The Nature Of The Yuejinshan Complex In The Eastern Part Of Heilongjiang Province And Its Evolution

The Yuejinshan complex, mainly exposed along Bawuer farm-Bawusan farm of Baoqing county, eastern of Heilongjiang Province, is located between Jiamusi Massif and Wandashan accretionary complex with distribution of nealy SN-trending in regional structure. The earlier researchers regarded it as a series of stratigraphic unit called as Yuejinshan Group. Some researchers considered that it is an exotic terrane accreted to eastern margin of Jiamusi Massif called Yuejinshan terrane. Others regarded it as a part of Mesozoic accretionary complex zone of Wandashan according to the series of complex located in south end of Wandashan mountain. There is wide controversy on property, evolution and age of Yuejinshan complex, because absence of research on careful petrology and tectonics, especially absence of evidence on petrogeochemistry and exact isotope chronology. According to research on detailed field geological investigation, petrology, geochemistry and isotope chronology, and couple with relative research achievement on Jiamusi Massif and Wandashan accretionary complex, this paper put forward new understanding on property and evolution of Yuejinshan compex.The field evidence show that the Yuejinshan complex is composed of strongly deformed matrix and rock mass mixed within it. The deformed matrix is mainly composed of felsic mylonite and felsic schist. The rock mass mainly contains basalt, marble and argillaceous siliceous rock. The rock mass with strongly foliated matrix is partly suffered strong deformation.The result of isotopic dating show that U-Pb age for 13 zircons from basalt range from 218 Ma to 242 Ma by LA-ICP-MS with a weighted mean age of 232 ± 5 Ma, which indicate it formed at Late Triassic. U-Pb ages of detrital zircons from felsic mylonite are mainly focus on two peaks by LA-ICP-MS: 496 Ma and 258 Ma, secondly are Precambian age of 738 Ma, 908 Ma, 1079 Ma, 1367 Ma and 1544 Ma, as well as age of youngest detrital zircon is 223 ± 7 Ma. The characteristics of detrital zircons age clearly show source of sedimentary material and character of deposition time of the series of greywacke. Two peak ages of 496 Ma and 258 Ma are ages of two important tectonic-thermal events occurred in Jiamusi massif. The former is Early Paleozoic advanced metamorphism period range from amphibolites facies to granulite facies, and the latter is formation period of Late Permian magmatic arc and show that material source of protolith for felsic mylonite is mainly from Jiamusi massif. The existence of part ages for detrital zircons greater than 738 Ma further prove older Precambrian curst exist in material source(Jiamusi massif). The age of youngest detrital is 223 ± 7 Ma, we define lower limit of deposition age for protolith is Late Triassic. The granitic rock vein mainly intruded along contact zone between matrix and rock mass. The result of zircon U-Pb dating indicate that age for these veins are range from 123 Ma to 125 Ma, and formation age is Early Cretaceous, which is accordance with Early Cretaceous magmatic activity event widely developed in the area.The results of petrogeochemistry show that basaltic rock mass is subalkali basalt with characteristic of back-arc basin mid ocean ridge basalt(MORB). The protolith of greenschist is alkali basalt with characteristic of ocean island basalt(OIB). The combination of marble indicate it formed in sea mountain environment. The content of SiO2 for rock mass of argillaceous silicecous rock is far lower than content of pure silicon, which indicate the component of argillaceous is more. The characteristic of geochemistry indicate it formed in non-biogenetic silicon in margin of crust. The protolith of felsic mylonite is a series of greywacke of calcareous deposition with geochemistry characteristic of tectonic setting of active continental margin, and it is a typical calcareous turbidite.We did research on geochemistry and isotopic chronology for granite widely developed in the Yuejinshan area and gabbro exposed in Dongfanghong area to the east of Yuejinshan complex. The result of isotopic dating show U-Pb age for 28 zircons from gabbro(13HL20-1) range from 263 Ma to 273 Ma by LA-ICP-MS with a weighted mean age of 266 ± 1 Ma. U-Pb age for 26 zircons from 13HL26-1 range from 268 Ma to 297 Ma by LA-ICP-MS with weighted mean age of 280 ± 3 Ma, which show that formation ages for gabbro is Early-Middle Permian. U-Pb age for 25 zircons from granite range from 116 Ma to 126 Ma by LA-ICP-MS with weighted mean age of 120 ± 1 Ma, which show formation age for granite is Early Cretaceous, and is same with the formation age for granitic vein rock intruded into Yuejinshan complex. The analysis for geochemical characteristics show gabbro formed in continental margin arc of active continental margin with characteristic of subalkaline island arc basalt.The formation age and environment are accordance with gabbro(266 Ma, 280 Ma) with characteristic of continental margin arc, Early-Middle Permian volcanic rock and granite widely developed in the east margin of Jiamusi massif. It show that the subduction of oceanic crust to the west in the Permian is exist in the east margin of Jiamusi massif, which provide powerful evidence to Early-Middle Permian magmatic arc developed in the east of Jiamusi massif. The existence of MORB type basalt(232 Ma) with characteristic of back-arc basin show that there is new oceanic crust formed to the east area of Jiamusi massif in Triassic. The characteristic of gabbro developed to the east of Yuejinshan complex further indicate that the Permian continental margin magma arc located in east margin of Jiamusi massif had splited after it formed, it means that back-arc basin is expand. It indicate that there is evolution trend angle of subduction is gradually steep from formation of Permian continental margin magma arc to Triassic back-arc spending. The intracontinental present extension with EW-trending, which result in split of east margin of Jiamusi massif. It makes stripping of earlier period of continental margin arc from continental margin. The age of basalt mainly focus on 218 Ma to 242 Ma indicate splitting of east margin of Jiamusi massif and back-arc spreading mainly happened in early period of Middle-Late Triassic.There is good corresponding relation between formation of Triassic oceanic crust and regional geological setting. A series of interactive marine and terrestrial deposit(Shuangyashan Group) of Late Triassic passive continental margin is generally developed in eastern margin of Jiamusi Massif, which is basically same on formation age with matrix from Yuejinshan complex. According to protolith of felsic melonite formed in instable sedimentary environment, which is clearly different with Nanshuangyashan Group contain Late Triassic Norian typical fossil, located in the east margin of Jiamusi massif, formed in stable sedimentary environment. We speculate formation age for the series of protolith of felsic melonite is later than Nanshuangyashan Group. The formation environment of argillaceous silica also is instable sedimentary environment, which indicate back-arc is stop to spread and begin to shrink, and formed a serried of fast and instable sedimentary formation.With gradually close of back-arc basin, the Late Triassic sedimentary rock transformed, and part of basalt and marble with oceanic crust is involved in transformed rock series as rock mass. According to data from Yang Zhongjin(1998), Rb-Sr isochron age of metamorphic-deformed rock from Yuejinshan complex is 188 Ma, which indicate formation age of metamorphic-deformed fabric expressed by current Yuejinshan complex is Early Jurassic.