The Metamorphic And Deformed Characteristics Of Heilongjiang Melange In Mudanjiang Area

The accretion and dispersion of the continental is two spectacular problems in the continental dynamics and continental geology. The long continental margin belts in the Eastern China possess very important position in the East Asia. Many important evidences of early crust-movements are kept in these belts. Constant renewal of oceanic lithosphere and unceasing change of continental lithosphere make it very important to study the continental margin in order to recover the evolution history of the old oceanic lithosphere and continental lithosphere. So, the Eastern China has become a natural laboratory in studying the early crust's formation and evolution.Jiamusi terrain is a very important geological structure unit in eastern Heilongjiang province. Its composition and tectonic attribute, especially its early evolution history has captured the attention of geotectologists and petrologist both at home and abroad. The research about Jiamusi terrain has been lasting from 1926, until 1980s, the research about metamorphic rocks has been developed fast. Researchers carried on studies about the metamorphic facies, metamorphic facies system that correlated with the tectonic setting and tectonic evolution. They also achieved opinions about Jiamusi terrain as follows: Jiamusi terrain is the microblock which divided from the north China plate (Xu etc, 1986); Jiamusi terrain is the residual of blocks divided from Jihei block (Ren, 1988, 1990) and the granite-greenstone belt (Zhao etc, 1986, 1996; Liu, 1988, 1991). In 1990s, through studies on metamorphic rocks of originic Heilongjiang group, Cao (1992), Zhang (1992) and Li (1999) discovered that in the Heilongjiang Group there existed rocks with the combination of dismembered ophiolite suite and low T and high P metamorphic mineral—sodium amphibole, they also found fadiolarian chert (Zhang, 1992) and chitinozoa fossil-beared phyllite (Li, 1999) in the rocks, which appearance proved that the originic Heilongjiang group is not normal metamorphic strata but the tectonic melange having the property of collision belt. It is the accretionary complex developed during the collision between Jiamusi terrain and adjacent terrain or between its interior microblocks (Zhang, 1992; Li, 1999; Cao, 1992). So far, the views about the property and the ascription of Jiamusi terrain began to unify. These progresses which domestic and foreign scholars have gotten in the composition and chronology research of Jiamusi terrain play a positive role in understanding of tectonic attribute and evolution of Jiamusi terrain.There are accretionary complexes outcropped between Jiamusi terrain and Xingkai terrain (Mudanjiang area), and these accretionary complexes are called Heilongjiang Group in the past and Heilongjiang melange in this text. In Mudanjiang, Luobei and Yilan area, Heilongjiang melange extend along south and north direction. In Mudanjiang area, it is leaped by Dun-Mi fracture. Previous researchers have not got identical opinion on high-P metamorphism and deformation of Heilongjiang melange in Mudanjiang area and its tectonic evolution. There also lack explanation of different characteristics of Heilongjiang melange among three areas on the stand of tectonic.This paper takes Heilongjiang melange in Mudanjiang area for object, bases on measured section and comprehensive analysis of the previous researchers' achievements of Heilongjiang melange on Jiamusi terrain, and combines the field work and the indoor work, and according to the geology, petrology, geochemistry, isotopic geochronology and genetic mineralogy, we do the research to the Heilongjiang melange that outcrop of Mudanjiang area include Space output, distribution characteristics, material composition, the original rock type, the tectonic setting , the era of the formation, metamorphism and deformation of Heilongjiang melange. After these studies of Heilongjiang melange, it provides some clear understandings of metamorphism and deformation and its tectonic evolution of this melange.Heilongjiang melange in Mudanjiang area distributes along both the south and the north of Dun-Mi fault, it mainly distributes in Mudanjiang-Muling area in the north and in Hulin area in the south.According to the study on rock association and mineral association, we can see that the Heilongjiang melange in Mudanjiang area is composed of the groundmass and the blocks, the groundmass is felisic schist, has 70% of congestion rock in Heilongjiang melange. Another one part survive in the felisic schist, among them ultrobasic rocks, metamorphic basic volcanic rock and metamorphic silicalite block that formed by deposition as a set of ophiolite combination.On the geochemistry characteristic, the felisic schist expresses the characteristic that a mainland edge deposition rock fasten, metamorphic basic volcanic rocks were the ocean island type basalt mainly, a little of the ridge basalt and these rocks grown symbiosis in the space in the ocean, their emergence is the important proof of the ocean existence.Ophiolite combination of Heilongjiang melange in Mudanjiang area means there was once an ocean basin before between Xingkai terrain and Jiamusi terrain. Heilongjiang melange is a suit of accrete rocks that formed when ocean crust released, basin closed and two terrains dived to collide.After comparison of Heilongjiang melange in three areas, we can see that Heilongjiang melange in Mudanjiang area is the product of collision between Jiamusi terrain and Xingkai terrain which is different from the other two areas.Combined the measured section and petrography characteristics of Heilongjiang melange in Mudanjiang area, it can be divided into three facies belts from the north to the south. They are blueschist belt, stilpnomelane belt and biotite belt. There are two strong ductile deformation belts between belt and belt. It means that these three belts are not successive metamorphic belt but tectonic belt.Through the analysis of metamorphism and deformation process of Heilongjiang melange in Mudanjiang area and the relationship between metamorphism and deformation, we made clear the type of metamorphism and pattern of deformation and determined that the Heilongjiang melange in Mudanjiang area experienced three periods metamorphism and deformation (blueschist facies—glaucophane greenschist facies—low greenschist facies). Blueschist facies is the first period metamorphism and deformation, and the formation of blueschist (glaucophane-beared) is related to subduction. Glaucophane greenschist facies is the second period metamorphism and deformation, and the transition from blueschist to glaucophane greenschist (from glaucophane to crossite) is related to structural uplift. The main uplift is thrust nappe which is caused by collision between Jiamusi terrain and Xingkai terrain and by the affection of western pacific tectonic domain. Partial high P metamorphism and deformation which is caused by large scale structure is the third period metaporphism and deformation, and the formation of vein crossite is related to sinistral strike-slip of thrust nappe. All of them indicate that metamorphism and deformation has clockwist P-T-t path.Tectonic evolution of Heilongjiang melange in Mudanjiang area has been discussed, and isotopic geochronology information has been analyzed in detail in this text. The research result showed that the tectonic evolution of Heilongjiang melange can be divided into three stage: (1) Before 305-296Ma: The subduction and closure of intercontinental sea basin. (2) 154-170.26Ma: Collision between Jiamusi terrain and Xingkai terrain and affection of western pacific tectonic domain (3) After 154Ma: Sinistral strike-slip of Dun-Mi fault and modification of Heilongjiang melange by sinistral strike-slip. These research results have important significance in understanding the formation and evolution of East Asia continental margin.