Paleozoic Metamorphism And Tectonic Evolution Of Jiamusi Block

With the growth, movement and evolution of lithospheric plates, there were various types of metamorphic rocks developing in its different parts. Different types of metamorphic rocks reflect the features as follows: different metamorphism in different parts of lithospheric plates; different metamorphism in inner of different types of plates(fore example:oceanic plate and continental plate); Different types of metamorphism in different margin of the same plate. Because of recycling of oceanic lithosphere, much of the history record of the Earth's evolution preserved only in continental lithosphere. As a basic structure of continental plate, the orogenic belt is an ideal area to explore the evolution of continental crust and is a natural laboratory of study of metamorphic geology, too.Jaimusi Block, located in the eastern part of Heilongjiang Province, which belongs to the sub-tectonic units of Jia-Meng continental block, are composed of a lot of kinds of rocks. Most of prior researchers generally thought they were formed in Proterozoi or Caledonian period. On the east of Mudanjiang fault, there are north-south distributions of all kinds of metamorphic rocks, which are believed to form in Precambrian. Based on plate tectonics theory, we discussed the output and distribution characteristics of space, material compostion, the original rock type, the main metamorphic mineral types and mechanism of formation, tectonics environment and time of metamorphism. According to study on the geology, palaeontology, petrology, geochemistry new information, we believe that so-called Mashan and Heilongjiang Group are not the traditional view of the layered metamorphic strata, but formed at different times in different tectonic environments; Majiajie Group is not the formation of Precambrian, but rather a set of formed in the late Paleozoic with hot contact metamorphism of sedimentary strata, and experienced regional dynamic metamorphism in Jurassic.Different types of rocks of Mashan complex feature different metamorphis grade in different area. Some rocks located in Jixi region is of highter degree of metamorphism to granulite facies, others in Boli and Huanan region is lightly lower metamorphism grade to amphibolite facies. Although the Mashan complex has different tectonic lineation in different region, most of all rocks of Mashan complex feature by Al-rich and carbon-rich. Research of petrology and geochemistry indicate that the original rocks of Mashan complex have the same sedimentary environment. Isotope geochronology analysis showed that the extent of different metamorphic rocks of Mashan complex is in line with the U-Pb age. The relationship between different rocks with different metamorphism grade of Mashan complex in Jixi and Huanan area are tectonic contacts. These research results above indicate that: so-called Mashan Group and Xingdong Group formed in the same tectonics environment at Cambrian, of which the original rocks were the same sedimentation before metamorphism, and these rocks are feartured by different metmamorphic characteristics because they located in different region during the metamorphic period.So-called Heilongjiang Group located in Luobei, Yilan, Huanan and Mudanjiang areas are not traditionally recognized as a set of metamorphic layered strata, but rather a set of tectonic melange feature ophiolite. This set of mélange is composed of two parts, one is matrix with the nature of continental crust, and the other is massive rocks with the nature of the oceanic crust. There are many types of sodic amphiboles with different micro-structural characteristics in different rocks of Heilongjiang mélange. Lithofacies study research showed these sodic amphiboles formed in different tectonics environment during different period. Some sodic amphiboles without directional structure in metamorphic basalt vocalic rocks which had original rock structure characteristics may be formed in the ocean-continent subduction-collision early, while the others with directional structure in green schists may be formed in different tectonics environment. Isotope geochronology studies have shown that the latest high-pressure low-temperature metamorphism of Heilongjiang mélange occurred in the early Jurassic period, and the U-Pb zircon age (264±7Ma)of granite dikes which intruded the Heilongjiang mélange and experienced the same metamorphism in early Jurassic both limited that the first high P/T metamorphism occurred before middle Permian, in another way, the paleo-ocean which the Heilongjiang mélange represents had been closed before 264±7Ma. The Heilongjiang mélange located in Luobei, Yilan, Huanan and Mudanjiang areas indicates that there was a paleo-ocean basin between Jiamusi block and Songnen block before Pemian, and the different occurrence feature of Heilongjiang mélange at different tectonic area are just because of transformation of faulting in Jurassic.Majiajie group is a set of Al-rich and carbon-rich sedimentary strata exposed in Huanan area, including a lot of andalusite, and staurolite empty spar which reflects hot contact metamorphism. In addition, Majiajie group experienced a later stage of the regional dynamic metamorphism. The zircon U-Pb dating results show that Meizuo granite intruded Majiajie group and made it metamorphose at about 259Ma, hence the sedimentation age of Majiajie group was before late Permian. Geochemical studies have shown that Mashan complex may be an important source area of Majiajie group. The granulite facies metamorphism of Mashan complex taken place at about 509Ma before a wide range of migmatization on Jiamusi block-eastern part of Jia-Meng continental block. According to the 500Ma tectonomagmatic case around in Erguna, Xingan and Songnen block, we believe that there was a unified landmass formed in early Paleozoic on the northeast China. Heilongjiang mélange, with nature of ophiolite, are the real trace of the paleo-ocean, although we can't make sure what time it did exist, we can limit the time to Permian according to the isotopic dating results and other reasonable proof. Hence we predict that the early hight P/T metamorphism which reflected ocean-continent subduction collision taken place before Permian, and the Heilongjiang paleo-ocen could have valished in early Paleozoic. In other words, Jia-Meng continental block formed in early Paleozoic, and then a lot of strata developed on the margin of this unified landmass. We can predict that sedimentation of Majiajie group just developed south-east magin of Jia-meng block.Along with the Paleo-Asian Ocean gradually closed eastward, late Permian tectonomagmatic occurred about 259Ma on Jia-Meng continental block, which lead to volcanic eruptions magmatic intrusion at the south-eastern of Jia-Meng continental block. The contact metamorphism of Majiajie took place on this tectonic environment; meanwhile some granite veins intruded the Heilongjiang mélange at Yilan and Luobei area. After Paleo-Asian Ocean completely closed, the tectonics system changed from east-west extrusion to the north-south extension in North China in the early Jurassic. There was a very important north-south fault movement, and then the famous Tan-Lu Fualt formed since 190Ma. It started at Guangji, Hubei, by Lujiang, Anhui, Suqian, Jiangsu, Tancheng, Shandong and Bohai, via Shenyang and then entered the Mudanjiang Yilan, Luobei till the territory of Russia. The Heilongjiang mélange and Permian intrusion granite veins both underwent second hight P/T metamorphism at 185~174Ma, which features formation of crossite s and phengites. Under the influence of change of Izanagi plate's subduction, the Mudanjiang fault and Dun-Mi fault, as branches of Tan-Lu fault, vein-like sodic amphiboles formed widely again in these belts because of unique lithology and tectonic environment when it was about 154.7Ma.Mashan complex, Heilongjiang mélange and Majiajie group feature different rock series, different metamorphism grade, and that they all expose in eastern Mudanjiang fault belt is a result of tectonics movement. Petrology, geochemistry and isotope geochronology study of above three sets of different types of metamorphic rocks show the interaction process of Jaimusi block and adjacent blocks during 500Ma-154Ma.