The Tectonic Characteristics And Evolution Of Jiamusi Massif And Wandashan Complex

The researched region is located in the east of Heilongjiang Province,NE China,bordering with Russia across Heilongjiang and Wusulijiang rivers respectively.The geographic coordinates is in the range of E132°00?-E134°,N46°-N48 °,with a area of about 25000Km2.The area is divided into two tectonic units of the Jiamusi massif in west and Wandashan(Nadanhada)complex in east,bounded by the N-S trending Tongjiang-Mishan fault.The former is northwards linked together with Bulieya massif in Russia,and the latter is a part of the Sihote-Alin Mesozoic.accretionary complex in Russia.The tectonic significance is attracted more attention because Jiamusi massif is located in the mosteastern end of the Central Asian Teconic Belt and Wandashan Complex located in the West Pacific Continental margin.Although there has been many studies about evolution of the two units,it is still limited that the Jiamusi massif is a long-term stable Precambrian massif,and Wandashan complex is a suite of Mesozoic accretionary complex or exotic terrane.In recent years,the significant evidences on the Jiamusi massif have been achieved.Firstly,it was determined that the high-grade granulite facies metamorphism in the Jiamusi massif was formed in early Paleozoic,rather than traditionally reffered Paleoproterozoic or Archean;secondly,the Permian granite and volcanic rocks formed in a mobile congtinental magin setting have been widely discovered in the Jiamusi Massif and its east margin.These achievements reveal that Jiamusi Massif is not a long-term stable block and was an active continental margin setting in the Permian.Mean while,the evidences arouse two new problems: whether there existed Permian oceanic crust to the east of the Jiamusi massif,and whether whole Wandashan Complex is composed of Mesozoic complex or exotic terrane.In order to solve the new problems described above,the paper gives some new geological evidence and explaination on the tectonic evolution of the two units,according to the detail field investigation and relative test analysis.The new evidence show that the Permian granite originated in active continental margin is widely distributed in the east margin of the Jiamusi massif,where the granite is unconformably covered by a suit of conglomerate-arkose.The zircon ages ranging in268Ma-288 Ma have been obtained from the granite and the four groups of detrital zircon ages of >800Ma?500-494Ma?269-250 Ma and 221-218 Ma have been obtainedindicated from arkose.The former three groups of ages were the most common and important records of three tectonic thermal events in Jiamusi Massif,the latter one was the youngest age group which is only recorded in the early Mesozoic granite belt in the lesser Xing'anZhangguangcailing range to the west of Jiamusi Massif.The evidences above indicates the sedimentary age of the conglomerate-arkose unconformable on the Permain granite was the Late Triassic,almost simultaneous with a suite of marin-land facies siltstone-mudstone(Nanshuangyashan Formation)containing abundant fossils in adjoining areas.The characteristics of the detrital zircon ages and sedimentary facies demonstrate that the east margin of Jiamusi Massif was in a passive continental margin sedimentary setting in the Late Triassic,the sediment sources were mainly from Jiamusi Massif,partly from the lesser Xing'an range-Zhangguangcai range to the west of the Jiamusi massif.All views from previous researches whether exotic terrain or accretion complex suggest the Wandashan(Nadanhada)complex is composed mainly of Mesozoic oceanic crust association.In this study,we find that the west part of the complex consists mainly of a suite of strongly deformed and slightly metamorphosed blocks of basalt(some with pillow structure),gabbro,marble and siliceous rocks.The dating results show that basaltsand gabbro yield a weighted mean206Pb/238 U zircon age of 270Ma~279Ma and 287 Ma,respectively,indicating the protolish ages of the basalts and gabbro formed at Early-Middle Permian.Petrochemical features of the basalts are affinity of N-MORB.The results provide strong evidences for revealing existence of Permian oceanic crust to the east of the Jiamusi massif and reasonably explaning the formation of Permian magmatic arc at the eastern margin of themassif.In addition,a suite of conglomerate-sandstone was discovered in the area.The detrital zircons from the sandstone and conglomerate show the four population ages of 219-221Ma?255-268Ma?494-500 Ma ?> 800 Ma,completely consistent with the conglomerate-arkose unconformable on the Permain granite in the east margin of Jiamusi massif.The east part of the Wandashan complex is mainly of metamafic-ultramafic rocks and siliceous rocks with radiolarian,in which the zircon age of 166-167 Ma had been obtained from pillow basalt and gabbro with OIB affinity by many researchers,indicating they formed at Early-Middle Jurassic simultaneous with the youngest fossil age of radiolarian in siliceous rocks.The study got again the same zircon age of 168 Ma and the sam geochemical feature of the OIB affinity in the pillow basalts,further proving the metamafic-ultramafic rocks formed in the Middle Jurassic.Besides the different in the form time and tectonic setting,the east part of the Wandashan complex are oberveous diferrent from the west in deformation type and metamorphism.The east rock association are characterized by brittle fold deformation without metamorphism,in which the chevron-folds are widely developed in the siliceous rocks and siliceous mudstones.Many reearchers thought the chevron-fold is a typical deformation pattern in the front of the imbricate thrust nappe.In view of the evidences described above,the paper divided the so-called Mesozoic Wandashan(Nadanhada)complex or terrane into two parts of the “Yuejinshan complex”mianly in the west and “Raohe complex” in the east.The former is a suite of dubductionrelated accretion complex containing Permian oceanic crust,and the latter is composed of a series of westwards thrust imbricate nappe consisting of Mesozoic oceanic crust silices and blocks.The main conclution on the tectonic evolution of Jiamusi Massif and Wandashan Complex are summarized as follows:1.The east margin of the Jiamusi massif was an active continental margin in the Permian,marked by the Permian magmatic arc.At that time,a new crust was still forming to the east of the massif.2.The so-called Mesozoic Wandashan(Nadanhada)complex is composed of two suite of completely different crust associations in time,deformation pattern,metamorphic grade and tetonic evolution nature and so on.It can be divided into the “Yuejinshan complex” mianly in the west and “Raohe complex” in the east.The former is a suite of dubduction-related accretion complex containing Permian oceanic crust,the latter is a series of imbricate westwards thrust nappe consisting of Mesozoic oceanic crust silices and blocks.3.The Yuejinshan accretion complex had become a part of the eastern continental margin of the Jiamusi massif before the Late Triassic,characterized by the Late Triassic marine-land facies strata unconformable on the Permain granite and the Yuejinshan complex4.The Raohe Complex is westwards thrust nappe consisting of imbricate Mesozoic oceanic crust silices and blocks.The form time of the thrust nappe is inferred to be after Middle Jurassic and before Early Cretaceous,according the available data that the youngest ages of the zircon dating and fossils are Middle Jurassic and that the complex is intruded by the cordierite granite with the age of 131-128 Ma and covered by Early Cretaceous sedimentary basin.