Zircon Hf isotopic compositions of the basement gneiss in Bayan Obo: Constraints on crustal evolution of the Yinshan Block in the North China Craton

Abstract

Bayan Obo, located in the north margin of the Yinshan Block, North China Craton (NCC), contains the Archean to Paleoproterozoic basement rocks which may shed light on the early crustal evolution of the NCC. In this paper, to reveal the formation and modification of the basements, we report Hf isotope of zircons from the biotite plagiogneisses in Bayan Obo. Combined with the published U–Pb ages and Hf isotope compositions of the zircons from the Bayan Obo basement rocks, a comprehensive review was employed to provide new insights into the formation and modification of the basement in this area. The oldest formation age of the basement rocks was found to be approximately 2.6 Ga, with positive ε_Hf(t) values indicating the contribution from the depleted mantle to the rocks. The age of ∼2.0 Ga marks the major formation period of the basement rocks. The varied ε_Hf(t) values (−17.2 to +8.9) of the magmatic zircons suggest a mixture origin between the depleted mantle and crustal rocks for the gneiss. The basement went through extensive metamorphism during 1.95–1.9 Ga. In addition, zircon U–Pb–Hf isotope data of the Precambrian units in Guyang and Wuchuan, which are adjacent to Bayan Obo, were compiled for comparison. The comprehensive review on the data of the concordant magmatic zircons indicates the existence of 3.5 Ga crustal fragments in the Yinshan Block. The melting of the depleted mantle and the addition of juvenile materials into the crust at 2.6–2.9 Ga and 2.45–2.55 Ga were confirmed by the high positive ε_Hf(t) values of the magmatic and inherited zircons. An increase in remelting of the crust at 2.0 Ga is deduced from the extremely unradiogenic Hf isotope of the magmatic zircons.