Convergence History Of The Jiamusi And Songnen-Zhangguangcai Range Massifs:Geochronological And Geochemical Evidence

This thesis presents U-Pb zircon and rutile ages,whole-rock major and trace element data,and Hf isotope data for the Heilongjiang Complex and related intrusions along the boundary area of the Jiamusi and Songnen-Zhangguangcai Range massifs,which will not only place important constraints on the rock-forming ages,source characteristics and tectonic setting,but will also provide insights into understanding the affinities and tectonic evolution between the Jiamusi and the Songnen-Zhangguangcai Range massifs.The Heilongjiang Complex lies between the Jiamusi and Songnen-Zhangguangcai Range massifs,extending from Luobei-jiayin in the north,through Yilan-Huanan to Mudanjiang in the south.It is composed predominately of blueschist,amphibolite,greenschist,serpentinite,mica schist?quartz schist?,metasilicalite and marble.For the metasedimentary rocks from the Heilongjiang Complex in Yilan-Huanan counties,most detrital zircons show“magmatic-like”zircon rare earth element patterns,with enrichment in heavy rare earth elements and positive Ce and negative Eu anomalies.Zircon U-Pb analysis indicates that the youngest detrital zircons in these metasedimentary rocks have concordant ²⁰⁶Pb/²³⁸U ages of ca.230¹80 Ma.Combined with previous studies and the Early-Middle Jurassic metamorphic ages of the Heilongjiang Complex,we conclude that the metasedimentary rocks from the Heilongjiang Complex were deposited during the Late Triassic-Early Jurassic.Approximately 55%of the detrital zircons yield ages of 297to 180 Ma,with peaks at 270²45 Ma,230²00 Ma and 200¹80 Ma,while 27%of the detrital zircons yield ages of 544⁴12 Ma with a peak at 500⁴80 Ma.Proterozoic detrital zircons?¹5%?are also identified,yielding ages of 2352 to 595 Ma,with most being of Neoproterozoic age.These results,combined with field relationships and previously published ages,suggest that the detrital zircons were sourced mainly from igneous rocks in the Jiamusi and Songnen-Zhangguangcai Range massifs,and the protoliths of the metasedimentary rocks from the Heilongjiang Complex were most likely deposited in a forearc basin setting.In addition,the amphibolites from the Heilongjiang Complex in Yilan county yield ²⁰⁶Pb/²³⁸U age of 274±2 Ma,interpreted as protolithic age.The dating of rutile of the amphibolites resulted in ages of 177±11 Ma and 172±5 Ma,respectively,which may reflect the time when the rocks cooled down to the closure temperature of Pb in rutile.These amphibolites all exhibit the geochemical characteristics of tholeiitic series rocks,and are characterized by depletion in Nb and Ta,showing arc affinities.The studied amphibolites from the Yilan Marble Quarry,Longlangang and Tuanshanzi areas were formed by similar petrogenetic mechanism.Their magmas were derived from partial melting of mantle wedges metasomatized by subducted slab fluids,but the addition of sediments might have played an important role in generating magma of the amphibolites from the Yilan Marble Quarry.But they evolved from distinct magma sources.A garnet-to spinel-peridotite transition is considered as a possible source for the amphibolites from the Yilan Marble Quarry,and spinel-peridotites may be possible magma sources for amphibolites from the Longlangang and Tuanshanzi areas.These data,combined with previous studies on the Heilongjiang Complex,suggest that the Heilongjiang Complex belongs to subduction accretionary complex,and the protolith formation ages of the Heilongjiang Complex were mainly Late Paleozoic-Early Mesozoic.Moreover,the Heilongjiang Complex underwent epidote amphibolite-blueschist-greenschist facies metamorphism,and the peak metamorphism took place during the Early-Middle Jurassic.The basic and acidic intrusions along the boundary area of the Jiamusi and Songnen-Zhangguangcai Range massifs also give further insights into the complicated convergence history of the Jiamusi and Songnen-Zhangguangcai Range massifs.Zircon U-Pb dating indicates that the magmatic zircons from the Zhushan and Taiping plutons yield ²⁰⁶Pb/²³⁸U ages of 256±2 Ma and 259±3 Ma,respectively,interpreted as the emplacement ages of the basic intrusions.The metagabbros from the