U-Pb Geochronology Of Baddeleyite,zircon And Apatite From Mafic Dikes In The North China Craton

This doctoral thesis investigates U-Pb isotopic characteristics and crystallization rules of co-existing baddeleyite,zircon and apatite minerals in the mafic igneous system,targeting mainly several representative Proterozoic mafic dykes as well as a Mesozoic gabbro and a monzonite porphyry intruding in volcanic rocks of Majiahe formation of the Xiong'er group in the North China Craton.Mmechanism for secnondary zircon overgrowth was constrained by EPMA measurement on Zr-bearing accessory minerals of different occurrences and estimationof temperature and pressure using amphibole and plagioclase thermobarometry.On the basis of previous achievements,this thesis will further define origin and tectonic implications of Proterozoic mafic dykes by means of major,trace elemental and Sr-Nd-Hf isotopic analyses.The 207Pb/206Pb age of baddeleyite yielded by SHRIMP well defined magmatic events of a series of Proterozoic mafic dykes(1752-1765 Ma,1661 Ma,1315-1301 Ma and 915-909 Ma)and a monzonite porphyry(1779 Ma).SHRIMP and TIMS U-Pb analyses on baddeleyite from the Mesozoic gabbro yield 206Pb/238U ages at 128.2± 1.5 Ma and 125.1 ± Ma.In this study,baddeleyite offers pratical reference for discussing U-Pb isotopic characteristics of corresponding zircon and apatite.Zircons in the mafic dikes are more complex,displaying both magmatic and hydrothermal origins.Magmatic zircon is commonly affected by abundant radiation damage as indicated by high alpha dose(D?).During SIMS measurement,metamict zircons commonly show high D? effect or lead loss,which translates into positive or negative correlations between 206Pb/238U age and D?,recording fluid alteration.The high D? effect would not affect the 207Pb/206Pb ages of zircons,whereas lead loss may result in a discordia line or even generate younger,but geologically meaningless concordant ages.Hydrothermal zircon is characterized by oscillatory zoning and core-rim texture in CL image and much younger concordia age.LA-Q-ICP-MS apatite U-Pb analytical spots are distributed along a discordia in the Tera-Wasserburg diagram and define a lower-intercept age which is consistent with that of co-existing baddeleyite within errors.Some measurements are suffered from large analytical uncertainties associated with low U concentrations(0.5-8.5 ppm)and low radiogenic 206Pb(or high common Pb).Secondary zircon polycrystalline aggregates were observed in some mafic dykes and the monzonite porphyry.They are mainly distributed along with rims and fractures of baddeleyite or distributed regularly in hornblende,showing close relationships with other secnodnary minerals(biotite,chlorite,epidote,apatite,titanite,etc.).Secondary zircon overgrowthes on baddeleyite have the largest Zr/Hf ratio which is the lowest for those associated with hornblende,suggesting that baddeleyite and hornblende have served as the predominant source for their growth,respectively.Complex anions(F-,Cl-,OH-,PO43-,etc.)are essential for activation and migration of Zr4+.Temperatures(470-580?)estimated by amphibole-plagioclase thermobarometry indicated that secnondary zircon overgrowth in the monzonite porphyry has formed during medium-to high-temperature hydrothermal alteration.The 1.8-1.6 Ga mafic dyke swarms are of enormous scale in the North China Craton.They have complex major,trace elemental and Sr-Nd-Hf isotopic composition,recording rejuvenation and evolution of the lithospheric and asthenospheric mantle.Mesoproterozoic(1.4-1.1 Ga)dyke swarms,which are plotted in the field of continental rift and associated with crustal contamination,are closely related to breakup of the NCC from the Columbia supercontinent.Neoproterozoic(1.0-0.8 Ga)series dyke swarms are widely distributed in the NCC and may have originated from asthenospheric mantle.They may constitute a large igneous province(LIP)in the NCC to respond to circulation of the supercontinent Rodinia.