U-Pb and Hf isotopic study of detrital zircons from the Wulashan khondalites: Constraints on the evolution of the Ordos Terrane, Western Block of the North China Craton

摘要

Detrital zircons from the Wulashan khondalites in the Ordos Terrane, western block of the North China Craton, give U-Pb ages between 1.84 and 2.32 Ga, showing that their provenance was dominated by Paleoproterozoic rocks, much younger than those exposed in the Eastern Block. These zircons have oscillatory growth zoning and possess ε Hf values between -8 and +9, suggesting derivation of their precursor magmas from old crust and juvenile materials from the mantle. The lowest ε Hf values of zircons with different ages give a line that intersects the depleted mantle line at about 2.6 Ga in a ε Hf vs. time diagram. This may imply that the precursor magma source in the deep level of the crust was largely separated from the mantle ∼2.6 Ga ago and therefore 2.6 Ga may be an important crustal formation period for the terrane. One khondalite sample has detrital zircons with a single age population at ∼2.0 Ga and positive ε Hf values from +1 to +9, clearly recording the significant addition of juvenile materials at ∼2.0 Ga in the source region. This may inspire us to seek whether there was a connection between a mantle superevent and the formation of Columbia supercontinent. Data of this study exclusively demonstrate that the Ordos Terrane was developed independently from the Eastern Block until assembly of the Columbia supercontinent. Therefore, tectonic models involving that the whole North China Craton was cratonized 2.5 Ga ago should be dismissed. © 2005 Elsevier B.V. All rights reserved.