Early Crustal Evolution Of The Yangtze Block: Constraints From Zircon U-Pb-Hf Isotope Systematics Of 3.1-1.9 Ga Granitoids In The Cuoke Complex, Sw China

The newly identified Cuoke Complex preserves voluminous early Precambrian granitoids, bearing key information for the early evolution of the Yangtze Block, South China. Here, we report new field observations, petrology, zircon U-Pb-Hf isotopes and trace elements for eight granitoids from this complex. The analyzed zircon domains are of magmatic origin, as evidenced by their oscillatory zoning, high Th/U ratios and magmatic rare earth element patterns. Three trondhjemitic gneisses yield crystallization ages of 3.11-3.06 Ga, documenting the oldest known rocks in the southern Yangtze Block. Their epsilon(Hf)(t) values vary from -3.2 to + 2.6 with depleted mantle two-stage (T-DM2) model ages of 3.6-3.3 Ga, indicating reworking of Paleoarchean crustal materials. Three gneissic granitoids crystallized at 2.86-2.85 Ga and reveal a significant episode of granitoid magmatism in the late Mesoarchean. They possess positive epsilon(Hf)(t) values (+0.2 to +5.0) and slightly older T-DM2 model ages (3.3-3.0 Ga), suggesting that these granitoids were most likely derived from remelting of juvenile continental crust. Two monzogranites with crystallization ages of 1.94 and 1.89 Ga display negative e epsilon(Hf)(t) values (-13.0 to -8.1) and older T-DM2 model ages (3.3-3.1 Ga), indicating substantial involvement of ancient crustal materials. Available zircon U-Pb-Hf isotopic data from the Cuoke Complex have revealed that the earliest crustal growth probably occurred in the early Paleoarchean, with significant crustal growth and reworking during the early Mesoarchean, followed by multistage crustal reworking during the late Mesoarchean to Paleoproterozoic. Integrating the results of this study with records from other Archean basement complexes in the Yangtze Block (i.e., Kongling, Zhongxiang, Douling, Yudongzi and Phan Si Pan), we propose that these complexes have diverse formation histories and generally experienced tectonothermal events at ca. 2.0-1.9 Ga, probably responding to the global-scale assembly of the Nuna supercontinent.