Petrogensis Of The Intermediate-acid Intrusive Rocks Of Xiong'ershan And Funiushan Area And Its Constraints On The Late Mesozoic Tectonic Evolution In The Southern Margin Of The North China Craton

This thesis studies on the Late Mesozoic intermediate-acid intrusive rock-mass(such as Wuzhangshan,Huashan,Haoping,Banzhusi and Funiushan)of Xiong'ershan and Funiushan areas in the southern margin of the North China Craton(NCC).Based on the systematic field investigation,petrology,LA-ICP-MS zircon U-Pb ages,geochemistry and whole-rock Sr-Nd-Pb isotopic,apatite Nd and zircon Hf-O isotopic analysis,this thesis defines geochronological framework of the Late Mesozoic intermediate-acid intrusive rocks and discusses their petrogenesis and magma source.Combined with the published data of the southern margin of the NCC,the thesis reveals changes of the continental crustal structure and tectonic evolution of the Late Mesozoic in the southern margin of the NCC.The main achievements of the thesis are as follows: 1.The geochronological framework of the Late Mesozoic intermediate-acid intrusive rocks in the Xiong'ershan and Funiushan areas The LA-ICP-MS zircon U-Pb data indicates that the Late Mesozoic granites in the Xiong'ershan areas can be divided into two stages,including the Late Jurassic to early Early Cretaceous(157 to 130 Ma)and the late Early Cretaceous(?124 Ma).The LA-ICP-MS zircon U-Pb data show that the crystallization age of the intermediate intrusive rocks in Funiushan area are mainly in the early Early Cretaceous(144 to 135 Ma).The granites in Funiushan area can be divided into two stages,including the early Early Cretaceous(?144 Ma)and the late Early Cretaceous(130 to 116 Ma).Combined with the published data of the southern margin of the NCC,the Late Mesozoic intermediate-acid intrusive rocks can be divided into two major periods,i.e.,the Late Jurassic to early Early Cretaceous(160 to 130 Ma)and the late Early Cretaceous(130 to 110 Ma).2.Nature of the magma source and petrogenesis of the Late Mesozoic granites in Xiong'ershan area The Late Jurassic to early Early Cretaceous granites in the Xiong'ershan area are mainly composed of monzogranite and granitic porphyry.They have high SiO₂ content and low MgO contents,and are enriched in light rare earth elements(LREEs)and large-ion lithophile elements(LILEs),and depleted in heavy rare earth elements(HREEs)and high-field strength elements(HFSEs).They exhibit high Sr contents and Sr/Y ratios,low Y and Yb contents,indicating the properties of adakitic rocks.Together with the relatively high initial 206Pb/204 Pb ratios and low ?¹⁸O values,the primary magma of the granites were supposed to be derived from reconstruction of the subducted continental crust of the Yangtze Craton(YC)with minor partial melting of the NCC basement.The late Early Cretaceous granites are dominated by monzogranites with relatively low SiO₂ and MgO contents.They are enriched in LREEs and LILEs,and depleted in HREEs and HFSEs,especially with obvious negative anomalies of Eu,P and Ti.Moreover,they show low initial ⁸⁷Sr/⁸⁶ Sr ratios and ?¹⁸O values,relatively high initial Pb isotopic composition,whole-rock,apatite ?Nd(t)values and zircon ?Hf(t)values.These characteristics suggest that the late Early Cretaceous monzogranites were derived from partial melting of the YC basement that has been modified by mantle material.3.Nature of the magma source and Petrogenesis of the Late Mesozoic intermediateacid intrusive rocks in Funiushan area The Funiushan intermediate intrusive rocks are composed of quartz diorite and quartz monzonite.They have high SiO₂ content and low TFeO and MgO content,high whole-rock initial ⁸⁷Sr/⁸⁶ Sr ratios,low whole-rock and apatite ?Nd(t)values and zircon ?Hf(t)values.Combined with high initial Pb isotopic composition and low ?¹⁸O values,the primary magma of quartz diorite and quartz monzonite should be derived from the reconstruction of the subducted YC basement.The early Early Cretaceous granites in Funiushan area mainly include syenogranite and monzogranite.They exhibit high SiO₂,Sr content and Sr/Y ratios and low MgO,Y and Yb content,and no obvious negative Eu anomalies,which suggest their affiliation to I-type granite with adakitic traits.The features of the granites including the inherited Neoproterozoic(851 to 734 Ma)zircons,high initial Pb isotopic composition and low ?¹⁸O values,suggest that the Early Cretaceous granites were derived from the reconstruction of material from the subducted YC basement.The late Early Cretaceous granites can be divided into two groups,including the late Early Cretaceous granites(130 to 116 Ma)and the highly fractionated granites of late Early Cretaceous(126 to 125 Ma).The late Early Cretaceous granites are characterized by high SiO₂,Al₂O₃ and low MgO contents,obvious negative Eu anomalies,and low Sr content and Sr/Y ratios.Combined with existence of the detrital Paleoproterozoic and Neoproterozoic zircons,the relatively high initial Pb isotopic compositions and low ?¹⁸O values,it is suggested that the magma source of the late Early Cretaceous granites contains both the YC and NCC basement materials.The highly fractionated granites of the late Early Cretaceous have high SiO₂ content,low MgO content and Zr/Hf ratios with negative Eu anomalies.In combination with the relatively high ?Nd(t),?Hf(t)values and initial Pb isotopic composition and low ?¹⁸O values,the highly fractionated granites of the late Early Cretaceous were derived from partial melting of the YC basement material with contamination of mantle-derived materials.4.Late Mesozoic changes of the continental crust structure and tectonic evolution in the southern margin of the NCC The intermediate-acid intrusive rocks of the Late Jurassic to early Early Cretaceous(160 to 130 Ma),with the properties of adakitic rocks,were derived from reconstruction of the thickened lower crust.The intermediate-acid intrusive rocks of the late Early Cretaceous(130 to 110 Ma)have no obvious adakitic traits,which were derived from partial melting of the normal continental crust with the involvement of mantle-derived materials.It is consequently supposed that the crustal structure of the southern margin of the NCC was changed during ca.130 Ma,from the thickened crust in the Late Jurassic-early Early Cretaceous to the normal continental crust in the late Early Cretaceous.The existence of the YC material in the southern margin of the NCC suggests that the YC subducted northward beneath the NCC during the Middle-Late Triassic,resulting in the thickening of the continental crust in the southern margin of the NCC.The Paleo-Pacific Plate was subducted westward beneath the NCC and dehydrated during the Late Jurassic to the early Early Cretaceous,resulting in partial melting of the fossil subducted YC slabs and/or thickened NCC,forming the intermediate-acid intrusive rocks with adakitic properties.During the late Early Cretaceous,the rollback of the Paleo-Pacific Plate with higher angles resulted in the reconstruction of normal continental crust that has been structurally changed,forming the intermediate-acid intrusive rocks.