1.41-0.95 mm/y with total uplift reaching~5900 m. The final stage is related to surface evolution since the Pliocene.

The exhumation history and tectonic evolution of the Qilian Shan at the north‐eastern margin of the Tibetan Plateau has been widely debated. Here, we present apatite fission‐track (AFT) data for 12 Ordovician granodiorite samples along a vertical transect in the eastern Qilian Shan. These thermochronometry data indicate that the eastern Qilian Shan experienced a three‐stage cooling history, including: (i) rapid initial cooling in the late Cretaceous; (ii) a stage of quasi isothermal quiescence from ~ 80 to 24 Ma; and (iii) rapid subsequent cooling beginning in the early Miocene. The inferred cooling rates for the three stages are 6.8 ± 4.9 °C Ma−1, 0.6 ± 0.2 °C Ma−1 and 2.7 ± 0.9 °C Ma−1 respectively (±1 σ). Assuming a geothermal gradient of 25 °C km−1, the exhumation rates for the three stages are 0.27 ± 0.20 mm a−1, 0.017 ± 0.007 mm a−1 and 0.11 ± 0.04 mm a−1 respectively (±1 σ). We suggest that the late Cretaceous cooling records collision of the Lhasa block with the Eurasian continent and that the Miocene cooling represents uplift/exhumation of the Qilian Shan.

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The mineralization ages reported in the past in the Tuwu-Yandong copper district not only are different, but also fall into the Hercynian epoch. This study has achieved 9 zircon and 7 apatite fission track analysis results. The zircon fission track ages range from 158 Ma to 289 Ma and the apatite ages are between 64 Ma and 140 Ma. The mineralization accords with the regional tectonics in the copper district. We consider that the zircon fission track age could reveal the mineralization age based on annealing zone temperature of 140-300℃ and retention temperature of ~250℃ for zircon fission track, and metallogenetic temperature of 120-350℃ in this ore district. Total three mineralization epochs have been identified, i.e., 289-276 Ma，232-200 Ma and 165-158 Ma, and indicate occurrence of the mineralization in the Indosinian and Yanshan epochs. Corresponding to apatite fission track ages, the three tectonic-mineralizing epochs are 140-132 Ma, 109-97 Ma and 64 Ma, which means age at about 100℃ after the mineralization. The three epochs lasted 146 Ma, 108 Ma and about 100 Ma from ~250℃ to ~100℃ and trend decrease from early to late. It is shown by the fission track modeling that this district underwent three stages of geological thermal histories, stable in Cretaceous and cooling both before Cretaceous and after 20 Ma. mineralization age, tectonic epoch, fission track, zircon, apatite, Tuwu-Yandong, Xinjiang