Composition Of The Wandashan Complex In Northeast China And Its Tectonic Implications

Wandashan Complex(also called Nadanhada Terrane)in Northeast China is the western part of the Sikhote-Alin accretionary complex,located along the eastern margin of the Eurasian continent.It is one of the most important parts of the Mesozoic circum-Pacific orogenic belt related to paleo-Pacific subduction and accretion.The block located west side of the Wandashan Complex is the Jiamusi Block,which is on east edge of the Central Asian Orogenic Belt(CAOB).The CAOB was generated by the closure of the paleo-Asian Ocean.Therefore,the Wandashan Complex is a so special tectonic transition zone between the paleo-Pacific tectonic system and the paleo-Asian Ocean tectonic system,that it has been always subjected to wide attention.Consequently,research on the Wandashan Complex contributes to a clearer understanding of the composition of the complex belt and its forming process,and on the other hand,it has an important significance for the restoration of Mesozoic tectonic evolution in this region.Wandashan Complex is mainly composed of two parts.The western part is the Yuejinshan Complex,and the eastern part is the Rao Complex.In this paper,a detailed research work is focused on the Raohe Complex.Through classification of matrix and rock blocks in Raohe Complex,and geochemistry study on these rocks,detrital zircon geochronology of sandstone in Raohe Complex,and combination with previous research results on Wandashan areas,it clarified the composition of the Raohe Complex and its formation environment,determined the age and provenance of clastic sedimentary rocks in Raohe Complex,and restored sequence stratigraphic framework,re-examined the formation age and the formation process of the Wandashan Complex,cleaned up the response relationship between Jiamusi Block and Wandashan Complex,and finally proposed a new tectonic model for the formation process of Wandashan Complex.Two sedimentary sequence units were reconstructed through classification of matrix and exotic rock blocks in Raohe Complex.One is the Mesozoic back-arc basin system according to the recovery of matrix,followed by pyroclastic deposits and olistostromes and volcano lavas developed in the period of rifting,and chert and silicon mudstone characterized by deep water sedimentary characteristics which formed in the time of the expansion,and the upper clastic rocks and turbidite which formed in the time of migration to the land and subduction period.The overall stratigraphic sequence displays coarse-fine-coarse graded characteristics.The other is the late Paleozoic seamount system in paleo-Pacific according to the recovery of exotic blocks in Raohe Complex,the seamount has obvious two-layer structure,with carbonates cap on the top and lower basalt base,and are also developed slope facies with slump breccia.Geochemical characteristics of cherts,limestones,siliceous mudstone of the matrix in Raohe Complex show high MnO/TiO2 value and total rare earth content,with no negative Ce anomaly.In sedimentary tectonic setting discrimination diagrams of Fe2O3/TiO2-Al2O3/(Al2O3+Fe2O3)and LaN/CeN-Al2O3/(Al2O3+Fe2O3),the samples are indicative of continental margin.It indicates the chert,siliceous mudstone and limestone were deposited by great impact of sediments from continental margin.It was possibly formed in the setting of continental margin or back-arc basin,rather than the pelagic depositional environmen as previous researchers considered.However,the exotic limestone blocks show distinct geochemical characteristics compared with those above matrix.The lower MnO/TiO2 value and total rare earth content,and high w(LaN)/w(CeN)values,and the obvious Ce negative anomaly indicate that limestone blocks deposited in one kind of offshore depositional environment.In addition,exotic basalt blocks in Raohe Complex show high TiO2 content,high content of Nb and Ta,weak LREE enrichment,and(La/Yb)N value is 1.31-8.25.REE and trace elements models and tectonic discrimination diagrams of Zr/Yb-Zr and Nb/Yb-Th/Yb are all indicative of OIB features which were within plate oceanic basalt.The geochemical characteristics confirm the above extrapolation of seamount inferred from sedimentary characteristics.The results of zircon laser-ablation ICP-MS U-Pb ages from sandstone in Raohe Complex show that the spectra ages from the Upper Triassic sandstone span in 210-320 Ma,with a peak age of 240-250 Ma and a a unimodal pattern age spectrum,demonstrating active magmatic activity in the paleo-Pacific Ocean from the late Paleozoic to the early Mesozoic and a peak magmatic episode in the early Triassic.Intraoceanic geological landmasses(such as oceanic arcs,islands/seamounts/plateaus)are possible sources for the late Triassic clastics in the Wandashan Complex.The minimum age of 165 Ma from the flysh turbidite indicates that the Raohe Complex were replaced at the East Asia continental margin as early as late mid-Jurassic and received detritus from Jiamusi Block and the adjacent continents.The age spectrum from Cretaceous sandstone is similar with Jurassic flysh,it could be subdivided into seven assemblage groups as 160-220Ma,220-290Ma,350-400Ma,450-500Ma,530-560Ma,0.6-1.1Ga,and 1.5-2.5Ga.The spectrum of Phanerozoic ages shows great similarity with Phanerozoic granite ages spectrum from Northeast China that all demonstrate that the provenance of the Jurassic and Cretaceous clastics was mainly the Jiamusi-Bureya-Khanka Massif and the blocks in Northeast China.Comprehensive results of this study and in combination with previous research datas in this region illustrate that the Wandashan Complex formed in Early Jurassic-Early Cretaceous.Finally,a new model was proposed that the Mesozoic east asia continental margin was an multi-island(arc)oceanic basin structure that experienced multi-stage subduction-accretion process.The arc-continent collision in EarlyMiddle Jurassic formed the Yuejinshan Complex,the back-arc basin subduction in late Middle JurassicEarly Cretaceous formed the Raohe Complex.This model can well explain the formation process of Wandashan Complex,and well explain the characteristics differences between the Raohe Complex and Yuejinshan Complex,and also well explain the Mesozoic deposition-tectonic-magmatic evolution in Jiamusi Block that were related with the Wandashan Complex.