The Petrology And Tectonic Implications Of The Dongjiao Group In Hebei

The North China Craton(NCC)is the largest and oldest craton in China,and also one of the oldest cratonic nucleis in the world.It is dominantly composed of Archean to Paleoproterozoic metamorphic basement,which are overlain by Mesoproterozoic to Cenozoic cover sequences.As an ideal object to probe the cartonic evolution,it has been attracting much attention of many Chinese and foreign geologists.The Taihang Mountain is located in the eastern margin of the middle segment of the Trans-North China Orogen(TNCO),which is one of the typical regions of the Early Precambrian succession in the NCC.In the region,the late Archean to Paleoproterozoic metamo-rphosed strata were exposed fairly well,including the Dongjiao Group in west of the central Hebei Province.On the basis of previous work,this paper carried out a detailed study of stratigraphy and petrology of the Dongjiao Group,mainly on the meta-rhyolites of this Group exposed in Quyang region.With systematically studying of the isotopic chronology,the petrology,and the geochemistry,the geochemical characteristics and petrogenesis and its tectonic setting have been discussed,which may provide new evidence for the Early Precambrian tectonic evolution of the NCC.Some important progresses have been improved in this study:(1)As the first time,two LA-MC-ICP-MS zircon U-Pb ages,2059±8 Ma and2068±10Ma,have been obtained from the meta-rhyolites of the Dongjiao Group,which limited the depositional age of this Group is not less than?2060Ma.Combining the formation age of basalt at the bottom of Gantaohe group,we deduce that the age of Gantaohe Group is between?2.09Ga and?2.06Ga.It suggests that both the Gantaohe and the Dongjiao Groups in the east Taihang Mountains can correlate with the Qingshancun Formation and the Wenshan Formation,in the midlle of the Hutuo Supergroup in Shanxi Province,west of Taihang Mountains.(2)The geochemical studies show that the meta-rhyolites of Dongjiao Groups is high in SiO₂(>72%)and K₂O+Na₂O(7.62%?9.25%),and low in Ca O,Mg O,and TiO₂,with high TFe O/Mg O ratios;The total rare earth element contents(?REE)of the rhyolites are between 156.09?562.78ppm,show obvious negative Eu anomaly(?Eu=0.21?0.28).In the diagram of the trace elements distribution pattern diagram,the meta-rhyolites shows enrichment in LILEs(Rb,Th,K and U)and depletion in Ti,Sr,Nb,P and Ta,The characteristics of REEs and trace elements normalized pattern of the rhyolites shows that the parental magma should be derived from partial melting of the crust.(3)Combining with the typical characteristics of within plate granites of the meta-rhyolites of Dongjiao Group and the sedimentary features of the lower Doujiao Group,as well as the tectonic setting of the underlying Gantaohe Group,the entire Dongjiao Group in Quyang County should be formed in an extensional rift environment.The meta-rhyolites at the top of Dongjiao Group derived from the partial melting of late Archean TTG gneisses in Taihangshan region.(4)Combining with the Gantaohe group-the Dongjiao group and adjacent the Hutuo Group,a collision uplift event during the period of?2.06Ga,then the eastern margin of TNCO should be in the passive continental margin,until the?1.85Ga,the Eastern and Western blocks collisioned,which related to the formation of the supercontinent Columbia...