Ca. 3.06 Ga granodioritic gneiss in Dunhuang block

We analyze gneisses from the Dongbatu Mountain, Gangou area, which is part of the Dunhuang block. Petrographic and geochemical data show that these rocks are granodiorite with Archean tonalite-trondhjemite-granodiorite(TTG) characteristics. Zircon U-Pb dating yielded an age of ca. 3.06 Ga (Mesoarchean) for zircon cores, making these rocks the oldest yet discovered in the Dunhuang area. The ages of ca. 1.9-1.86 Ga for the zircon rim indicate a late Paleoproterozoic tectonothermal event. Hf isotopic compositions for the zircon cores show that two spots have positive e Hf ( t ) values of 5.6 and 1.2, respectively, corresponding to two-stage Hf model ages of 3091 Ma and 3356 Ma; other samples have negative e Hf ( t ) values of -9.4 to -0.5 and correspond to two-stage Hf model ages of 4.00-3.45 Ga, with a peak age of ca. 3.82 Ga. This indicates that the ca.3.06 Ga granodioritic gneisses were produced by reworking ancient continental crust with added juvenile crust, and that the early continental crust in the Dunhuang area began forming in the Eoarchean era (3.80-4.00 Ga). Therefore, these factors, combined with the relational evidence obtained from the Tarim Craton and the North China Craton, indicate that the Milan Complex is not the same tectonic rock unit as the Dunhuang Complex and the Milan Complex does not belong to the Dunhuang block; The Dunhuang block may be part of the North China Craton rather than the Tarim Craton.