The Geochronology And Provenances Of Detrital Zircons From Paleozoic Basins In The Sonid Youqi Area, Inner Mongolia

The Sonid Youqi area, which is located between the Solonker suture zone in the north and the margin of the North China craton (NCC) in the south, covers the north margin of the NCC, Boin Sum arc and Ondor Sum subduction-accretion belt in terms of its tectonic framework. The Boin Sum arc is one of key zones to study the tectonic evolution and interaction between the Paleo-Asian ocean and the NCC in the southern part of the Sonid Youqi.Detrital zircons were extensively deposited in various sedimentary rocks, and thus can be used to trace the sources by determining their U-Pb zircon ages and Hf isotopes, which play an important role in the study of sedimentary ages, provenance analysis and tectonic evolution.In this study, I collected a number of detrital zircon samples from the Baiyunebo, Xuniwusu, Amushan and Sanmianjing Formations, collected from the Paleozoic basins in and around the Boin Sum arc belt in order to undertake detrital LA-ICPMS U-Pb zircon dating and Lu-Hf isotopic analysis, which can help to determine the provenances of detrital zircons and infer the interrelationship among geologic terrains outcropped in the study area.The ages of detrital zircons collected from the Halahuogete Formation range widely from 2891±22.2Ma to 1144±51.1Ma, which have four age peaks at 1200Ma,1770Ma,2500Ma and 2800Ma, recording the geological events occurring in the Pre-Cambrian NCC. Detrital zircons collected from the Xuniwusu Formation have a wide age range from 3353±33.8Ma to 422± 3.9Ma, which form five age peaks at 440Ma,960Ma,1200Ma,1650Ma and 2500Ma, and in addition, four dated zircons give an age range from 900Ma to 600Ma, which suggests an exotic terrane below the Boin Sum arc belt. Detrital zircons collected from the Amushan Formation have also a wide age range from 2790±26.1Ma to 396±3.8Ma, which have two age peaks at 1450Ma and 430Ma. Two samples collected on both sides of the Boin Sum arc belt have different provenances and show different sedimentary environments. Moreover, Hf isotopic data of detrital zircons provide more detailed evidence for the tectonic settings.With combining the previous data and new LA-ICP-MS U-Pb detrital zircon data and Lu- Hf isotopic data for samples collected from Paleozoic basins in the Boin Sum area, I conclude that (1) the north margin of the NCC represented a passive continental margin from early Cambrian to Neoproterozoic; (2) in late Cambrian, the north margin of the NCC changed from a passive continental margin into an active continental margin due to the subduction of an oceanic crust subducted beneath the NCC, which formed a continental marginal arc and a parallel island arc belt (Boin Sum arc belt); (3) the Boin Sum arc belt was accreted to the north margin of the NCC in late Silurian and the Ondor Sum group merged with the Boin Sum arc belt in the late Carboniferous; and (4) the final closure of the Paleo-Asian ocean occurred at some time later than the early Permian.