The Nature Of Late Archean To Paleoproterozoic Basement In The Northern Yangtze And Its Geological Implication

Late Achean to Paleoproterozoic?2.8-1.6 Ga?witness pronounced transition of the composition of earth's continental crust,and accompanly with the cratonazation processes,while the Paleoproterozoic period is an important stage for the assembly of the supercontinent Columbia.The North China Block,Tarim Block and South China Block are the three lagerest blocks in China;among them,the South China Block can be divided into Yangtze Block and Cathaysia Block.Due to the sporadic outcrups of Achean to Paleoproterozoic basement rocks in the Yangtze Block,its early evolution history has not been well constrainted.In this study,we studied the late Archean to Paleoproterozoic rocks from the Zhongxiang,Yudongzi and Tongbai orogen in the northern Yangtze Block,with detail petrology,whole-rock geochemistry,zircon/baddeleyite U-Pb dating and Hf-O isotope analyses.The main achievements are as follows:?1?U–Pb dating of magmatic zircons from Huji pluton in Zhongxiang area yielded a weighted mean age of 2656±6 Ma,representing the timing of magmatic emplacement.The granite samples show relatively evaluated?K₂O+Na₂O?/CaO,FeOt/MgO ratios,and Zr,Nb,Ce,Ga and Y contents,sharing the same features as the A-type granite.They have pronounced Eu,Sr and Ba negative anomalies on the primitive mantle-normalized trace element spidergram,implying the residual of plagioclase in the magma sources or fractional crystallization phases.Their high Zr contents of 440-529 ppm correspond to initial magma temperatures of 887-910oC.The zircons have negative?_Hf?t?values of–2.7 to 0.1,and Hf model ages of ca.3.2 Ga,respectively.The?¹⁸O values of the zircons range between 4.94 and 5.72‰and give an average of 5.26±0.14‰,suggesting no contribution of supracrustal rock into the host magma.Therefore,the Huji granite was generated by reworking of ca.3.2 Ga juvenile crustal rocks at relatively high temperatures and low pressures.The occurrence of the ca.2.65 Ga Huji granites likely records the thermally stable lithosphere and thus heralds the craton stabilization of the northeastern part of the Yangtze Block.?2?The Huashanguan complex in Zhongxiang area is a granite complex comprised of monzogranite in the marginal facies and K-feldspar granite in the central facies.U–Pb dating of magmatic zircons from the monzogranite and K-feldspar granite yielded ages of 1816±50 Ma and 1843±9 Ma respectively.The granites have uniform negative zircon?_Hf?t?values of-20.2 to-15.8,with corresponding T_DM2 ages of ca.3.6-3.5 Ga.The?¹⁸O values average at 6.80±0.09‰.These indicate that the granites were generated by partly melting of Archean rocks.Both the monzogranite and K-feldspar granite have A-type granite affinities,while the monzogranite display lower FeOt and MgO contents and evolved trace element features,suggesting their formation by more fractional crystallization from similar parent magma of the K-feldspar granites.The occurrence of the Huashanguan granite complex indicates that the Yangtze Block has experienced a tectonic transition from tectonic convergence to extension at ca.1.85 Ga,possibly due to the collapse of the ca.2.0 Ga orogeny.?3?The granitic gneisses in the Yudongzi complex have the TTG affinity characterized by relatively high Na₂O/K₂O and Sr/Y ratios,and fractionated REE pattern with relatively high LREE but low HREE concentrations.Zircon grains in the granitic gneisses show magmatic cores and metamorphic rims.U-Pb dating on zircon core of four granitic gneiss samples yielded their formation ages of 2660-2632 Ma with a weighted average of 2650±21 Ma?MSWD=1.6;n=4?.Their average?_Hf?t?values range from-0.6 to 0.5 and two-stage model ages from 3.2 to 3.1 Ga,suggesting that the granitic gneisses were likely generated by remelting of thickened Mesoarchean proto-mafic crust.The metamorphic zircon rims have relatively high Th/U ratios and identical Hf isotope compositions to their corresponding cores,indicating that the rims were formed via solid-state recrystallization process.The metamorphic zircons recorded a metamorphic event at ca.2.5 Ga.U-Pb dating and Lu-Hf isotopes on magmatic zircons from a K-feldspar sample revealed a magmatic event at 2477±18 Ma.The recognization of the 2.65 Ga TTG magmatism and the 2.48 Ga intercrustal remelting event in the Yudongzi complex demonstrates a distinct crustal evolution history compare with the Paleoarchean continent nucleus in the eastern part of the Yangtze Block,suggesting that the continent nucleus might have subjected to lateral growth by accretion processes.?4?The SIMS dating of magmatic zircon from amphibolite of Tongbai orogen yields an age of 1869±30 Ma,representing the emplacement age of the mafic magma.The SIMS dating of the zircon and baddeleyite from the host rock marble yields ages of1840±13 Ma and 1848±3 Ma,representing the formation ages.Their relatively low trace element contents,unfractionated REE patterns and indentical Hf isotope compositions indicate they are formed by contact metamorphism.The zircons in marble have high?¹⁸O values with a weighted average of 20.37±0.13‰,which might be generated by detrital zircon dissolution reaction and a subsequent transition from baddeleyite into zircon.These data indicate that a part of the sedimentary rocks in Tongbai orogen were deposited before 1.85 Ga,and intruded by the 1.85 Ga mafic magma,which is consistent with the geological records of the Kongling area.?5?Comparing the timing and nature of magmatism in each basement in Yangtze Block,we propose that the Kongling area represents the early continental nuclear of Yangtze block that had went through crustal growth of>3.6 Ga and inter-crustal evolution during 3.44-3.20 Ga;The Zhongxiang area might have been collage to the Kongling nuclear before 2.7-2.6 Ga and shear a similar crustal evolution history since then,representing the initial cratonization of the eastern Yangtze Block;The western part of the Yangtze Block have a distinct crustal growth and reworking periods with the eastern continental nuclear,and represents the later accretion to the northeastern nuclear at Paleoproterozoic.According to the records of the Paleoproterozoic orogen in the Yangtze Block and the nature of basement rocks,we infer that the Yangtze Block might be linked with Sibera and Laurentia continents in the supercontinent Columbia.The Yangtze Block was extensively intruded by the 1.85 Ga bimodal magmatic rocks,indicated the Yangtze Block might have accomplished the final cratonization.