A comparative study on zircon Hf isotopes of neoarchean tonalite-trondhjemite-granodiorite (TTG) in Trans-North China Orogen and eastern block of the North China craton

Abstract

The North China Craton (NCC) is regarded as the largest craton in east Asia and can be arbitarily divided into the Eastern and Western Blocks with the Trans-North China Orogen in between. Numerous studies with the aid of zircon Hf isotope had been done on the crustal growth of the Eastern Block (EB) but relatively less in amount regarding the Trans-North China Orogen (TNCO). Hf isotope is a newly developed technique which is useful in tracing the nature of crustal accretion and the time when the magma is extracted from mantle. In this contribution, here I had made a comprehensive review on literatures which use zircon Hf isotope data supplemented with zircon U-Pb geochronology to study the Neoarchean crustal growth of the TNCO and the EB with the focus on tonalite–trondhjemite–granodiorite (TTG). It is my objective to find out any features of the crustal implications of TNCO and EB by comparing the collective Hf isotope data respectively. It is found that in the TNCO, the zircon εHf(t) values and Hf model ages suggest that the 2.8-2.7 Ga TTG gneisses in Taihua were derived mainly from older crust while those in other regions such as Zhongtiao and Fuping were mainly from juvenile crust. On the other hand, the high-grade TTG gneisses in regions such as Fuping, Hengshan and Huai’an were formed rather shortly after differentiated from depleted mantle while the low-grade granite-greenstone complexes in regions such as Zhongtiao, Dengfeng and Wutai were formed from reworking of older crust. In EB, Hf isotope data indicates that both ~2.75-2.65 Ga and ~2.55-2.50 Ga rock associations are mainly derived from juvenile crust. Furthermore, the Hf model ages of the later points towards the rock-forming age of the former, indicating that the former was reworked to form the latter. The rocks in TNCO shows similar Hf isotope results with those in EB, especially rocks forming in early Neoarchean. This is consistent with the proposal of eastward subduction in which TNCO was developed on the basis of subduction into EB.