The Geological-geochemical Composition And Evolution Of The Wandashan Orogenic Belt

Wandashan orogenic belt distributed in the eastern northeast China, where was amalgamation and accretion between the east margin of Central Asia-Mongolia giant orogenic belt and the Pacific tectonic domain. Therefore, it is important for reveal the geological significance of superposition and divertion between the two testonic domains. Wandashan orogen is the typical accerationary complex belt result from the subduction and collision of the Pacific plate, corresponding to the Nadanada terrane or Wandashan terrane. Many domestic and foreign geologists had made geological research of different subjects and different degree around Wandashan orogenic belt, they acquired significant progress on many fields especially on paleontology, structural feature of terrane and paleomagnetism, confirmed the compositon of rocks in Wandashan orogen was not stratum units, but a accerationary complexes belt result from strong deformation and tectonic amalgamation, it is have common characteristics with Mino terrane of Japan in rock association, fossil association and paleomagnetic characteristic. They are all the accretion-tectonic complexs result from the Pacific plate subduction in Mesozoic. There are still many fundamental geological problems wait for resolve, despite aquire some significant achievements, such as the problem of collision age between Wandashan terrane and Jiamusi terrane, the field geological characteristics, geochemical characteristics and geological significance etc.Based on summarizing predecessors` informations, we investigated the workaround carefully, rock masses include clastic rock, siltstone, silty slate, siliceous rock, slate, limestone etc. The ophiolites in the region of Raohe are mainly composed of mafic-ultramafic cumulate complex, cumulate crystallized gabbro, pillow lava, a mass of radiolarian fossils were found in siliceous rock,besides above, early Cretaceous volcanian and Mesozoic granite were distributed here.The structures are well developed and magmatism was very intense in here, all sorts of rock underwent strongly ductile shear-deformation. They are enclosed by many faults and touch structurally each other, outcrops and rock mass disturbed traverse in field, it is difficult to finding the clear boundary of stratum, and they are a suit of typical accretion-tectonic complexes according to the field occurrence characteristics. We investigated the structure-rock association, at the same time, we selected representative samples from the workaround to examine and analyse their geochemistry order to explore and discuss their occurrence setting. According to the analyze result, the formational tectonic setting of sedimentary rocks in Wandashan orogen is very complicated, some derive in continental island ares, some derive in active continental margin, others derive in passive continental margin. The material sources of the sedimentary rocks are complicated too. Above analysis indicate that Wandashan orogen underwent serial tectonic collision and changed the tectonic setting from continental island arcs to continental margin .We use adaptive protolith discriminating diagrams to discriminate the metamorphic rocks in Yuejinshan complexes and the result show that protoliths of the granitic mylonites and the mica-quartz schists belong to pelitic-arenaceous sedimentary rocks. According to Roser and Korsch`s provenance discriminating diagram, their protolith mainly come from granitic-igneous rocks provenance, according to tectonic setting discriminating diagram of the arenaceous rocks, some derive in continental island ares, some derive in active continental margin, others derive in passive continental margin. They have the similar geochemical characteristics compared with sedimentary rocks of Wandashan terrane. All these indicate that Yuejinshan complexes is the collisional belt where between Wandashan terrane and Jiamusi terrane, Wandashan terrane move to here result from the process of western Pacific plate thrusting under the continent of northeast China. The original rock of meta-basic rocks in ophiolite complex of Wandashan orogenic belt can be divided into two sorts. The meta-basalt rocks in ophiolite nappe where around the Dadingzishan and Tuoyaoshan show following geochemical characteristics: with high content of TiO2, HFSE such as Nb,Ta,Zr,Hf , enrichment in LILE, such as Rb,Sr,Ba and LREE. The geochemical characteristics indicate that the meta-basalt rocks belong to the OIB. The geochemical characteristics of the samples of the meta-basic rocks in the Yuejinshan metamorphic complex belt where near to the Yuejinshan block fault indicate that the meta-basalt rocks belong to the tholeiite series of MORB, but enrichment in Rb,Ba,Th etc. The fact that the elements of Rb,Ba,Th are rich and actived in the crust indicate that the rocks were subjected to erosive metamorphism during the collision of two blocks.For the sake of accurately confirm the formation ages of Raohe ophiolite and collision ages of Wandashan terrane, we selected two representative samples date them by SHRIMP zircon U-Pb dating, and the dating results indicate that the formation ages of Raohe ophiolite is 228Ma and Raohe granite is 130Ma, based on this information, we can obtain the cognition that the composition of the accerationary complex belt is Indosinian sedimentary rocks. The main collision ages between 145Ma and 130Ma, and finished in 130Ma and cover into island arc tectonic setting.By studies of this region we obtained following cognitions: Raohe ophiolite that mainly consist of pillow lava and cumulate crystallized complexes in Wandashan orogen represent the fundic rocks of subducted oceanic crust. following are detailed analysis: the basic rocks in Raohe ophiolite represent the oceanic island of the Pacific plate, the basic rocks in Yuejinshan ophiolite complexes reperesent the upper oceanic crust of the Pacific plate.others were consisted in the Wandashan orogen such as Carboniferous-permian limestone, Triassic bedded chert, middle Jurassic siliceous shale, later Jurassic- early Cretaceous sandstone and shale etc. Represent the surficial sediment and sedimentary rocks of the Pacific plate. Analyse the composition of the accretionary complexes that mainly composed of seamount, oceanic island and surficial sediment etc.they were scraped down easily during the process of subduction because of the light density of those rocks and protuberance of topography, at the same time, a small quantity of typical oceanic crust and surficial sediment were scraped and distributed in the boundary of subduction belt. They composed the main body of the accretionary wedge together that result from the subduction of Pacific plate. Therefore, the rock associations of the Wandashan orogenic belt represent the material of the complete subduction belt. This text selected representative samples from the workaround to examine and analyse their geochemistry in detail, the cognition has significant theoretical acceptation to inversion the accretionary evolutional history of the Wandashan orogenic belt.