Detrital zircons U-Pb age and constraints for regional tectonic evolution of the ophiolitic mélange in Mandula area, Inner Mongolia

Zircon U-Pb geochronology and Lu-Hf isotope of matrix clastic rocks in the Mandula ophiolitic mélange were studied in this paper.Most of the detrital zircons in greywacks(D18003RZ1)are magmatic crystallization origin.The zircon U-Pb ages range from 272 Ma to 2302 Ma with two peak ages of 282 Ma and 442.8 Ma.The ¹⁷⁶Hf / ¹⁷⁷Hf ratios of zircons from 431 Ma to 456 Ma range from 0.282661 to 0.282797.The values of ε_Hf(t)range from 6.4 to 10.7, and T_DM2 range from 801 Ma to 1155 Ma.The zircon ¹⁷⁶Hf / ¹⁷⁷Hf ratios of 272~286 Ma range from 0.282554 to 0.282768.The values of ε_Hf(t)is between - 1.5 and 5.9, and T_DM2 is between 1069 Ma and 1575 Ma.The zircon ages of the silty sericite bearing siliceous slate(DC01TW1)range from 260 Ma to 2185 Ma.The zircons are magmatic origin and metamorphic origin.The main peak age is 270 Ma, and the secondary peaks are 1600 Ma and 1850 Ma.The zircon ¹⁷⁶Hf / ¹⁷⁷Hf ratios of 263 ~ 293 Ma range from 0.282434 to 0.282805.The values of ε_Hf(t)range from -6.0 to 7.1, and T_DM2 range from 930 Ma to 1894 Ma.The detrital zircons in greywack reflect that the provenance is mainly Xingmeng orogenic belt.Siliceous slate are mainly Late Paleozoic detrital zircons and Paleo-Meso Proterozoic detrital zircons, that show the provenance area might be the Xingmeng orogenic belt and the North China block.Combined with the rock composition, zircon age distribution and provenance analysis of the two lithology, the greywacks may have formed a little earlier and close to the island arc, while the sericite siliceous slate formed a little later and its environment is closer to the continental block.The clastic rocks in Mandula area might have formed in the Late Permian and not be later than 256 Ma.They might have formed in the transformation background from subduction to collision, which further limited that the Paleo Asian Ocean in central Inner Mongolia still existed in the Late Permian, and its evolution might have continued to the end of the Late Permian.