Detrital zircon U-Pb geochronology of the Ordovician sandstone and its constrain to the tectonic evolution of the northern Great Xing'an Range

The combined NE China blocks are the major component of the Central Asian Orogenic Belt ( CAOB). There has been a continued debate on the derivation of the old Precambrian crustal basements and Paleozoic tectonic division and evolutions of this orogen. Due to the absence of the Precambrian tectono-magmatic events, the Ordovician strata and magmatism play a key role in the reconstruction of the tectonic evolution of the Xing'an accretionary Terrane located in the west of the combined NE China blocks. In this study, we conducted U-Pb dating, geochemical analyses and Lu-Hf isotope analysis for the detrital zircon grains from metasandstone and tuff samples in the Duobaoshan Formation. The youngest ages of 481 5Ma ( D9088), 462 5Ma (296NJ-1) and 473 11Ma ( HDG06) and similar to the youngest weighted mean 206Pb/238U ages of 482 3Ma ( n =12), 475 6Ma ( n =10) and 483 8Ma ( n = 7), and thus give the lower limit age of sediments, indicating the Duobaoshan Formation developed in the Early -Middle Ordovician. The ages of detrital zircons from the Duobaoshan Formation range ( <1. OGa) are mainly concentrated in 462 520Ma ( with peaks at ca. 516Ma, 497Ma and 482Ma), and 790 980Ma ( peaks at ca. 969Ma, 830Ma, 788Ma and 760Ma). The ages of detrital zircons ( >1. OGa) have a minor population of 1321.2410Ma, with the peaks at ca. 1882Ma and 2410Ma. All the peaks are consistent with the contemporaneous magmatism of the Erguna Block, which reveal that the provenance of the metasandstone and tuff samples is mainly derived from the Erguna Block. By comparing the detrital zircon Hf isotopes of the Duobaoshan Formation in different areas, a rule can be observed that the farther the Duobaoshan Formation are away from the Erguna Block, the smaller e Hf (t) values and the older Hf model ages ( tD,2) of the detrital zircons in it will be, which suggests that there are more older crustal source from the Erguna Block from east to west. Together with the geochemical characteristics of the Ordovician strata, the Duobaoshan Formation was deposited in an environment related to an active continental margin.