Archean TTGs and sanukitoids from the Jiaobei terrain, North China craton: Insights into crustal growth and mantle metasomatism

Abstract U–Pb zircon dating for 10 rock samples collected from the Archean Jiaobei terrain of the eastern block of the North China craton (NCC) yield three groups of ages: ∼2.9 Ga, ∼2.7 Ga and ∼2.5 Ga. Thirteen rock samples studied can be classified into a low Mg# (molecular Mg/(Mg + Fe)) (Mg# = 34–52) and a high Mg# group (Mg# = 52–61), respectively. The low Mg# rocks are typical Archean TTGs with emplacement ages of ∼2.9 Ga, ∼2.7 Ga and ∼2.5 Ga whereas the high Mg# rocks are Archean sanukitoids with age of ∼2.5 Ga. Despite having different ages, all the Jiaobei TTGs have zircon δ 18 O values between 5.6‰ and 6.3‰, essentially identical to those of the ∼2.7 Ga TTGs from adjourning Taishan area and Archean TTGs from other parts of the world. Magmatic zircons from the Jiaobei sanukitoids have δ 18 O values between 6.4‰ and 7.5‰, significantly higher than the Archean TTGs. The whole-rock Nd and the zircon Hf isotopic compositions indicate that the ∼2.9 Ga and ∼2.5 Ga TTGs and ∼2.5 Ga sanukitoids represent crustal growth at ∼2.9 Ga and ∼2.5 Ga in the Jiaobei terrain. Unlike the ∼2.7 Ga TTGs in the adjourning Taishan area that are considered to reflect juvenile crust, the Jiaobei ∼2.7 Ga TTGs, however, are most likely resulted from the reworking of the pre-existing ∼3.0 Ga crust. This is in contrast to the previous suggestion that ∼2.7 Ga is the most significant crust-forming episode and ∼2.5 Ga a period of crustal reworking in the Jiaobei terrain. The high Mg# and high contents of Cr, Ni, LREE and LILE of the ∼2.5 Ga Jiaobei sanukitoids, require a metasomatized peridotitic mantle source for their origin. The significant negative Zr and Hf anomalies and the elevated δ 18 O values of the ∼2.5 Ga sanukitoids, are inconsistent with a mantle source metasomatized solely by the TTG melts. Instead, we suggest that the Jiaobei sanukitoids could have originated from a mantle wedge metasomatized by a high δ 18 O component, most likely the slab-derived fluids. The similarity of multi-element patterns and the zircon δ 18 O values between the Jiaobei and Superior Province sanukitoids may imply similar processes involved to form sanukitoids in both Jiaobei and Superior Province terrains.