Eoarchean evolution of the North China Craton: Zircon Hf-O isotopic evidence of the ca. 3.8Ga gneiss from eastern Hebei Province

The formation of continental crust is the key to understanding the evolution of the Earth. However, the genesis of continental crust on early Earth has been poorly constrained due to the scarcity of ancient continental crustal remnants. A new Eoarchean outcrop was recently discovered in eastern Hebei Province, which provided a critical object for understanding the nature of the early continental crust of the North China Craton. In this study, we carried out an integrated study of zircon U-Pb age and Hf-O isotopes on the newly discovered Eoarchean granodioritic gneiss from the Labashan area, Lulong City, eastern Hebei Province. The zircons from the granodioritic gneiss show euhedral to subhedral shapes and oscillatory zoning in CL images, and have relatively high Th and U contents and Th/U ratios, which coincide with the characteristics of magmatic zircons. Seven SIMS U-Pb dating analyses on these zircons gave a weighted mean Pb-207/Pb-206 age of 3776 +/- 11Ma (MSWD = 3.1), which was interpreted as the crystallization age of the granodioritic magma. This indicates that there were Eoarchean continental basements distributed in eastern Hebei Province. The studied zircons had a weighted mean delta O-18 value of 5.81 +/- 0.13 parts per thousand, consistent with that of normal mantle zircon within the analytical error, and gave a weighted mean epsilon(Hf)(t) value of 0.09 +/- 0.31, similar to that of the chondrite uniform reservoir (CHUR). These Hf-O isotopic features were evidently different from those of the Eoarchean detrital zircons in the Caozhuang Formation, eastern Hebei Province, suggesting that the Labashan granodioritic gneiss did not provide the detritus for the Caozhuang Formation. The Eoarchean rocks in the Anshan gneissic complex were the most probable source rocks for the Eoarchean detrital zircons in the Caozhuang Formation due to their similar Hf-O isotopic features. This implies that there was a unified basement beneath both eastern Hebei Province and the Anshan area during the Eoarchean. The Eoarchean rocks detected in the North China Craton are comparable with those exposed in other main cratons like North Atlantic Craton, Pilbara Craton, and Tarim Craton in terms of zircon Hf-O isotopes. The presence of these ancient rocks suggests that the oldest continental crust originated from a primitive and undifferentiated mantle source.