The Geological Genesis And Tectonic Significances Of The Heilongjiang Complex Belt

There distribute Songnen terrane, Jiamusi terrane and Wandashan terranefrom west to east in Jiamusi area locating on the east segment of Xingmengorogenic belt. The study on the amalgamation between the terranes is always thefocused problem and hot point, and made certain progresses on it, but there is astrong dispute on the era and the mechanism of the amalgamation between Jiamusiterrane and Songnen terrane. The Heilongjiang complexes belt occurring on thewest margin of the Jiamusi terrane provides a good opportunity for the study ofabove geological problems.Heilongjiang complexes belt that we speak of is the tectonic complexes beltoccurring on the west margin of the Jiamusi terrane, and extendsNorthsouth-direction like a belt shape along Mudanjiang river, occurs inMudanjiang, Yilan and Luobei etc areas in Heilongjiang province, correspondingto the Heilongjiang Group or Heilongjiang tectonic complexes. The Heilongjiangcomplexes result from the subduction and amalgamation of the Jiamusi terrane,and represents the suture between Jiamusi terrane and Songnen terrane. Therefore,we make our mind to research the complexes. Although many geologists had madesome research on the complexes, in generally speaking, the research on thecomplexes is not enough, there are still many fundamental geological problems,the main geological problems follow: 1.the field geological characteristics, outcropplaces and typical amalgamation marks of the Heilongjiang complexes. 2. theprotolith association and protolith ages of the Heilongjiang complexes. 3. theformation era of the Heilongjiang complexes.On the basis of summarizing predecessors` information, we survey carefullyHeilongjiang complexes belt, and found that Mudanjiang area, Yilan area andLuobei area etc are the main occurrence locations of the Heilongjiang complexes.The rocks constituting the Heilongjiang complexes develop commonly ductiledeformation fabrics, this indicates that they underwent strongly ductileshear-deformation,and they consist of all kinds of deformational rocks ofgreenschist-facies metamorphic minerals association, and all kinds of rock massesof different era, different property and different metamorphic characteristicsmingle within the complex belt, they are enclosed by many faults around them andtouch structurally each other. Rock masses include ultramafic rocks, metamorphicgabbros, metamorphic mafic lavas, siliceous rocks, marbles, blueschists,amphibolites and hornblende schists etc. The in-situ matrix masses mainly includegranitic mylonites, garnet-mica-quartz schists, slates and metamorphic sandstonesetc. Though these rock masses underwent multiple metamorphism anddeformation processes, and led to change the inner textures of the rock masses,there are many rock masses that preserve extently original textures. They are a suitof typical accretion-tectonic complexes according to the field occurrencecharacteristics and their inner structure-rock combination features.We investigated the structure-rock association, at the same time, we selectedrepresentative samples from the Heilongjiang complexes belt to examine andanalyse their geochemistry order to explore and discuss their occurrence setting.We use adaptive protolith discriminating diagrams to discriminate their protolithproperties, and the result shows that the protoliths of the granitic mylonites and themica-quartz schists belong to pelitic-arenaceous sedimentary rocks;According toRoser and Korsch`s(1988) provenance discriminating diagram, their protolithmainly come from granitic-igneous rocks provenance;they are considered to formin active continental margin by tectonic setting discriminating diagram of theSiO2-K2O/Na2O duality of the arenaceous rocks put forward by Bhatia andCrook(1986). Analysis results of their REE shows that they are of high∑REE(∑REE = 75-340×10-6)and a little higher LREE/HREE ratio(∑LREE/∑HREE=6.2-13.9);and are of harmonious chondrite-normalized REEpattern of alleciated right-leaned trend with negative abnormity of Eu element,their whole feature of REE pattern also indicate that they came from activecontinental margin. In addition, their MORB–normalized incompatible elementsspider diagram models resemble those of rocks constituting upper crust studied byWeaver et al(1984). Therefore, we think that the protolith of granitic mylonitesmica-quartz schists as host rocks from Heilongjiang complexes belt arepelitic-arenaceous sedimentary rocks, and are considered to form in activecontinental margin. the results