Petrological And Geochronological Research Of Blueschist In Heilongjiang Complex And Its Tectonic Signification

Northeastern China was traditionally considered as the eastern part of the Paleozoic Central Asian Orogenic Belt(CAOB),located between the Siberian Craton and the North China Craton.The Jiamusi Massif(block)is one of the four microcontinental blocks in the CAOB,located in the eastern part of the NE China.My study area,Yilan-Mudanjiang blueschist zone,is probably an ancient subduction zone in the Jiamusi Massif.It is characterized by～200 km long,and situated in the east of the Mudanjiang fault that extends from north to south.The Heilongjiang complex consists of greenschist,blueschist,carbonate rocks,ultramafic rocks,quartzite and pelitic schist,whereas the remainder are most likely constituents of ophiolite.These rocks are now tectonically interleaved and were deformed together.In recent studies,for the metamorphic condition,the metamorphic rocks including blueschist have not described petrologically in modem sense.Bai(1988) gave the pressure of the Heiiongjiang complex yielded 0.6-0.7 Gpa,using the diagram of b₀ value of potassic-micas;and temperature is Ca.350-450℃,according to the temperature range of the glaucophane schists.In addition,a number of isotopic ages were reported from the Jiamusi Massif especially blueschists:599-664Ma of whole rock Ar-Ar dating,suggesting one collision between the Jiamusi Massif and the Songnen Massif but not fully closed,till 410-440Ma the final collision(Zhang 1992);497-530Ma ages from Jiamusi Massif are considered as the age of the Paleozoic Pan-African tectonic events(Ca.500 Ma)(wilde 1997,1999,2000,2003; Wu 2007).270-254Ma ages of SHRIMP single zircon from granites and granitic gneisses indicated the Permian batholiths in the Jiamusi Massif(Wu 2000,2001); 174-184Ma of ⁴⁰Ar/³⁹Ar dating from biotites and phengites,indicate the metamorphic age of Heilongjiang complex(Wu 2007);～154Ma plateau ages of ⁴⁰Ar-³⁹Ar from crossites vein,suggesting the effect of an activity of the Dunhua-Mishan strike-slip fault.Samples were collected from the Yilan and the Mudanjiang areas for petrological study including two samples for ⁴⁰Ar/³⁹dating.Field investigations show that, basic schists occurred as lenses in the country rocks(pelitic schists).Generally,the basic schist shows weak deformed and/or almost non-deformed,but the country rocks exhibit strong deformation.Blueschists usually show the residual pillow lava structure,sometimes,we can also see the relic vesicular structure in the blueschist suits.These should be the evidences of protolish deriving from pillow basalts.All the samples,according to the field investigations and the petrographical researches,can be divided into several groups,the detailed descriptions as follows: Metabasite is characterized by the greenschist and blueschist,separating by the series of the Amp(Na-amp occurs in the blueschist and Ca-amp in the greenschit),pelitic schist,quartzite,carbonate rocks,ultramafic rocks.In the blueschist,the typical mineral assemblage of the blueschist is represented by,Na-amphibole + epidote + albite + chlorite + calcite + quartz + paragonite + phengite + hematite±garnet±stilpnomelane; Pelitic schist is characterized by two-mica schist,stilpnomelane-quartz schist, garnet-bearing quartz schist,chlorite-mica-albite schist.Two main typical mineral assemblages in the pelitic schist are:albite + epidote+ chlorite + quartz + amphibole + phengite + titanite + apatite + opaque mineral±turmaline,and quartz + albite + stilpnomelane + chlorite + phengite + biotite±epidote±titanite±apatite±opaque mineral±turmaline.Amphiboles,in the blueschist,are classified into sodic amphibole,sodic-calcic amphibole and calcic amphibole(Leake 1997).The amphiboles occurred in the samples,with the compositional zonations of glaucophane cores to winchite rims in Mudanjiang area,and the samples collected in Yilan area with the zoning form glaucophane cores to barroisite,and from barroisite to the actinolite rims,which the zonation represents the retrograde metamorphic stage.Calcic amphiboles in the pelitic schists show the zonation of magnesiohornblende to actinolite,and also with the high Ti content in the cores.Combining with the texture of this kind of amp, which shows brown color anhedral core part and colorless euhedral rim part,we suggest that the amphibole may be relic igneous amp or more high-grade metamorphic rocks,i.e.Mashan complex.Na_B(Na in M4 site)vs.Si in the formula unit in amphibole barometer,shows the amps from blueschists are mostly stable at over 7 kbar,Calcic amphiboles in the pelitic schists mostly stable at 2-3 Kbar.The mineral assemblage in the blueschists, glaucophane+ paragonite occur as inclusions in the prophyroblastic epodite, suggesting the peak metamorphic stage(blueschist facies stage).The Pg-in reaction in blueschist facies and the epidote-glaucophane stability field(Evans 1990)bounded the peak stage stability field,which limits the P-T condition of the peak metamorphic stage yielding T=350-520℃,and the P=9-16 kbar.The retrograde metamorphic stage of blueschist is different in Mudanjiang area and Yilan area.In Mudanjiang area, compositional changing from glaucophane or ferroglaucophane to winchite,gives the P-T conditions following T=300-450℃,and P=4-8 kbar(Otsuki & Banno,1990);and in the Yilan area,amp shows from glaucophane to barroisite to actinolite gives the clockwise P-T evolutional history,Those retrograde path also can indicate the exhumation history after the peak collision.The 144-146Ma ⁴⁰Ar/³⁹Ar isochronal ages of phengites in blueschist from Yilan area were obtained,indicating one metamorphic thermal event.Base on the investigation of the field,that the foliation of the blueschist is accordant with the occurrence of the Jiamusi-Yilan strike-slip fault.Thereforeâ… suggest this metamorphic thermal event is affected by the activity of the fault.Combining the discussion above with the previous works,a simple tectonic evolutional model of the Heilongjiang Complex and the Jiamusi Massif can be established,follows as:～500Ma ages from the basement of the Jiamusi Massif, indicate that the Jiamusi Massif belong to the Gandwanaland,after separated from Gandwana,it drifted northward;185-270Ma,the collision of oceanic block and continental block happened,and were accompanied by the a great mount of granite intrusions;185-174Ma,the conversion of collision from oceanic-continental to continental-continental occurred,and the principal part of Heilongjiang complex formed;154-145Ma,one metamorphic thermal event affected by the activity of the Jiamusi-Yilan Sticke-Slip Fault.In short,the Heilongjiang complex,especially the blueschists,was formed by the collision of the Jiamusi Massif and the Songnen Massif.The research of the blueschists makes an important role for discuss the evolution of not only the Heilongjiang Complex,but also the Jiamusi Massif and the whole NE China.