Abstract:

U-Pb dating and Hf isotopic composition of zircons from the northern margin of North China Platform, the Great HinganMongolian Orogen, and the Yangtze PlatformQinlingDabieSulu Orogen around the Ordos Basin show that its surrounding tectonic units have different histories of formation and evolution. Average age is 1837 Ma based on U-Pb dating of 885 zircon grains from the northern margin of North China Platform. The most intensive magmatism peak occurred between 2200 and 1800 Ma; the zircon grains within this range amount to 40% of the total. The secondary intensive period is between 2800 and 2200 Ma; percentage of zircon grains in this range is ~30%. The zircons crystallized in 15001200 Ma and 500100 Ma occupy ~15% respectively. The northern margin of North China Platform is characterized by a lack of 1000700 Ma zircons. The zircons older than 3000 Ma are very rare. The averaged Hf isotopic depletion mantle model age of zircons is 2.55 Ga, which is older than that of U-Pb ages, suggesting that zircons crystallized between 2200 and 1800 Ma contain older recycled precursor crustal components. The most intensive peak of Hf isotopic depletion model age is ~2.8 Ga, consistent with that of Nd isotopic depletion mantle model ages. The zircon grains with Hf model ages between 3.0 and 2.25 Ga account for ~95%, which implies that the crustal accretion from the depleted mantle took place between the Archean and the Paleoproterozoic. Other ~5% zircon grains were formed by Mesoproterozoic crustal accretion event. The Late PaleozoicMesozoic magmatism was predominately controlled by anatexis of precursor crustal components, especially the Mesoproterozoic juvenile crust. The percentage of zircon grains with the Hf model ages older than 3.0 Ga is less than 0.1%. The average U-Pb age of zircons in the Great HinganMongolian Orogen is 497 Ma; the most intensive peak is around the Carboniferous, i.e. ~320 Ma. CarboniferousPermian zircons account for over 30% in the orogen. Another population of zircon U-Pb ages is around the NeoproteozoicEarly Paleozoic, i.e. 600440 Ma, whose percentage is >55% of the total. The percentage of Late MesoproterozoicNeoproterozoic (1200600 Ma) zircons is ~4%. The corresponding average Hf isotopic depletion model age of the zircons in the orogen is 1.13 Ga, and the most intensive peak is around ~0.6 Ga. The Hf model ages of zircons dominantly distribute between 0.7 and 0.28 Ga; the zircons within this range account for 57% of the total, which implies the most intensive accretion period of the orogenic crust from the depleted mantle. Other intensive accretion episode is between the Mesoproterozoic and the Neoproterozoic (1.50.7 Ga); the percentage of zircons in this range is up to ~38%. The zircons with Hf model age between 3.3 and 1.5 Ga is less than 5%. The orogenic crust almost has no juvenile component from the depleted mantle since the Paleozoic. The average U-Pb age of zircons in the Yangtze PlatformQinlingDabieSulu Orogen is 799 Ma. The zircon population between 1300 and 750 Ma is ~70% of the total. The other intensive magmatism period is between the Late Paleozoic and the Yanshanian; the zircons formed in this period account for ~20%. PaleoarcheanPaleoproterozoic (35002650 Ma) and PaleoproterozoicMesoproterozoic (21181680 Ma) zircons account for ~5%, respectively. On the basis of averaged Nd isotopic depletion mantle model age of 1.56 Ga, it is suggested that much old precursor crustal components were involved in the magmatism event between 1300 and 750 Ma. The tectonic units around the Ordos Basin have different histories of formation and evolution. Their crustal growth has been episodic. But the age and intensity of each episode show significant differences among the tectonic units. Thus, the strata of the basin may decipher corresponding provenances through U-Pb dating and Hf isotopic composition of detrital zircons in them.

Key words:  U-Pb ages, Hf model ages, crustal evolution, the Ordos Basin, the Great HinganMongolian Orogen, North China Platform