Mesozoic and Cenozoic exhumation history and magmatic-hydrothermal events of the central Tianshan Mt. Range, NW China: Evidence from (U–Th)/He and 40Ar/39Ar dating

Abstract

The thermal history of rocks in the Awulale area, central Chinese Tianshan, is significant to understanding the formation and tectonothermal evolution of the Tianshan orogen. K-feldspar 40Ar/39Ar and apatite (U–Th)/He age data reveal at least four Mesozoic–Cenozoic tectonothermal events in the central Tianshan, which occurred during the Middle–Late Triassic (244.4–216.5 Ma), Early Jurassic (ca. 180.3 Ma), Early Cretaceous (99.1–98.1 Ma), and early Miocene (18.6–13.7 Ma). This thermal history is consistent with that throughout the Tianshan orogenic belt and surrounding blocks. Modeling of the thermal history shows that intrusive rocks in the central Tianshan have experienced four stages of cooling in the Mesozoic–Cenozoic. Stage I occurred from ca. 310 to 230 Ma, after magma emplacement, and resulted in cooling to the 40Ar/39Ar closure temperature of K-feldspar (200–180 °C). During this time, tectonism in the central Tianshan was not intense and the rapid cooling was due mainly to thermal exchange, with a maximum cooling rate of ~7.1 °C/Myr. Stage II occurred from ca. 230 to 180 Ma. Collision between the Qiangtang microcontinent and the Eurasia continental margin resulted in active tectonism in the central Tianshan, and cooling was caused mainly by tectonic uplift, with the maximum uplift of intrusive rocks exceeding 9 km. Stage III occurred from ca. 180 to 45 Ma, when regional tectonism was weak and cooling was gradual. During this stage, the apatites in the syenite-porphyry remained in the He partial retention zone for a long time. This indicates that during this stage the central Tianshan was only weakly influenced by the far-field collisions of Lassa and Karakoram–Pamir with the southern margin of Eurasia. Stage IV occurred from 45 Ma to the present-day, and was related to the far-field influence of the middle–late Himalayan Orogeny, during which tectonism in the central Tianshan became more intense. In this final stage, many types of intrusive rock were exhumed to the surface, with the maximum uplift exceeding 2 km. Furthermore, the cooling history and regional structural trends indicate that since the Mesozoic, under the influence of tectonic compression, there has been differential uplift on the northern and southern sides of the east Awulale area. The cooling and exhumation rates of intrusive rocks are much higher on the northern side of the area than the southern side. This was related to reverse movement on the Yuximolegai Fault, as exhumation rate of the hanging wall (northern region) was higher than that of the footwall (southern region) under N–S compression. These findings, combined with the inversion model results, indicate that this differential uplift occurred mainly from the Late Triassic to Early Jurassic.