Fission track thermochronology of the Tuwu-Yandong porphyry Cu deposits, NW China: Constraints on preservation and exhumation

摘要

The Carboniferous Tuwu-Yandong porphyry Cu deposits, located in the eastern Tianshan, are the largest porphyry Cu deposits in the Xinjiang province, China. This work presents new zircon and apatite fission track (ZFT and AFT) data from this porphyry Cu deposit, combined with inverse thermal modeling and previously published ages to reveal their preservation condition and exhumation history. Specifically, four cooling episodes are recognized: rapid cooling (24-44 degrees C/Ma) during the early Carboniferous (340-332 Ma), thick burial ( > 4 km) and reheating from the late Carboniferous to late Triassic, slow cooling (1.45-2.21 degrees C/Ma) from the late Triassic to the Paleocene and final protracted slow cooling (0.47-0.28 degrees C/Ma) from Paleocene to the present day. The first phase of cooling can be mainly attributed to magmatic heat transfer and thermal relaxation, whereas the second reheating phase was likely related to post-deposition burial. The latter two phases of cooling were predominantly controlled by uplift and denudation processes. Overall, our results indicate that the Tuwu-Yandong porphyry Cu deposits experienced two stages of slow post-burial exhumation at rates of similar to 48-64 m/Ma and similar to 9-16 m/Ma, respectively. We estimate that at least 8 km thickness of cover has been removed from above the Tuwu-Yandong porphyry Cu deposits. We conclude that early thick burial and later extremely slow exhumation were two key factors that led to preservation of this ancient porphyry system (i.e. Tuwu-Yandong porphyry Cu deposit).