ABSTRACT

Recently, how to quantitatively estimate the growth rate of continental crust is an enigmatic issue. With the development of ICP-MS technology, the U-Pb dating and Lu-Hf isotopic compositions of detrital zircons from the fluvial sediments can provide an effective and simple approach to constrain the growth rate and evolutionary history of continental crust. In this paper, 189 concordant detrital zircons from the Laoha River have been analyzed for U-Pb ages and Lu-Hf isotopic compositions by excimer LA-MC-ICP-MS. Detrital zircons from samples LH and LH2 show three major age groups, i.e., 2370 Ma~2572 Ma, 1728~2087 Ma, 127~376 Ma and 2374~2598 Ma, 1765~2087 Ma, 119~405 Ma, respectively. They have the common prominent two stage Hf model ages with a peak at ca. 2.7 Ga, which is consistent with the global continental crust. These indicate that the timing of the strongest magmatic events is at ca. 2.5 Ga and 1.8 Ga, and the best estimation age of mantle extraction of the northeast margin of North China Craton is ca. 2.7 Ga. The detrital zircons with U-Pb ages of ~1.8 Ga and ~2.5 Ga have the two stage Hf model ages of ca. 2.7 Ga, whereas ε_Hf(t) values are different from those of depleted mantle. These indicate that the majority of continental crust of the northeast margin of North China Craton at time of ~1.8 Ga originated from the reworking of ~2.7 Ga crust. About 5% of the present crustal volume in the northeast margin of North China Craton was formed at 2.9 Ga; whilst ~64% of the present crustal volume in the northeast margin of North China Craton has been formed at 2.5 Ga, which is higher than that of previous studies. It suggests that the continental crust growth in the North China Craton is not uniform, but it is consistent with the episodic growth of global continental crust (60%). Moreover, the majority continental crust of the northeast margin of North China Craton has been formed at 1.8 Ga (84%) as previously interpreted. Finally, we have used formulas to quantitatively calculate the reworking rate and give a suggestion that the time of 2.5 Ga is also the main growth period of continental crust of the North China Craton, and the time of 1.8 Ga is the strongest reworking period of the continental crust in the craton. In addition, the reworking rate began to drop after reactivating in the North China Craton, and the addition of depleted mantle was gradually increased.