In situ U-Pb dating and Lu-Hf isotopic analysis were carried out for detrital zircons from quartzite in the Paleoproterozoic Songshan Group on the southern margin of the North China Craton (NCC). The re- sults provide further constraints on the crustal formation and evolution history of NCC. Four 207Pb/206Pb age populations were obtained from 99 analyses, with clusters at ~3.40 Ga, 2.77―2.80 Ga, ~2.50 Ga and 2.34 Ga, respectively. The 3.40 Ga old zircons have similar Hf isotopic compositions to those from Ar- chean rocks in the Jidong and Anshan areas of NCC. However, crustal remnants older than 3.6 Ga have been identified in the southern margin of NCC, the South China Craton, the northwestern part of the Qinling Orogen and its adjacent area. Thus, it is not easy to trace the source rock from which the 3.40 Ga detrital zircons were derived. It can be inferred that the crustal remnants older than 3.40 Ga might have been widely distributed in the North China Craton. The 2.77―2.80 Ga zircons make up a relatively small proportion and have the highest εHf (t) values (up to 6.1±1.6), consistent with the Hf isotopic composition of the depleted mantle at 2.83 Ga. Their single-stage Hf model age of 2.83 Ga is close to their crystallized age, suggesting that their source rocks were extracted from the contemporaneous depleted mantle. The ~2.50 Ga zircon grains constitute about 85% of the total grain population and their Hf isotopic compositions indicate major growth of juvenile crust at ~2.50 Ga but minor reworking of ancient crust. The youngest zircon dated in this study gave an U-Pb age of 2337±23 Ma, which can be considered the maximum depositional age of the formation of the Songshan Group.
DIWU ChunRong SUN Yong YUAN HongLin WANG HongLiang ZHONG XingPing LIU XiaoMing
Widely distributed on the southern margin of the North China Craton,the Taihua complex extends roughly in an east-west direction and the relatively complete successions are found in Lushan County,Henan Province.Like many other Archean terranes,the Taihua Complex can be divided into two major lithological units along the Dangze River,namely gneisses series and supracrustal rocks.The former is located on the north side of the river and chiefly composed of TTG gneisses and amphibolites;the latter on the south side of the river is mainly supracrustal rock.Coupled with the previous studies,the results obtained by this study show that the Taihua complex was formed in a large time span from Neoarchean to Palaeoproterozoic.The TTG gneisses and amphibolites are dated at Neoarchean (2794–2752 Ma).The 2.9 Ga and 3.1 Ga zircons in amphibolites could be xenocrysts.In the North China Craton,the 2.8–2.7 Ga old rocks crop out in several areas e.g.western Shandong Province,Jiaodong Peninsula and Lushan area of Henan Province.In addition,2.8–2.7 Ga detrital zircons or xenocryst zircons have been recognized in Huai’an,Fuping,Wutai areas of North China Craton and also reported in the Early Paleozoic diamondiferous kimberlites in Mengyin and Fuxian.All these age results indicate that the 2.8–2.7 Ga rocks may have been developed much broader region than today’s outcrops.Zircon Hf and whole rock Nd isotopes show that the 2.8–2.7 Ga tectono-thermal event represents an important period of crustal growth with minor ancient crust reworked in the North China Craton.However the formation of supracrustal rocks is limited to 2.2–2.0 Ga in the Palaeoproterozoic time,not the Archaean,as previously believed.Combined with the chronological data of aluminium-rich metamorphic rocks (Khondalite series) on the southern margin of the North China Craton and adjacent areas,it is suggested that the above areas have widely developed Paleoproterozoic passive continental margin environment.
