The Discovery Of Yongli Carbonatites In Fengning,northern Hebei Province,and Its Geological Significance

The North China Craton,the largest and the oldest craton in China,has been the focus of geological research for a long time.The exact time of tectonic system transition from crystalline basement to breaking up during the Paleoproterozoic and Mesoproterozoic remain disputed.Research on an outcrop of carbonatites near Yongli village in Sichakou,Fengning,northern Hebei Province,was carried out,which provides reliable geological evidence for the initial breaking time of the North China Craton.Yongli carbonatite is distributed in the NW trend in a lenticular shape,with a width of 300-400 m and a length of about 1km.A number of small carbonatite outcrops scattered around it.A large sum of plagioclase amphibolite and pyroxene biotite plagioclase gneiss xenoliths was captured in the rock mass.The main mineral composition is calcite with a content of more than 90%,which constrains the Yongli carbonatite to typical calcite carbonatite.The carbonatite is mainly composed of Ca O and CO2.,with an average Ca O content of 54.01%,and the content of CO2(42.60%)was reflected by the loss of ignition,which is in accordance with the feature of typical calcareous carbonatite.The Primitive Mantle-normalized REE partition curve has a gentle trend.The ratio of LREE / HREE is between 2.041~3.999?g/g,with an the average content of 2.623?g/g.In C and O isotopes,The value of ?13CV-PDB is 0~-0.2‰,with an average value of-0.1‰.The value of ?18OV-SMOW is 14.68‰~18.80‰,with an average value of 16.74‰.LA-ICPMS zircon U-Pb dating result yielded an age of 1806±36Ma(MSWD=3.3),which is interpreted as the formation age of the cabonatite,meanwhile an age of 1848±12Ma(MSWD=0.88)was considered as the age of plagioclase amphibolite xenoliths.The Carbonatite is generated in relatively stable intraplate environments,and most of them are related to rifting and intraplate breaking up events.Generally cabonatite derived from a deep material source,so it is an ideal rock to explore the tectonic environment of lithosphere.Based on the detailed study on Yongli carbonatite a conclusion can be drawn that the carbonatite magma probably was derived from the primitive mantle,and contaminated by the lower crust.It revealed that the magma originated from the melting of the mantle plume and the lower crust.Based on the analysis of tectonic setting diagrams,the samples were all plotted within or near the WPB plate,indicating that its formation was related to intraplate continental rifting.Yongli carbonatites formed at 1806±36Ma,suggesting the North China craton started to breaking up at 1.8Ga.It provides a favorable channel for the emplacement of carbonatite magma by deep fracture,so the carbonate can quickly rise to the surface.It also indicated that the transition from basement formation to extensional system of the NCC tectonic occurred at about 1.8Ga,which is reliable evidence for defining the initial breaking time of the North China Craton.