APATITE (U-TH)/HE AGES FROM THE PENEPLAIN IN THE EASTERNMOST TIENSHAN, CHINA

Tienshan area, located at the southwestern margin of the Central Asian Orogenic Belt (CAOB)—the largest accretional orogenic belt on the Earth—provides crucial information about accretional history and long-term mountain formation processes. After the last major accretion related to the south Tienshan Ocean’s closing during the Carboniferous-Permian, the area experienced tectonic quiescence, forming a large-scale peneplain during the late Mesozoic–early Cenozoic. The peneplain is well preserved in the eastern Tienshan (regional slopes <0.5°, current elevations between ~500–1100 m), whereas the western Tienshan (peak elevation > 7000 m) experienced more extensive intra-plate orogenesis during Cenozoic developments of the Tibetan Plateau and Himalayas. Our samples were collected from plutonic and metamorphic outcrops in the easternmost Tienshan peneplain, where the only topographic barrier is the Bogda Mountains to the north.

Apatite (U-Th)/He (AHe) ages were widely scattered, mainly in the range of ~50 Ma–200 Ma, with a few outliers. The intra-sample AHe ages were also scattered, in some cases over a 100% age dispersion. Such a large AHe age dispersion is commonly attributed to differential He diffusion kinetics driven by effective Uranium (eU = U + 0.235×Th), particularly for slowly cooled samples. However, only 5 samples out of 11 yielded positive age-eU relationship, suggesting the age spread cannot be explained exclusively by the eU effect. The remaining six samples yielded either negative or random age-eU relationship. Inclusions are observed in some samples but further investigation is required to understand the age distribution. Although what caused the large age spreads is still unclear, the average AHe ages for individual samples were generally within ~100–150 Ma. This indicates the study area experienced slow exhumation since the late Jurassic–early Cretaceous, presenting long-term average exhumation rates ranging from 0.01–0.02 km/Ma forming the peneplain.