现代河流碎屑锆石U-Pb年龄与Hf同位素对中国华北大陆地壳幕式生长的研究

来源 :中国地质大学(武汉) | 被引量 : 0次 | 上传用户:yangzhengm
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Clastic sedimentary rocks are samples of the exposed continental crust. In orderto characterize the crustal growth history of North China and its possible regionalvariations, 479 concordant detrital zircons in three sand samples from the lowerreach of the Yellow River (which drains the Tibet-Qinghai Plateau, the WesternQinling Orogen, the Qilian Orogen and the North China Craton) and two sandsamples from the Luan River and the Yongding River (which run entirely within theNorth China Craton) were measured for U-Pb age and Lu-Hf isotopic compositionsby excimer laser-ablation ICPMS and MC-ICP-MS. Although regional variationsexist, concordant detrital zircons from the Yellow River reveal three major agegroups of 2.1-2.5 Ga, 1.6-2.0 Ga, and 150-500 Ma. Detrital zircons from thesmaller Luan and Yongding Rivers show three broadly similar major age groups at2.3-2.6 Ga, 1.6-2.0 Ga, and 140-350 Ma, but with narrower age ranges. Comparedto the Luan and Yongding River zircons, the Yellow River zircons are characterizedby a significant number of Neoproterozoic grains. Although Hf isotopiccompositions show both juvenile crustal growth and crustal reworking for all agegroups, much of the crustal growth of North China occurred in the Neoarchean andMesoproterozoic. All three rivers are characterized by a common prominent group ofHf crust formation model ages at 2.4-2.9 Ga with a peak at 2.7-2.8 Ga. A lesssignificant age group lies between 1.4 and 1.8 Ga for the Yellow River, and between1.6 and 1.9 Ga for the Yongding River and Luan River. Crustal growth rates based onHf continental crust formation model ages suggest 45% and 90% of the presentcrustal volume was formed by 2.5 Ga and 1.0 Ga, respectively, for the drainage areaof the Yellow River. In comparison, 60% and 98% of the present crustal volume ofthe North China Craton was generated by 2.5 Ga and 1.0 Ga, respectively, for theLuan and Yongding Rivers. The 2.7-2.8 Ga age peak observed in all river samplesagrees well with the coeval major peak for global crustal growth. However, the othersuggested global peaks of crustal growth at 3.4 and 3.8 Ga are insignificant in NorthChina. Taken together with our previous studies of the Yangtze Craton, which showinsignificant crustal growth at 2.7-2.8 Ga, we suggest that these advocatedworldwide crust formation peaks be re-examined and treated carefully. Our resultsalso show that Phanerozoic zircons may have been derived from crustal sourcesseparated from the mantle up to 2.0 Ga ago before the zircons crystallized,suggesting long-term preservation, reworking and recycling of the continental crust.
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