Late Paleoproterozoic Tectonic Evolution In The Middle/eastern Part Of The North China Craton:Evidence From A-type Granites And Plagiogranites

In this thesis,A-type granites in Songxian-Ruyang region in Trans-North China Orogen(TNCO),southern margin of the North China Craton(NCC),A-type granites in Huanren region and plagiogranites in Gaoliduntai region in the Jiao-Liao-Ji belt(JLJB)of the eastern NCC are selected for research.Based on the systematic petrology,petrography,geochronology,geochemistry and whole-rock Sr-Nd-Pb isotopic,apatite Nd and zircon Hf-O isotopic analyses,as well as plagioclase mineral chemistry,this thesis defines the formation ages,petrogenesis and tectonic implications of the Late Paleoproterozoic granites.Combined with previous researches,the Late Paleoproterozoic tectonic evolution in the middle/eastern part of the NCC can be further constrained.The main achievements are as follows.1.The geochronological framework of the Late Paleoproterozoic magmatism in the middle/eastern part of the NCCThe LA-ICP-MS zircon U-Pb dating results indicate that the Songxian-Ruyang granite-porphyries in the southern NCC were emplaced at 1813?1789 Ma.Zircon U-Pb dating results for Huanren porphyry monzogranites yield 207Pb/206Pb ages of 1863?1842 Ma.The Gaoliduntai plagiogranites yielded zircon U-Pb ages of 1853?1802 Ma,indicating emplacement at the Late Paleoproterozoic.Combined with previous researches,it is shown that there was a Late Paleoproterozoic granitic magmatism in the southern margin of the NCC and the JLJB.2.Natures of the magma source and petrogenesis of the A-type granites in the Songxian-Ruyang regionThe Paleoproterozoic granites in Songxian-Ruyang region are mainly composed of granite-porphyries.They have high SiO2(67.51?74.42 wt.%),K2O contents(4.27?9.60 wt.%),low MgO contents(0.09?1.3 1 wt.%),and show enrichtment in large-ion lithophile elements(LILEs)and depletion in high-field strength elements(HFSEs),indicating a crustal-derived geochemical characteristics.They have high Zr+Nb+C e+Y contents(566?835 ppm),high 10000Ga/Al ratios(1.57?3.15)and high zirconium saturation temperature(Tzr),indicating that these granites show the characteristics of A-type granites.The A-type granites also have negative whole-rock ?Nd(t)(-8.62--6.10),apatite ?Nd(t)(-8.31?-6.18)and zircon ?Ht(t)values(-13.76?-7.69),different from the mantle-derived A-type granites which generally show high and positive Nd-Hf isotopic values.In combination with their Mesoarchean-Neoarchean two-stage model ages,it suggests that these A-type granites were derived from partial melting of the crustal calc-alkaline granitoids from Taihua Group in TNCO,southern margin of the NCC.3.Natures of the magma source and petrogenesis of the A-type granites and plagiogranites in the JLJBThe Paleoproterozoic granites in Huanren region are mainly composed of porphyry monzogranites.They have high SiO2(64.54?69.06 wt.%)and K2O contents(5.69?7.26 wt.%),low MgO contents(0.53?1.03 wt.%),and are enriched in LILEs and depleted in HFSEs.In addition,they have high Zr+Nb+Ce+Y contents(381?599 ppm)and high 10000Ga/Al ratios(2.73?2.97).Combined with their high Tzr,these granites show the characteristics of A-type granites.Their zircon ?Hf(t)(-1.62?+2.50)and apatite ?Nd(t)values(-2.69?-1.65)are low and relatively concordant,suggesting that the Huanren A-type granites were derived from the partial melting of mafic lower crust.Paleoproterozoic plagiogranites were identified in the Gaoliduntai region of the JLJB Granites from Gaoliduntai region have high SiO2(71.06?79.40 and NazO(6.60?7.89 wt.%)contents,low K2O(0.10?0.70 wt.%)and Rb(1.86?18.0 ppm)contents,low K2O/Na2O and Rb/Sr ratios.As well as the plagioclases are all albite,the Gaoliduntai rocks can be classified as plagiogranites.They exhibit low whole-rock and apatite ?Nd(t)values,low radiogenic Pb isotopes,low zircon ?Hf(t)(+0.90?+6.55)and ?18O values(+2.1 ‰?+4.5 ‰),indicating that the Gaoliduntai plagiogranites were formed by partial melting of high-temperature altered mafic oceanic crust,with contributions from delaminated crustal components.4.The Late Paleoproterozoic tectonic evolution in the middle/eastern part of the NCCTHe existence of Songxian-Ruyang A-type granites in tHe southern margin of TNCO suggests that they were fonned in the extensional tectonic environment after the collision of the Eastern and Western blocks of the NCC.The Huanren A-type granites in JLJB,eastern NCC,were formed in the extensional tectonic environment after the continental collision between the Longgang and Nangrim blocks.We suggest that the Paleoproterozoic Gaoliduntai plagiogranites provide evidence for the involvement of ancient oceanic crust in the tectonic evolution of the JLJB,and they were formed in an extensional tectonic setting of the oceanic slab break-off.The post-collision extensional tectonic environment of the TNCO and the JLJB in the Late Paleoproterozoic is the response of the breakup of the Columbia supercontinent in the NCC.