| This thesis deals with the supracrustal rocks of the Mashan Complex,as well as the orthogneisses and various types of intrusive rocks that intrude the supracrustal rocks from the Linkou-Boli and Mishan areas in the Jiamusi Block.Systematic studies on petrology,zircon LA-ICP-MS and SIMS geochronology,whole-rock geochemistry,and zircon Hf isotope obtain four aspects of achievements,i.e.,1)testify the existence of the Neoproterozoic magmatism,and further ascertain its stage,association and petrogensis;2)limit the peak and retrograde timing of the granulite facies metamorphism,as well as the depositional age of the supracrustal rocks within the Mashan Complex;3)determine the formation mechanism of the early Paleozoic magmatism and its relationship with the Late Pan-African high-grade metamorphism;and 4)discuss the tectonic affinity between the Jiamusi Block and the Kuunga orogenic belt of East Gondwana,and accordingly propose a preliminary tectonic-magmatic evolution model for the Jiamusi Block.Zircon U-Pb dating for the orthogneisses from the Mishan and Linkou-Boli areas yields protolith ages of 898~891Ma and 757~751Ma,respectively,this is the first evidence to show that at least two stages of Neoproterozoic magmatism did occur in the Jiamusi Block.The ~895Ma magmatism is mainly composed of syenogranites,they display geochemical features of high silicon and potassium,peraluminous and low Sr/Y with ?Hf(t)values of-4.2~+0.9(mainly negative),suggesting that the ~895Ma syenogranites could be derived from a ancient crustal source possibly with high maturity,coupled with wall-rock(juvenile crustal material)assimilation during magma evolution.This stage of syenogranites was emplaced in a subduction-related setting that was undergoing transition from compression to extension,probably corresponding to the accretion of Rodinia.The ~752Ma magmatism mainly consists a suite of monzogranites,syenogranites and alkali feldspar granites,they display similar characteristics to A-type granites with zircon saturation temperature of 788~845°C and ?Hf(t)values of-2.8~+4.5(mainly positive),which,together with the linear correlation between the major/trace elements with SiO2,suggests that the ~752Ma granitic magma was probably derived from dominantly juvenile crustal rocks with both the heat and material involvement of a depleted asthenosphere mantle at relatively low pressures,which then underwent fractional crystallization and ancient wall-rock assimilation.The ~752Ma granites belong to high-fractionated I-type granites,they together with the ~757Ma meta-basalts of the Khanka Block display a bimodal igneous association,and most likely formed within an rift-related extensional environment,perhaps corresponding to the break-up of Rodinia.The detrital zircons from the paragneisses of the Mashan Complex in the Mishan area yield age spectra of 2548~1064Ma,combined with the geological fact that the ~895Ma orthogneisses intrude those paragneisses,indicating that the protolith of the supracrustal rocks within the Mashan Complex deposited during the 1064~898Ma.In addition,the metamorphic zircons from the ortho-and paragneisses yield concordant ages of 568~476Ma.According to zircon genetic types and field observations,these metamorphic ages can be divided into two groups: 1)569~560Ma(~563Ma),this group was analyzed from zircon domains formed by solid-state recrystallization and replacement processes,and their isotopic systems basically have not been disturbed by late thermal events,implying that ~563Ma ages can represent the timing of peak P-T conditions of the granulite facies metamorphism;and 2)546~476Ma,this group was analyzed from zircon domains formed by coupled dissolution-reprecipitation,solidus growth,subsolidus growth,reorganization,and replacement processes,corresponding to those retrograde metamorphism stages such as metamict,melting and generation of migmatites,granites,etc,indicating that the ages of 546~476Ma reflect the timing of retrograde metamorphism.After the peak stage of the granulite facies metamorphism,a large-scale early Paleozoic magmatism with ages of 541~476Ma(duration: ~65Ma)occurred in the Jiamusi Block.The early Paleozoic intrusive rocks consist of ~500Ma gabbros,~515Ma quartz syenites,and 541~476Ma granodiorites,monzogranites,and syenogranites.The ~500Ma gabbros were generated from the partial melting of an enriched mantle source that had been metasomatized by slab-derived fluids,with insignificant influence of contamination by continental materials.The 541~476Ma granitoids(including the ~515Ma quartz syenites)are normal or high-fractionated I-type granites,and display variable geochemical characteristics,suggesting multiple sources in their origin,including juvenile crust,ancient crust,and depleted mantle.They were mainly from partial melting of crustal rocks under relatively low pressure conditions,coupled with the involvement of depleted mantle-derived magma as well as wall-rock assimilation.Based on rock association,geochemical feature,and P-T path of the granulite facies metamorphism,it is concluded that the 541~476Ma intrusive rocks within the Jiamusi Block were emplaced in a post-orogenic extensional setting that resulted from the collapse of a late Pan-African global orogen after the near-isothermal decompression metamorphic stage.The good correlation on the 755~115Ma magmatic and metamorphic events between the Jiamusi and Khanka blocks suggest they formed a contiguous crustal unit,and the Mesozoic Dunhua–Mishan Fault cannot be a boundary fault to separate two Precambrian blocks.Taking into account of the above research results,published regional geological data,and global correlations,it is inferred that the combined Jiamusi-Khanka Block belong to an exotic terrane of the Central Asian Orogenic Belt(CAOB),and can not be directly derived from the Siberia,North China,and South China cratons.Instead,it might be either a rifted portion of the Sayang-Baikal orogen or derived from a peri-Gondwana position(i.e.,the Kuunga orogen).Considering the finding that in many aspects,including tectonic setting of Neoproterozoic igneous rocks,early Paleozoic metamorphic-magmatic mechanism,as well as age and source of supracrustal rocks,the Jiamusi-Khanka Block show perfect correlations with East Antarctica and West Australia,it is thus proposed here that the Jiamusi-Khanka Block may have originated as part of the Kuunga orogen within East Gondwana during the Late Pan-African period.In details,during the 1000~750Ma,the Jiamusi-Khanka Block along with East Antarctica-Australia witnessed the assembly,growth and break-up of Rodinia.During the 570~470Ma,the Jiamusi-Khanka Block experienced the whole evolution processes(including collision,rapid uplift and collapse)of the Late Pan-African Kuunga orogen.Subsequently,at sometime between ca.470 and 433 Ma,the Jiamusi-Khanka Bock rifted away from East Gondwana and began its northward journey towards what is now NE China.The final docking with the Songliao Block occurred during the 210~185Ma as a result of the Pacific-plate subduction.Ultimately,the Jiamusi-Khanka Bock forms part of the easternmost segment of the CAOB. |
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