Geochemistry Of Mesozoic Granitic Magmatism From The Dabie-Sulu Orogen And Research On Several Fractional Mechanisms Of Mg-Fe Isotopes In Granites

The first part of this thesis focuses on the Jurassic granitoids in the Sulu orogen.Systematic petrology,geochronology,major and trace elements,Sr-Nd-Pb-Mg-O isotopes analyses of the Jurassic granitoids were carried out and several conclusions have been obtained.?1?The ages of the Jurassic magmatism range from 166.2Ma to156.7Ma.An obvious magmatism‘calm period'of ca.25My has been recognized previous the magma activities in Cretaceous in the Sulu orogen.?2?The Sulu Jurassic granitoids are characterized by high SiO₂,Al₂O₃,low FeO_t+MgO contents,low Mg#,low Rb/Sr,weak peraluminous,high Sr/CaO,low REE and Th contents,high Eu/Eu*and strong correlations among REE,Th and Eu/Eu*,which can be best explained by partial melting of intermediate-felsic orthogneissic sources under high pressure?>1.5GPa?,leaving garnet,epidote-group minerals,clinopyroxene and quartz/coesite as residuum.The above features can be applied on identifying high-pressure partial melts of intermediate-felsic sources.A thickened continental crust?>50km?is suggested to extensively exist during Jurassic in the north Sulu orogen.?3?The thickened continental crust has a superimposed structure.The underlying is the subducted South China upper-middle crust,while the overlying layer is the obducted North China crust.In the second part,post-collisional granitoids from the Dabie orogen were selected for investigating Mg and Fe isotope fractionations.Analyses on mineral-pairs of granite show that the order of heavy iron isotope enrichment in I-type granitoids is feldspar>pyrite>magnetite>biotite?hornblende.The fractionation among feldspar,biotite and magnetite are correlated with mineral composition,which reflects the compositional effect of mineral composition on the inter-mineral fractionation of Fe isotopes.Significant magnesium and iron isotope fractionations were observed in an adjacent gabbro and granite profile from the Dabie orogen.The negatively correlated Mg and Fe isotopic compositions as well as co-variations of Mg and Fe isotopes with Mg#reflect Mg-Fe inter-diffusion driven isotope fractionation.The study suggests diffusion profiles can be a powerful geospeedometry.The observed isotope fractionations also indicate that Mg-Fe inter-diffusion can produce large stable isotope fractionations at least on a decimeter scale,with implications for Mg and Fe isotope study of mantle xenoliths,mafic dikes,and inter-bedded lavas.