U-Pb Detrital Zircon Geochronology、Hf Isotope Analysis Of The Floodplain Sediments In Nen River And Their Geological Implications

The formation and evolution of continental crust have become one of the forefront areas forgeoscientists since1980s, which are mainly based on the age and distribution of the rocks,howerver, the early crust suffered post-transformation and erosion, and many of which have beennon-existent. The clastic sediments and sedimentary rocks, which are from the weathering,erosion, transport and deposition of the outcropping rock, have become a ideal research object ofcontinental crust formation, evolution and chemical composition.Daxinanling locates in eastern Xing-Meng orogenic belt, and is mainly composed of Ergunblock, Xing’an block and Songnen block, part of which is in Jiamusi block. The region hasalways been a key research region for Xingmeng orogenic belt. The detrital zircon U-Pb ages canbe used to study the magmatic events of eastern Daxinanling and western Xiaoxing’anling; Hfisotope data can be used to analyse the information of juvenile crust and old crust remodeled inthe magmatic events, and also can be used to estimate the generation time of juvenile crust;combined with the Nd isotope and trace elements, the basic composition characteristics, theformation of historical evolution and geochemical significance of regional crust will be acquired.The conclusions are as follows:1) The detrital zircon U-Pb ages of Nen River floodplain sediments are mainly divided into80~225Ma (70%),250~494Ma and1746~2300Ma, plus one Neoproterozoic Age (718Ma)and two late Archean ages (2611±6Ma,2776±6Ma respectively). The substances of theMesozoic detrital zircons provide the primary provenance.2) The two old ages of detrital zircons of South urn River and Naduli River (1798Ma,1799Ma respectively), and the two old ages of Gan River (1909Ma,1991Ma respectively) indicatethat there are Precambrian basement rocks in Ergun block and Xing’an block. The three~2300Ma deep samples from Nen River may all come from the western Xiaoxing’anling or SongliaoBasin.3) The floodplain sediments of Nen River depleted HFSE (such as Nb, Ta) apparently andenriched LILE (such as Cs, Rb, Nd, La) slightly. Over the past century, human activities andmining development did not have great impact on the composition of crust.4) The zircons of U-Pb ages in1746~2300Ma, their two-stage Hf isotope model agesindicate that there a mixing between accretion crust and old crust in Mesoarchean-Neoarchean;The zircons of U-Pb ages in494~718Ma, their two-stage Hf isotope model ages indicate thatthere a mixing of old crust between late Paleoproterozoic and early Mesoproterozoic;108~339Ma zircons, their two-stage Hf isotope model ages indicate that there a accretion event of juvenilcrust between Neoproterozoic and early Archean.5) The cumulative percent of the detrital zircons ages between1039~550Ma has reached71%, indicating that the crust accretion of Nen River source region mainly occurred inNeoproterozoic, which experienced a small-scale crustal accretion event again in414Ma.6) Through the Nen river Nd model ages distribution characteristics, the proterozoic to thehyperplasia of neoproterozoic material in Nen river floodplain deposits to provide main source ofmaterial, the material may come from the young crust mantle or depleted mantle.