Talk Description
Evolution of the early Earth, especially the formation of continental crust and related geodynamics, is still not clear due to the scare of ancient rock samples. While, detrital zircon hosted in the younger rocks, to some extent, makes up for the regret. It has strong resistance to physical and chemical changes and can be survived from multiple sedimentary and metamorphic cycles, and then records some information of early Earth. Here, we report U-Pb dating, Hf-O isotopes and trace elements of detrital zircon grains in the Caozhuang quartzite from the North China craton (NCC), and try to constrain the formation and evolution of continental crust. The Caozhuang quartzite had the youngest concordant age cluster for detrital zircons at 3.55 Ga and underwent high-grade metamorphism at 3.5 Ga, which makes the Caozhuang quartzite among the oldest clastic sedimentary rocks around the world. The detrital zircons were euhedral or subhedral in shape, indicating they were of local provenance. The detrital zircon U-Pb age spectra of the Caozhuang quartzite show continued records from 3.89 Ga to 3.55 Ga with two main peaks at 3.80 and 3.68 Ga, which resemble those of the Anshan gneissic complex. In addition, the concordant detrital zircons in the Caozhuang quartzite show variable δ18O (mainly from 3.37 to 8.12) and εHf(t) (mainly from -4 to +4) values, which are also comparable with those of the Anshan gneissic complex. Accordingly, the Anshan gneissic complex was the most probable source rocks for the Eoarchean to early Paleoarchean zircons in the Caozhuang quartzite. Importantly, the Caozhuang detrital zircons show continuous and smooth evolution trends without obvious shift for εHf(t), δ18O, Ti-in-zircon temperatures, oxygen fugacity, Eu/Eu*, Th/Nb, and the inferred SiO2 contents of their host rocks. This indicates the lack of global simultaneity of geodynamic transitions during Eoarchean, arguring against the onset of modern-style plate tectonics in the NCC during Eoarchean. The binned averages of εHf(t) values for every 20 m.y. interval decrease monotonically from +1 at 3870 Ma to -2 at 3570 Ma, and binned average δ18O values are elevated in relative to the normal mantle zircon and fall in a narrow range of 6 to 7. The oldest model ages of Hf isotopes are about 4.0 Ga, indicating that the earliest continental crust of the NCC was possibly formed at 4.0 Ga. The isotopic characteristics indicated that the Eoarchean felsic continental crust of the North China Craton formed by partial melting of mafic rocks that underwent low-temperature water-rock interaction after separation from chondritic mantle source.