Geochemistry Of The Jinpan Complex In The Northern Margin Of The Yangtze Craton

Studying the formation and evolution of the Precambrian basement of ancient cratons is an important way to understand the formation and evolution of continental crust,the origin of plate tectonics,the assembly and breakup of supercontinent.In this thesis,the magmatic rocks from the Jinpan complex in the northern Yangtze Block were analyzed for zircon U-Pb ages and Lu-Hf isotopes,whole-rock major and trace element compositions,Sr-Nd-Hf isotopes to reveal their source materials and the petrogenesis.By combining the results with previous data obtained from coeval igneous rocks in other areas of the northern Yangtze Block,how the Yangtze Craton response to the assembly of the Columbia supercontinent and its position in supercontinent is discussed.The 2.5 Ga magmatism in the Jinpan complex occurred as monzogranite and trondhjemite.Zircon U-Pb ages of the monzogranite are from 2460±20 Ma to 2480±11 Ma.They have high SiO2 contents(62.20?69.61 wt.%),high K2O contents(2.84?3.09 wt.%),high K2O/Na2O ratios(0.83?0.91),and show enrichment in Ba,Th,Zr,Hf,K,depletion in P,Ti,Nb,Ta.The zircon ?Hf(t)values are from-4.03 to 0.63,which is the result of partial melting of ancient continental crust materials,and there may be a small contribution from juvenile materials.The zircon U-Pb age of the coeval trondhjemite is 2456±17 Ma.The trondhjemite has high SiO2 content(78.30 wt.%)and Na2O(5.47 wt.%),low content of K2O(1.32 wt.%)and MgO(0.27 wt.%)with low K2O/Na2O ratio(0.24).It has a negative zircon ?Hf(t)value(-2.73±0.30).And the whole-rock ?Nd(t)and ?Hf(t)values are-1.5 and-0.3.It has a highly fractionated REE pattern with significant Eu positive anomaly.Whole rock(La/Yb)N value is 33.7 and Sr/Y value is 71.1.Its major and trace element compositions are similar to those of the mid-pressure TTG suite.It is speculated that the 2.5 Ga trondhjemite was formed by partial melting of ancient crust under pressure greater than 1.2 GPa with the presence of garnet but basically absence of plagioclase in the residual.This episode of magmatism at the Archean/Paleoproterozoic boundary marks a period of crustal thickening,possibly related to the convergent process.The middle Paleoproterozoic magmatism of the Jinpan complex consists of monzogranite,trondhjemite and mafic veins.Zircon U-Pb ages of the monzogranites range from 1999±17 Ma to 2013±26 Ma.They have high SiO2 contents(71.69?72.82 wt.%)and K2O contents(4.29?4.37 wt.%),low Na2O contents(2.24?2.72 wt.%).They are enriched in Ba,Th,Zr,Hf,K,depleted in P,Ti,Nb,Ta,and have obvious negative Eu anomalies.They show negative zircon ?Hf(t)values of-10.0 to-7.1.And the whole-rock ?Nd(t)and ?Hf(t)values of-6.4 to-4.9 and-8.8 to 3.8 respectively,which is the result of partial melting of ancient continental crust materials,and there is crystallization effect in the process.Zircon U-Pb dating of trondhjemite yields age of 2028 ± 23 Ma.The trondhjemites have high SiO2 contents(75,19?77.11 wt.%)and Na2O contents(5.19?5.96 wt.%),low K2O contents(0.37?1.65 wt.%)with low K2O/Na2O ratio(0.06-0.31).In addition,they show significant depletion in Nb,Ta,Ti,P,and enrichment in Ba,U,Pb,with positive Eu anomalies.Whole rock(La/Yb)N value of 73.2 to 122.7 and Sr/Y value of 19.2 to 151.5.And the whole-rock ?Nd(t)and ?Hf(t)values are-8.2 to-6.3 and-8.4 to-5.9.Their major and trace element compositions resemble those of the mid-low pressure TTG.Therefore,their petrogenesis is ascribed to the partial melting of ancient thickened mafic crust,with residual mineral assemblages contains garnet,but basically no plagioclase or amphibole.This episode of crustal thickening is related to the widespread 2.0-1,9 Ga subduction and accretion process in the northern Yangtze Block.The zircon U-Pb age of the mafic vein is also around 2.0 Ga.They have low SiO2 contents(46.47?55.25 wt%),variable K2O/Na2O ratios,and Mg#of 31.52-49.65.The 2.0 Ga mafic veins are enriched in light rare-earth element and large-ion lithophile elements,but depleted in HREE and HFSE.Zircons from the mafic vein display negative ?Hf(t)values of about-6.0.There are some 2.5 Ga zircons in the mafic veins,likely captured from the country rocks.Geochemical signatures suggest that the mafic veins were derived from an enriched mantle source and underwent various degrees of crustal contamination during upwelling and emplacement.And the tectonic environment is related to the extension.Combined with previous data,two phases of crustal thickening are identified in the northern Yangtze block,one at 2.5 Ga and another at 2.0 Ga.The mid-Paleoproterozoic crustal thickening event is related to subduction and collision which is related to supercontinent Columbia assembly.