Clastic sediments and sedimentary rocks are widely used for understanding the formation and evolution of the continental crust.The Tietonggou Formation outcrops in the Xiaoqinling region at the southern margin of the North China Craton(NCC)and has unconformable contacts with the underlying Taihua Complex and overlying Paleoproterozoic Xiong’er Group.It mainly consists of quartzite and its protoliths are mature terrigenous clastic rocks.On the basis of the ages of the youngest detrital zircons from the quartzites and the ages of the Xiong’er Group,the depositional ages of the protoliths of the Tietonggou Formation were well constrained to 1.91–1.80 Ga.The U-Pb isotopic data of detrital zircons from the Formation show a major age peak at^2.1 Ga,which is consistent with 2.2–2.0 Ga magmatism in the Trans-North China Orogen of the NCC.Taking into account the texctural and compositional maturity of the Tietonggou Formation quartzite,the^2.1 Ga lithologic units in the Trans-North China Orogen are interpreted as the major source of the Tietonggou Formation.The majority of these^2.1 Ga detrital zircons mostly have highδ18O values(>6.5‰)and negativeεHf(t)values(–7.8–0.0),with corresponding Hf model ages significantly older than their crystallization ages,indicating that these zircons formed from the partial melting of ancient continental crust.The majority of the2.8–2.7 Ga and^2.5 Ga detrital zircons from the Tietonggou Formation had positiveεHf(t)values,and mantle-likeδ18O values,suggesting that the NCC has experienced two stages of significant crustal growth in the Neoarchaean at 2.7 and 2.5 Ga,respectively.The Hf isotopic data of detrital zircons from Paleoproterozoic metasedimentary rocks in the Trans-North China Orogen varied mainly toward the reduction of the radiogenic Hf isotope and gradually show a similar trend of the isotope trajectories of crustal evolution.This reveals that the NCC probably has not developed a long-lived subduction to complete the final assembly of the NCC.Alternatively,the
Inductively coupled plasma mass spectrometry(ICP-MS) has become a powerful tool for providing reliable analytical results in many laboratories around the word. In this study, the mixture of HF and HNO3 acids in high-temperature and high-pressure closed-vessel digestion technique were used to decompose some Chinese reference materials, and thirty seven elements were determined by ICP-MS. Most of the results for Chinese soil reference materials were found to be in reasonable agreement with the reference values, except Cs, Ta, Li, Ge, Zn, Nd, Tb and Ta whose values need to be revised. Their precisions were typically lower than 5% RSD. However, the Precisions of Chinese clay reference materials, especially for GBW03102 and GBW03102a, were significantly different with reference values, probably reflecting the existence of a coarser-grained fraction(>70 μm) in samples, and the formation of fluorides in Al-rich samples during sample decomposition by using the mixture of HF and HNO3 acids. Moreover, thirty-seven trace elements covering the mass range from Li to U in four Chinese clay reference materials were firstly provided with good precision and accuracy in this study.
LIU YeDIWU ChunrongZHAO YanLIU XiaomingYUAN HonglinWANG Jianqi
The Qinling Complex of central China is thought to be the oldest rock unit and the inner core of the North Qinling Orogenic Belt （NQOB）. Therefore, the Qinling Complex is the key to understanding the pre- Paleozoic evolution of the NQOB. The complex, which consists of metagraywackes and marbles, un- derwent regional amphibolite-facies metamorphism. In this study, we constrained the formation age of the Qinling Complex to the period between the late Mesoproterozoic and the early Neoproterozoic （ca. 1062-962 Ma）, rather than the Paleoproterozoic as previously thought. The LA-ICP-MS data show two major metamorphic ages （ca. 499 and ca. 420-400 Ma） for the Qinling Complex. The former age is consistent with the peak metamorphic age of the high- and ultra-high pressure （HP-UHP） rocks in the Qinling Complex, indicating that both the HP-UHP rocks and their country rocks experienced intensive regional metamorphism during the Ordovician. The latter age may constrain the time of partial melting in the NQOB between the late Silurian and early Devonian. The Qinling Complex is mostly affiliated with subduction-accretion processes along an active continental margin, and should contain detritus deposited in a forearc basin.
Chunrong DiwuYong SunYan ZhaoBingXiang LiuShaocong Lai
