Nature Of The Early Cretaceous Deep Mantle In The Middle Part Of Southern North China Block:Constraints From Intermediate-basic Igneous Rocks

This thesis focuses on the Early Cretaceous intermediate-basic igneous rocks composed of Jingwan,Sunjiagou,Shangdian,Putaozhaung,Nianpanzhuang,Niujiaowan,Hujiazhuang,Laojunshan intrusive rocks and volcanic rocks from Jiudian and Daying Formation in the middle part of southern North China Block(SNCB).Based on the field investigation and petrology,this thesis presents new LA-ICP-MS zircon U-Pb ages,major and trace elements,whole-rock Sr-Nd-Pb-Hf isotopic,apatite Nd and zircon Hf-O isotopic data,with aim to define temporal and spatial distrution of Early Cretaceous intermediate-basic igneous rocks and petrognesis as well as further to discuss the nature of the deep mantle and crust-mantle interaction.This thesis reveals the background of the intermediate-basic igneous rocks and Meosozoic tectonic evolution of the SNCB.Main achievements are listed below:1.The rock assemblages,temporal and spatial distribution of the Early Cretaceous intermediate-basic igneous rocks within the SNCBThe Early Cretaceous intermediate-basic igneous rocks are north-south distribution within the SNCB.The new geochronological data indicate that the Early Cretaceous intermediate-basic igneous rocks were formed in the three periods,i.e.,140～129 Ma(intermediate-basic intrusive rocks),130～120 Ma(intermediate-basic volcanic rocks)and～112 Ma(minette).The intermediate-basic intrusive rocks consist of meta-hornblende gabbro,metadiabase,hornblende diorite and minette;The intermediate-basic volcanic rocks are composed of pyroxene andesite,hornblende andesite,stomatal-almond andesite and andesitic tuff.2.Petrogenesis of the Early Cretaceous intermediate-basic igneous rocks and nature of deep mantle within the SNCBThe geochemistry of the Early Cretaceous intermediate-basic igneous rocks are not affected by fractional crystallization,crustal contamination process and low-grade metamorphism,which could be used to trace the nature of mantle source.According to the distinct trace element compositions,the intermediate-basic intrusive rocks could be divided into two types,including the Mid-Ocean Ridge Basalt(MORB)-like basic rocks and arc-like intermediate-basic rocks.MORB-like basic rocks exhibit extremely low SiO2,K2O,total alkali(Na2O+K2O)contents,as well as high MgO and Mg#values.They show Enriched-MORB(E-MORB)and Normal-MORB(N-MORB)rare earth element(REE)patterns with enrichment of large ion lithophile elements(LILEs)and depletion of high field strength elements(HFSEs).Considering their high Cr and Ni concentrations and depleted isotopic compositions,the MORB-like basic rocks originated from a depleted mantle source via metasomatism by melts derived from pelagic sediments and altered basaltic oceanic crust.Arc-like basic rocks have relatively low SiO2,and high K2O,total alkali,MgO contents and Mg#values.They are enriched in light REE(LREE)and LILEs with depletion of heavy REE(HREE)and HFSEs.Among them,Hujiazhuang hornblende diorite has relatively enriched whole-rock Sr-Nd-Hf isotopes,suggesting an origin of the ancient lithospheric mantle metasomatized by aqueous fluids derived from Yangtze Blcok lower crustal materials.Laojunshan minette has relatively high TiO2 and Nb contents with Nb/U ratios,similar to Nb-riched basalt.Considering their relatively depleted whole-rock Sr-Nd-Hf isotopes,it indicates that minette originated from partial melting of the lithospheric mantle with incorporation of the asthenspheric materials.Intermediate-basic volcanic rocks have relatively high SiO2,K2O,total alkali and low MgO contents and Mg#values.The intermediate-basic volcanic rocks have arc-like REE distribution patterns and trace element abandunce,with enrichment of LREE and LILEs as well as depletion of HREE and HFSEs.Combined with significantly enriched whole-rock Sr-Nd isotopes((87Sr/86Sr)i=0.7072～0.7246,εNd(t)=-25.71～-16.84),they show an EMI-type enriched lithospheric mantle origin.This indicates that they are derived from a lithospheric mantle source with incorporation of Yangtze Block and North China Block lower crustal materials.3.The existence of recycled carbonate in mantle sourceArc-like basic rocks(Hujiazhuang hornblende diorite and Laojunshan minette)show relatively heavy whole rock Zn isotopic compositions(0.14‰～0.41‰).In addition,they have relatively high Zn concentrations and weakly enriched whole-rock Sr isotopic compositions,which is consistent with those of dolomite.It indicates that Hujiazhuang hornblende diorite originated from a mantle source metasomatized by dolomite and calcite.Whereas,the mantle source of minette is modified by dolomite.The whole-rock Sr-Zn modeling results suggest that the deep mantle is a storage of subducting carbon via metasomatism by dolomite-dominated species.4.Tectonic evolution of the SNCB during the MesozoicBoth MORB-like basic intrusive rocks and minette show significant contribution of asthenosphere mantle,indicating that the lithosphere of the SNCB was under the strong extensional tectonic setting in the Early Cretaceous,which is supported by regional large-scale A-type granites,extensional basins,conglomerate formations and metamorphic core complexes.The information of pelagic sediments and altered basaltic oceanic crust recoreded by the intermediate-basic igneous rocks indicates that along with the northward subduction of Paleo-Tethys oceanic plate during the Ordovician,the asthenospheric mantle was modified by oceanic crust-derived melts.The existence of Yangtze Block crustal materials in mantle source suggests that the ancient lithospheric mantle was fertilized by the aqueous fluid released by the Yangtze Block during the subduction process.In the Early Cretaceous(140～129 Ma),due to the far-field effects of the Paleo-Pacific Plate,the asthenosphere mantle convection and upwelling formed the MORB-like basic intrusive rocks along the fault zone.This process further caused partial melting of the lithospheric mantle,forming arc-like intermediate-basic intrusive rocks and volcanic rocks.In the late Early Cretaceous(～112 Ma),corresponding to progressive lithospheric thinning,the parital melting of lithospheric mantle with addition of asthernospheric materials formed the Laojunshan minette.