捕虏体麻粒岩锆石U-Pb年龄和铪同位素: 华北地块下地壳的形成与再造

How did the continental lower crust form and evolve? The in situ U-Pb dating ages and Hf isotopic compositions are obtained for zircons collected from the basic granulitic xenoliths in Fuxian, Liaoning, the acidic granulitic xenoliths in Xinyang, Henan, and the intermediate granulitic xenoliths in Hannuoba, Hebei respectively. The U-Pb dating ages of 2620-2430 Ma and 1904-1842 Ma for most zircons from the basic granulitic xenoliths in Fuxian, Liaoning represent the age of magma crystallization and the age of metamorphism respectively. The U-Pb dating ages of 1927 Ma and 605 Ma for one zircon sample represent the age of metamorphism and the age of overgrowth respectively. The ε _Hf values of old zircon with model age of 2.8 Ga are higher than zero, while those of the young zircon with model age of 1.6-1.7 Ga are varied from 7.0 to 34.1. The U-Pb dating ages of most zircons from the acidic granulitic xenoliths in Xinyang, Henan are 3655-3670 Ma and 1767 ∼ 1981 Ma respectively, with low ε _Hf values (0 - -41.1‰, average -13‰) and high model ages (3.8-4.0 Ga). One sample recorded the remelting process of the 2.5 Ga lower crust in the period of 2.1-1.9 Ga. The U-Pb dating ages of zircons from the intermediate granulitic xenoliths in Hannuoba, Hebei are 2489-2447 Ma and 1842 ± 40 Ma with ε_Hf values close to zero (occasionally up to +9.2‰ - +10.2‰). These results show that the Lower Archean crust existed not only in the northern part of the North China, but also in its southern edge. It is believed that the original substances of the lower crust older than 3.6 Ga beneath the North China were derived from the 4.0 Ga primary mantle. Since the formation of the early Archean nucleus, the lower crust of the North China had been undergone various processes of the re-melting in the period of 3.8-3.6 Ga, the re-growing with the input of large amount of mantle materials in the periods of 2.7-2.5 Ga and 1.9-1.8 Ga respectively, the re-working by heat of the asthenospheric upwelling in the period of 600-700 Ma, and the further re-working by the strong interaction between anthenosphere and lithosphere (mantle metasomatism) in Late Mesozoic.