U-Pb Geochronology And Lu-Hf Isotopic Geochemistry Of Detrital Zircons From Modern Rivers In The Kongling Terrane,yangtze Craton

The Neoarchean is one of the most important periods for studying the formation of the Earth's early continental crust,when the dynamic mechanism of the Earth's interior might get changed,and which is one of the hot topic in Precambrian research.Thus far,the corresponding records of the ~2.7-2.6 Ga magmatism and metamorphism have been widely documented in many Archean cratons worldwide.However,as limited by the heavy vegetation covering and uncommon Archean outcrops,the distribution of ~2.7-2.6 Ga magmatism in Yangtze craton and its geological significance is poorly understood.Recently,Zircon U-Pb and Lu-Hf isotopic analysis technologies have greatly matured and get widely applied,thus providing a vast amount of data for studying the Precambrian history of the Yangtze craton.On that basis,this thesis studies the U-Pb geochronology and Lu-Hf isotopic geochemistry on detrital zircons from three rivers(Gongjia River,Wudu River,and Bianyuchi Stream)crossing the northern part of the Kongling Terrane,which is the Archean nucleus of the Yangtze craton,preserves Paleoarchean-Proterozoic rocks as old as 3.45 Ga.In this study,323 U-Pb and 190 Lu-Hf isotopic data are obtained from those three samples.And the detrital zircons from different local rivers share similar age distributions.Together,they yield age peaks at 3.3-3.1 Ga,3.0-2.8 Ga,2.7-2.6 Ga,2.6-2.2 Ga,2.0-1.9 Ga,1.9-1.7 Ga,1.7-1.5 Ga,and 1.0-0.8 Ga,which account for 5%,18%,30%,15%,27%,1%,2%,and 1% among the total concordnt zircons,respectively.Most zircons show linear or oscillatory zoning patterns in cathodoluminescence(CL)images and have high Th/U ratios(>0.3),indicating magmatic origins.And yet some 2.0-1.9 Ga zircons exhibit sector or structureless zoning in CL images,low Th/U ratios(<0.1),and similar initial Hf isotopic compositions as those of the 2.7-2.6 Ga zircons,suggesting a metamorphic recrystallization origin.Besides,the U-Pb and Lu-Hf isotopic characteristics of our detrital zircons are roughly consistent with local granitoid records,thus which could effectively reflect the formation and evolution history of the Kongling Terrane.The above age distribution implies that the North Kongling Terrane formed primarily during Meso-to Neoarchean(3.0-2.6 Ga).Most Mesoarchean zircons(3.0-2.8 Ga)exhibit predominant subchondritic to chondritic ?Hf(t)values(-11.3 to 0),suggesting the dominant role of crustal reworking in its parental magma.In comparison,most Neoarchean zircons(43%)have near chondritic to suprachondritic values(up to +7.4),indicating significant juvenile crustal addtions during this period.The high proportion of 2.7-2.6 Ga detrital zircons in modern riversands(this study),is consistent with those previous detrital zircon data from the local metasedimentary rocks and the peripheral sedimentary strata.Combined with the predominant chondritic to suprachondritic ?Hf(t)values of 2.7-2.6 Ga zircons,we propose that the 2.7-2.6 Ga magmatism may have played a dominant role in forming the North Kongling Terrane.In addition,previous studies have reported several 2.7-2.6 Ga Archean outcrops,minor detrital zircons in Precambrian sedimentary rocks and xenocrystal zircons in volcanic rocks from the Yangtze craton,it seems that the 2.7-2.6 Ga tectono-thermal events may have widely affected the entire Yangtze craton,similar to many other Archean cratons worldwide.And the 2.7-2.6 Ga may correspond to the initial stabilization process of the Archean nucleus of the Yangtze craton.Thereafter,the Yangtze craton experienced a series of tectono-thermal events from Neoarchean to Paleoproterozoic,which might be related to the formation and fragmentation of the Sclavia/Nunavutia supercraton(2.6-2.2 Ga)and the Columbia supercontinent(2.1-1.5 Ga).