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: ∼2.9Ga, ∼2.7Ga and ∼2.5Ga. 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 ∼2.9Ga, ∼2.7Ga and ∼2.5Ga whereas the high Mg# rocks are Archean sanukitoids with age of ∼2.5Ga. Despite having different ages, all the Jiaobei TTGs have zircon δ18O values between 5.6‰ and 6.3‰, essentially identical to those of the ∼2.7Ga TTGs from adjourning Taishan area and Archean TTGs from other parts of the world. Magmatic zircons from the Jiaobei sanukitoids have δ18O 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 ∼2.9Ga and ∼2.5Ga TTGs and ∼2.5Ga sanukitoids represent crustal growth at ∼2.9Ga and ∼2.5Ga in the Jiaobei terrain. Unlike the ∼2.7Ga TTGs in the adjourning Taishan area that are considered to reflect juvenile crust, the Jiaobei ∼2.7Ga TTGs, however, are most likely resulted from the reworking of the pre-existing ∼3.0Ga crust. This is in contrast to the previous suggestion that ∼2.7Ga is the most significant crust-forming episode and ∼2.5Ga a period of crustal reworking in the Jiaobei terrain. The high Mg# and high contents of Cr, Ni, LREE and LILE of the ∼2.5Ga Jiaobei sanukitoids, require a metasomatized peridotitic mantle source for their origin. The significant negative Zr and Hf anomalies and the elevated δ18O values of the ∼2.5Ga 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 δ18O component, most likely the slab-derived fluids. The similarity of multi-element patterns and the zircon δ18O values between the Jiaobei and Superior Province sanukitoids may imply similar processes involved to form sanukitoids in both Jiaobei and Superior Province terrains.