Geochronology And Geochemistry Of Carboniferous Intrusions In The Central Great Xing'an Range And Its Geodynamic Implications

This thesis studies the Carboniferous intrusive rocks in the central Great Xing'an Range,northeast China.Zircon U–Pb dating results indicate that the Carboniferous magmatic events in the central Great Xing'an Range can be subdivided into three stages,i.e.,earliest Carboniferous(330~359 Ma),late early Carboniferous(325~328 Ma),and Late Carboniferous(299~322 Ma).According to the rock association,whole-rock major and trace element data,and zircon Hf isotopic data of the intrusive rocks and their spatial-temporal transition,further combined with the regional tectonic evolution history,this thesis studies the petrogenesis of the Carboniferous intrusive rocks,which further constrains the Carboniferous tectonic evolution of the central Great Xing'an Range and the amalgamation time of the Xing'an and Songnen–Zhangguangcai Range massifs.1.Earliest Carboniferous–an active continental margin settingThe earliest Carboniferous intrusive rocks have different rock association and geochemical characteristics in the eastern segment(Zhalantun area)and western segment(Wunuer area)of the central Great Xing'an Range.The intrusive rocks in the Zhalantun area consist of monzogranites,syenogranites,gabbro diorites and gabbros and they belong to high-K calc-alkaline series.In contrast,the intrusive rocks in the Wunuer area are low-K tholeiitic to medium-K calc-alkaline gabbros.The granitoids in the Zhalantun area are enriched in SiO2 and poor in MgO,with their ?Hf(t)values and two-stage model ages ranged from +4.59~+8.16 and 1043~1319 Ma,which imply that their primary magmascould be mainly derived from partial melting of the Mesoproterozoic accreted lower crust.Both of the gabbros show enrichment in the LREEs and LILEs,and depletions in the HREEs and HFSEs.Further together with the spatial variation in their zircon ?Hf(t)values(-0.53 to +0.84 in the east and +12.3 to +13.5 in the west),indicating that the gabbros were derived from partial melting of mantle wedge and asthenosphere mantle modified by subduction-related fluid and/or melt,respectively.Above all,the earliest Carboniferous intrusive rocks formed under an active continental margin setting,and the western segment were under a back-arc setting,related with northwest-towards subduction of the Paleo-Asian Ocean plate beneath the Xing'an Massif.2.Late early Carboniferous–an active continental margin settingThe late early Carboniferous intrusive rocks include granodiorites and quartz diorites.The granodiorites are high-K calc-alkaline I-type granites,and have zircon ?Hf(t)values and two-stage model ages of +13.1~+14.8 and 422~572 Ma,implying that their primary magma was mainly derived from partial melting of the Phanerozoic accreted lower crust with minor addition of mantle-detrived materials.The quartz diorites are geochemically similar to the Sanukitoid and exhibit higher zircon ?Hf(t)values(+7.89~+9.21),implying that their primary magma was generated by partial melting of a depleted mantle wedge metasomatized by subduction-related fluids and/or melt.Above all,the late early Carboniferous intrusive rocks were formed under an active continental margin setting,associated with northwest-towards subduction of the Paleo-Asian Ocean plate.3.Late Carboniferous–a syn-collisional to post-collisional environmentThe Late Carboniferous intrusive rocks are composed of high-K calc-alkaline syenogranites,monzogranites,granodiorites,quartz syenites and quartz diorites.The granidiorites and syenogranites(13XA27),with high K2 O contents and Rb/Sr values,low Cr,Ni contents and Mg# values,have an affinity to adakites.In addition,their zircon ?Hf(t)values range from +9.72 to +12.8.The above features indicate that they were generated by partial melting of thickened lower crust.The monzogranites and syenogranites(13XA28)are I-type granites and show zircon ?Hf(t)values and two-stage model ages of +8.53~+13.3and 544~980 Ma,which imply that their primary magma were derived from partial melting of the Phanerozoic-Neoproterozoic new-accreted lower crust.The above features,together with the widely occurrence of early late Carboniferous adakites,latest Carboniferous A type granites,and early Permian A-type granites in the Great Xing'an Range,we proposed that Late Carboniferous intrusive rocks were formed under a transition from syn-collisional to post-collisional setting,and the amalgamation of the Xing'an and Songnen–Zhangguangcai Range massifs occurred during the early late Carboniferous.