Zhushan pluton display the transitional characteristics of tholeiitic to calc-alkaline series rocks,and are enriched in light rare earth and large ion lithophile elements,and depleted in Nb,Ta,P,Zr and Hf.The _Hf?t?values of magmatic zircons in these metagabbros vary from–5.47 to+0.74.All these geochemical features indicate that the magma of the Zhushan pluton was derived from an enriched lithospheric mantle wedge that was metasomatized by subducted slab-derived fluids,and a garnet-to spinel-peridotite transition is considered as a possible source.The gabbros from the Taiping pluton have similar geochemical characteristics with those in the Zhushan pluton,characterized by enrichment in large ion lithophile elements and depletion in Nb-Ta-P,with _Hf?t?values of–4.02 to–1.70,suggesting that they have similar petrogenetic processes.But combined with geochemical and isotopic compositions,as well as the related studies,the magmas from the Zhushan and Taiping plutons are likely to be derived from two distinct magma sources.In addition,the acidic intrusions?Mingyi,Tuoyaozi,Mengjiagang,Hengtoushan,and Qingbei plutons?are composed of syengranites,monzogranites,and granodiorites,and they were emplaced in the Early-Middle Permian?278²63 Ma?.These acidic plutons are mostly high-K calc-alkaline and weakly peraluminous,and show consistent correlations of different oxides versus SiO₂.They are all enriched in large ion lithophile elements and light rare earth elements,and depleted in high field strength elements?e.g.,Nb,Ta,Ti?and heavy rare earth elements.And they have relatively homogeneous Hf isotopic compositions,with _Hf?t?values varying from–6.16 to+2.95 and two-stage model ages ranging from 1681 to 1111 Ma.Based on their geochronological and geochemical characteristics,and Hf isotopic compositions,we conclude that these granitoids might be originated from parental magmas with similar compositions but evolved different degrees of fractionation,and their magmas were derived from the partial melting of amphibolite-facies mafic lower crust.Taken together,we conclude that the generation of the Permian basic and acidic intrusions along the boundary area of the Jiamusi and Songnen-Zhangguangcai Range massifs was most likely related to the subduction.The eastwards subduction of the Mudanjiang oceanic plate beneath the Jiamusi Massif induced crustal melting to produce the studied Permian acidic intrusions,as well as the generation of the Taiping pluton.Furthermore,the westwards subduction of the Mudanjiang oceanic plate beneath the Songnen-Zhangguangcai Range Massif gave rise to Permian magmatism along eastern margin of the Songnen-Zhangguangcai Range Massif,including our studied Zhushan pluton.These data,combined with previous studies on the Heilongjiang Complex and contemporaneous magma-tectonic activities in the Jiamusi Massif and Songnen-Zhangguangcai Range Massif,indicate that two paralleled N-S trending Permian magmatic belts are distributed in these two massifs,which were associated with the evolution of the Mudanjiang Ocean.Taken together,we suggest that a double-side subduction model is favored for the tectonic evolution of the Mudanjiang Ocean,which can be summarized as follows:?1?Prior to 250 Ma,the Mudanjiang Ocean existed between the Jiamusi and Songnen-Zhangguangcai Range massifs.The double-side subduction evolution involved the development of protoliths of the Heilongjiang Complex and magmatic arcs on both the active continental margins.?2?During 250²30 Ma,continued double-side subduction resulted in the generation of younger Triassic arc rocks on the active continental margins of the Jiamusi and Songnen-Zhangguangcai Range massifs,as well as protoliths of the Heilongjiang Complex.?3?During 230¹80 Ma,double-side subduction beneath the Jiamusi and Songnen-Zhangguangcai Range massifs continued,forming the youngest arc rocks.The metasedimentary rocks from the Heilongjiang Complex were deposited in fore-arc basins during the continuous double-side subduction.?4?Continued convergence of the Jiamusi and Songnen-Zhangguangcai Range massifs led to closure of the Mudanjiang Ocean sometime between 180 and 160 Ma.Subsequent collision-accretion resulted in metamorphism and deformation of the Heilongjiang Complex.