Marginal Accretion Processes Of Jiamusi Block In NE China

The initiation and evolution of the Paleo-Pacific subduction under the East Asia directly affected and transformed the tectonic framework from E-W Paleo-Asian into NNE-NE Paleo-Pacific tectonic regime in the whole East Asia,which also controlled the Mesozoic tectonic evolution not only in the eastern part of China,but even the whole region of East Asia.However,a number of key scientific questions,such as the time when the Paleo-Pacific Plate began to subduct and its dynamic processes,are still controversial and have not been fully resolved.The Wandashan Terrane is located in the eastern margin of the Jiamusi Block in NE China.It is composed of two set of complexes?Yuejinshan Complex and Raohe Complex?,which becomes the ideal area of discussing the initiation and evolution of the Paleo-Pacific subduction in the continental margin of East Asia.In this thesis,we focused on geochronology in the clastic rocks from the Mesozoic Dalingqiao Fm.?J₂-₃d?and Yongfuqiao Fm.?J₃-K₁y?,by the detrital zircons which represents a time-capsule,together with the samples collected from the river sand from drainage channels near the Raohe and the Yuejinshan complexes respectively,and analyzed the potential provenance as well as its composition,and limited the emplacement time of the Raohe Complex by determining the deposition age of the Dalingqiao Fm.Combining observations in the field with the previous datasets of the research area,this thesis discussed the accretion processes on the eastern edge of the Jiamusi Block,then rebuilt the tectonic evolution in the eastern part of NE China from the Late Paleozoic to Mesozoic.The detrital zircons selected from the Dalingqiao and Yongfuqiao formations areIIImostly magmatic in nature,and the U-Pb dating results show that both the formations have the main peak ages of²70 Ma,460-510 Ma,and small age groups of⁸00 Ma,¹800 Ma and²500 Ma,respectively.Comparing with the magmatic age on the Jiamusi Block,it is found that this age spectrum is seemingly coincident with the magmatic activity of the Jiamusi Block,and thus its provenance can be linked tightly to the igneous rocks exposed in the Jiamusi Block.In addition,a part of the zircons with age group of¹70 Ma in the Dalingqiao Fm.show the feature of basic magmatic origin in CL images,indicating their ages of crystallization period,so the provenance of this age group may be the ocean islands or seamounts which later on involved in the Raohe Complex.Therefore,our research contends that the Dalingqiao Fm.deposited seemingly during or shortly after the Raohe Complex started to accrete,and thus,the Raohe Complex was not yet in the place during the deposition of the Dalingqiao Fm.Meanwhile,since the overlying Yongfuqiao Fm.began to deposit after the emplacement of the Raohe Complex,the deposition time of the Dalingqiao Fm.could limit the maximum time of the Raohe Complex emplacement.The youngest detrital zircon in the Dalingqiao Fm.shows the crystallization age of 142 Ma,which represents the deposition time of the Dalingqiao Fm.Thus illuminating that,the Raohe area was still under the environment of Paleo-Pacific subduction and accretion at that time,and the Raohe Complex was not yet fully evolved in place.Combined with the oldest age of post-collision granite reported so far in this area,131 Ma,we believe that the emplacement time of the Raohe Complex is approximately 142-131 Ma.This thesis summarized and analyzed previously reported data on some characteristic rocks exposed on the Yuejinshan Complex,Jiamusi and Songnen Blocks,and we believe that the Yuejinshan Complex accreted to the eastern edge of Jiamusi Block during the Late Carboniferous-Permian,and the subducted oceanic crust affected its accretion is not the Paleo-Pacific Plate or Paleo-Asian Plate,but an unknown oceanic crust?probably the Panthalassa?.Additionally,the oceanic subduction led to the gaint Permian igneous rocks on the Jiamusi Block,and resulted in the back-arc extension between the Jiamusi and Songnen blocks in the Late Permian-Late Triassic,forming the Mudanjiang oceanic crust.In the Late Triassic-Early Jurassic,the Paleo-Pacific plate began to subduct westward,IVresulting in the accretion of the Raohe Complex and the subduction of the Mudanjiang oceanic crust under the Songnen Block.As the Raohe Complex was in place in the Late Jurassic-Early Cretaceous,the final amalgamation between the Jiamusi and Songnen blocks happened which consequently resulted the evolution of the Mudanjiang Suture.