Continental Crust Growth And Evolution In Southern China

The formation and evolution of the continental crust is one of the most basic issues of geosciences.To investigate the crustal growth of the South China,detrital zircons of nine sand samples from upper,middle and lower streams of the Yangtze River?the world's third largest river?and its two largest tributaries,the Han and Jialing rivers were analyzed for U�Pb�Lu�Hf�O isotope compositions.Based on here present and previously reported data,the works and conclusions are listed in the following:?1?We systematically report the age distribution of detrital zircons from the Yangtze River basin.Detrital zircons from the upper Yangtze River show 6 main age groups: 0�100 Ma,200�300 Ma,400�500 Ma,700�1000 Ma,1800�1900 Ma and 2300�2500 Ma,with a striking peak at 41 Ma diagnostic of magmatism on the Tibetan Plateau,whereas those from the middle and lower Yangtze River and its tributaries exhibit broadly similar age groups at 100�300 Ma,400�500 Ma,700�900 Ma,1800�2000 Ma and 2400�2500 Ma,except for the lack of Cenozoic ages.?2?Quantitatively calculate the difference between model ages based on the depleted mantle and arc mantle models using zircon Hf isotope composition.We derived theoretical equations for relationships and differences between the two types of crust formation ages.Our data show excellent agreement with the theoretical derivations.They indicate linear relationships between the two types of ages and their differences ranging from ?160 Ma to ?300 Ma for our data set,depending on zircon crystallization age,initial Hf isotopic ratio and 176Lu/177Hf ratio of the crustal source.?3?Contraints on the nature of crustal sources of detrital zircons by zircons' Hf and O isotope compositions.Our zircons with mantle-like oxygen isotope composition(?¹⁸O = 4.7�6.0 �)show five linear arrays in a diagram of ?Hf?t?against crystallization ages,which intercept the depleted mantle evolution line at ?1.3 Ga,?1.35 Ga,?2.1 Ga,3.1 Ga and 4.0 Ga,respectively.Four arrays give 176Lu/177 Hf ratios ranging from 0.020 to 0.025,which indicate mafic crust sources.The 1.35 Ga array yields a 176Lu/177 Hf ratio of 0.016,suggesting a more evolved crustal source.These arrays intercept the depleted mantle evolution line,which are ?200 Ma older than those with the arc mantle evolution line.Magmatic zircons from the currently known oldest 3.45 Ga rocks and the oldest 3.5�3.8 Ga detrital zircons in South China plot along the array with the lowest ?Hf?t?.This array suggests that the oldest rocks that ever existed in South China would be no older than 4.0 Ga.?4?Our study reveals step-like crustal growth and its relationship with geodynamics.The Yangtze River catchment is covered by thick Phanerozoic sedimentary rocks and so relative to the outcrop volume of Archean rock,Archeanaged zircons would be under-represented in modern river sands.To overcome this problem,we apply the approach of Dhuime et al.?2012?to calculate crustal growth curve for South China.The resultant curve shows two inflections.The older inflection at ?2.8 Ga is slightly younger than that of Dhuime et al.?2012?,suggesting the onset of plate tectonic might be not simultaneous in different cratons.The younger inflection at ?1.8 Ga appears to mark the onset of another period of relatively high crustal growth rate similar to that before 2.8 Ga.This high growth rate in the Mesoproterozoic is well coeval with the assembly and breakup of the supercontinent Columbia,which may suggest that juvenile magmatic input was enhanced during extensional rifting and supercontinent dispersal in the Yangtze Craton.Comparison with other crustal growth curves based on worldwide detrital zircons indicates that those without constraints from oxygen isotopes tend to result in more smoothed out growth patterns.In contrast,the other detrital zircon-based models combining oxygen isotopes show more or less step-like patterns.We infer that the former smoothed out crustal growth patterns result from the effects of hybridization of material of different ages in supracrustal zircons.We propose that crustal growth is controlled by episodic formation of new crust from the mantle,caused by changes in geodynamics,leading to marked changes in rate and step-like patterns.?5?The relationships of the maximum ?¹⁸O values of detrital zircons with the atmospheric oxygen level and the breakup of the supercontinents.The variation of maximum ?¹⁸O values in our zircon well follows the estimated variations of atmospheric oxygen levels.We therefore suggest the atmospheric O₂ levels might link to the increasing incorporation of high ?¹⁸O chemical sediments in crustal source,which,consequently,raised the ?¹⁸O in zircons from the Late Archean onwards.Zircons from the periods of the supercontinents Superia,Nuna,Rodinia and Pangea have similar peaks in ?¹⁸O,and these peaks are followed by decreases of ?¹⁸O values.We interpret such decrease to signify breakup of the supercontinents.