Archean continental crustal growth and reworking of the North China Craton: Constraints from zircon U[sbnd]Pb age and Hf isotopic composition

Abstract

Although less than 7% of the continental crust with the ages older than 2.5 Ga is preserved today, the long-lived continental crust can be served as a long-term archive, unravelling the Earth history back at least 4.4 Ga. The North China Craton (NCC) is one of antique cratons on Earth, containing with oldest continental material traced back to Hadean (4.1 Ga). Archean rocks are extensively exposed in the craton and are characterized by significant tectonothermal events at ca. 2.51 Ga during the Archean, thus providing unique insight into the origin, growth, and evolution of the continental crust on Earth. Statistical modelling based on U–Pb ages and Hf isotopic compositions of zircons from a global database of the NCC reveals that more than 70% of the present crustal volume of the NCC was generated at the end of the Neoarchean. The oldest rocks with ages of ca. 3.8 Ga have so far been identified from the trondhjemite-dominated gneiss complexes in the Anshan and Eastern Hebei areas, northeastern NCC. Hf isotopic compositions in zircons reveal that the Eoarchean continental crust was extracted from depleted mantle reservoirs. The Paleoarchean to early Mesoarchean rocks are sporadically exposed in the Anshan and Benxi areas of the northeastern NCC. Hf isotope data of zircons from these rocks indicate that the relative proportion of juvenile material added to the crust gradually decreased, crustal reworking/recycling from older crustal reservoirs has in turn gradually dominated from 3.60 to 2.92 Ga, with the proportion increased from 5% to 45%. This suggests that long-term reworking of continental crust has occurred in the NCC during that period. The dramatical shift of the Hf isotopes in zircons from the early Precambrian rock occurred at ca. 2.92 Ga, which suggest that modern-style plate tectonics processes were probably initiated in the NCC. The statistical modellings of continental growth models highlights that the NCC show a relatively high net rate of continental growth in the time interval from ca. 2.92 to 2.50 Ga. At least 50–60% of the present volume of continental crust has established, which is notably different from the global-scale continental crust. The strongly episodic distribution of zircon U–Pb ages is tightly linked to tectonothermal events of the continental crust, with peak clusters centered at ca. 2.92, 2.71 and 2.51 Ga. This is also a prominent characteristic of crustal growth of the NCC. During the time interval of ca. 2.92–2.50 Ga, juvenile material extracted from the mantle input to continental crust has dominated in the NCC. The proportion of the newly generated crust exceeded that of reworked antient crust, with estimated cumulative juvenile proportion over 85%. Although the ca. 2.85–2.70 Ga rocks are sporadically distributed in the NCC, and have been only identified in more than ten localities, the ca. 2.71 Ga zircons from the NCC are characterized by higher positive εHf(t) values, almost close to those of the contemporaneous depleted mantle. This unarguably indicates that the 2.85–2.70 Ga time interval was one of the major periods of continental crust growth in the NCC. The ca. 2.55–2.50 Ga rock associations are widely distributed in the NCC, occupying more than 80–90% of the Archean basement. Some researchers have found that the ca. 2.55–2.50 Ga rocks present a prominent Hf and Nd crust model age cluster centered at ca. 2.8–2.7 Ga, and therefore proposed that the ca. 2.51 Ga tectono-thermal events in the NCC were mainly involved in crustal reworking or partial melting of the early Neoarchean mafic crust formed at 2.8–2.7 Ga. However, we propose that the ca. 2.55–2.50 Ga tectonothermal event represents another major period of crustal growth in the NCC, which is markedly different from most other typical cratons worldwide. A large volume of juvenile material, estimated to cumulative proportion of 85%, was extracted from the mantle during the time interval of ca. 2.55–2.50 Ga. Meanwhile, about 15% of the continental crust has been reworked or recycled from older crustal reservoirs, as highlighted by the obviously negative εHf(t) values of a number of ca. 2.51 Ga zircons.