of protolith discrimination of amphibolites,hornblende schists and blueschists derived from Heilongjiang complexes beltshow that majority of them are tholeiitic rocks and minority of them are alkalicrocks. The mafic rock masses from Heilongjiang complexes belt are MORB, WPBand alkalic basalts according to the tectonic setting discrimination diagrams offormation of basalts. In addition, their chondrite-normalized REE patterns havetwo kind of patterns: one is of enriched LREE, and of right-leaned trend withoutobvious abnormity, the characteristic resembles with that of ocean-island basalts;the other shows the smooth REE patterns, and one or two depleted in LREEwithout obvious abnormity, their characteristic resembles with that of MORB.Their MORB-normalized incompatible elements spider diagram models also showgenerally two type of models: one is of enriched elements contrast to MORB onthe left end of the model curves, and depleted obviously in Cr element, this showsthe features of Ocean-island basalts;the other model resembles with that ofMORB, and their analysis data show that Ta element is poor, its elementabundance is from 0.33 to 0.49μg/g, and are of low Ta/Yb ratio(0.04-0.15), whichindicates that they are derived from a relatively strongly depleted mantle sourcesarea, and according with the features of the typical MORB. Summarizing above all,we make sure that the mafic rock masses formed originally in Mid-Ocean Ridge orOcean islands, and are typical ocean crust matters. They resulted from thewestwards amalgamation of Jiamusi terrane, and underwent the dying out andsubduction process of the ocean crust.We selected seven blueschists and amphibolites samples from Heilongjiangcomplexes belt to date them by SHRIMP zircon U-Pb dating, and the datingresults showed 185-220Ma, 250-270Ma and 450-500Ma three ranges. Accordingto the inner texture and trace elements features of the dating zircons, the zircons ofrange from 185Ma to 220Ma and range from 250Ma to 270Ma show the featuresof magmatic zircon, we considered them to represent the formation ages of therocks constituting the Heilongjiang complexes belt. However, the zircons of rangefrom 450Ma to 500Ma are metamorphic zircons or captured sedimentary detritalzircons, and their ages accord with the age of granulite metamorphic facies eventin the interior of Jiamusi terrane, so we think that they are inherited zircons fromJiamusi terrane. Besides, we primarily built the tectonic evolution stages of thewestwards subduction and amalgamation of Jiamusi terrane resulting in formingthe Heilongjiang complexes.By the field geological investigation, the researches of geological features onHeilongjiang complexes belt, and the geochemistry analysis and chronologystudies on the representative rocks samples, we obtained presently followingcognitions(conclusions):1. based on the field geological evidence, geochemical characteristics andgeochronological data of Heilongjiang complexes belt, we think that Heilongjiangcomplexes belt is of the characteristics of the typical accretional complexes beltand resulted from the subduction and amalgamation of Jiamusi terrane, andrepresents the tectonic suture belt between Jiamusi terrane and Songnen terrane.2. The protolith ages of the rocks constituting the Heilongjiang complexesbelt are 185Ma-255Ma, so we think that the formation ages of Heilongjiangcomplexes belt is younger than 185Ma and may form later than late Indosinian toearly Jurassic.3. The protolith of host rocks in the Heilongjiang complexes belt arepelitic-arenaceous sedimentary rocks, and are considered to form in activecontinental margin setting;However, the protolith of the mafic rocks massesimmingled in Heilongjiang complexes belt belong to tholeiitic basalts, and most ofthem are considered to form originally in ocean tectonic setting, minority of themformed within plate setting.4. The Heilongjiang complexex belt formed during the subduction andaccretion of Jiamusi terrane towards Songnen terrane, and is the suture beltbetween Jiamusi terrane and Songnen terrane.5. The geochronological data indicate that the subduction and amalgalmationprocess between Jiamusi terrane and Songnen terrane began at later than 185Ma, itis later obviously than the time of orogenic event of the MidAsia-XingMengorogenic belt. As a result, the result challenges the idea that Jiamusi terranebelongs to the MidAsia-XingMeng orogenic belt, and this new viewpoint willprovide an important clue to discuss the tectonic attribute of NorthEast China andthe closure history of the paleo-asian ocean in NorthEast China.