SHRIMP zircon U-Pb ages of the tuffite beds in the Tuanshanzi Formation(Changcheng Group)of the Yanshan Mountains,North China Craton
The geochronological research of the Mesoproterozoic Changcheng Group,North China Craton(NCC),has been concentrated for several decades.Although lots of significant progress have been achieved in recent years,the exact age of some key horizons and the relevant units,including that of the Tuanshanzi Formation,are still somewhat ambiguous.So far,all the previous chronological studies of the Tuanshanzi Formation have been focused on the zircon U-Pb age dating of the intercalated alkalic volcanic rocks in this formation.However,due to the complicated category of the zircons from the eruptive alkalic rocks,to some extent,it resulted in dispute both about the precise depositional age of the Tuanshanzi Formation and that of the underlying Chuanlinggou Formation,especially while the porphyry dikes and the rapakivi-granite apophysis,truncated by the Changcheng Group in the Yanshan Mountains,have been dated successfully since early 2010's.Hence,further work is needed to verify the exact depositional age of the Tuanshanzi Formation.Recently,some interbedded tuffite beds in the lower part of the Tuanshanzi Formation are firstly recognized in the Miyun and the Pinggu districts,Beijing City,in the Yanshan Mountains.High-precision SHRIMP zircon U-Pb dating yields tuffite ages of 1634±9Ma and 1637±8Ma,and the depositional age of the Tuanshanzi Formation can be calibrated more precisely.Combining with the previous works,the bottom age of the Tuanshanzi Formation,the same with the top age of the underlying Chuanlinggou Formation,can be constrained more tightly as～1638Ma,and the chronostratigraphic framework of the Changcheng Group in the Yanshan Mountains can be revised,too.This may conform with the characteristic of the rapid depositional setting in this area during the latest Changchengian Period(1650～1600Ma),which should represent a sort of successions from the constantly extending rift-basin to the passive continental margin along the northern NCC.Further,it can lead a more credible stratigraphic correlation of all the Changchengian successions at the NCC,especially between the Luoyukou Formation at the southern margin of the NCC,and the Tuanshanzi-Dahongyu formations in the Yanshan Mountains.Moreover,it may be inferred that,corresponding to the deposition of the Tuanshanzi-Dahongyu and the Luoyukou formations,around～1638Ma,for the first time,the warm,clean,shallow carbonate sea covered both at the northern and southern margins of the NCC.Simultaneously,probably a narrow but unobstructed carbonate sea passage,roughly north-south,emerged in the central of the NCC.This indicated that,from this time on,the NCC had been not only developed in the more extensional tectonic setting,but also isolated enough with the surrounding newborn seas(juvenile oceans).All these would prevent the input of the terrigenous clasts both from the NCC and the previous converged cratons,i.e.the main body of the Supercontinent Columbia(Nuna),and be suitable for the formation of carbonate rocks.Therefore,it should mark the new stage of the break-up between the NCC and the Columbia,as well as the supercontinent itself.
SHRIMP zircon U-Pb agesTuffiteTuanshanzi FormationChangcheng GroupNorth China CratonSupercontinent Columbia(Nuna)
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维普
