Discussion On The Construction Properties Of Carbonatites

Carbonatite is a rare type of the igneous rock on the earth,this paper summarize the Carbonatite research of the world from the aspects of field geology,geomorphology,petrology,mineralogy,petrography,geochemistry and the origin of magma and tectonic environment,with the outcrop of Carbonatite in Dodarmo region of Qinghai Province for the example.The Carbonatite in Dodarmo region is closely associated with the sodium alkalinity basalt without obvious crustal contamination in the same area,and the Carbonatite always appears to be the sill shaped as the interbedded layer in the red bed or the ring shaped intrude into the red bed,and the Carbonatite is mainly composed with dolomite carbonatite,dolomite carbonatitic breccia ignimbrite,dolomite carbonatitic ignimbrite,dolomite carbonatitic breccia tuff,dolomite carbonatitic tuff,carbonatitic peperite,etc.The whole-rock chemical composition is characterized by high Mg^#,high CaO and low Na₂O+K₂O.In the CaO-MgO-(FeO+Fe₂O₃+MnO)projection graph,the Carbonatite is divided into two types:Magnesium Carbonatite and Calcium Large ion lithophile element Sr,Rb,Ba,U are relatively enrichment and high field-strength element Nb,Ti,P are relatively depleted.The concentration of Sr and Ba vary greatly.Total rare earth elements concentration is low(TREE=4.83×10^-6-85.3×10^-6).6 samples have slightly Eu negative anomaly(?Eu=0.72?0.91)and slightly Ce negative anomaly(?Ce=0.81?0.99).Light and heavy rare earth elements separation is not strong((La/Yb)N=1.12?12.76).The characteristic of the geochemical data analysis that high Mg^#data indicate the feature of mantle origination and low total rare earth elements concentration indicate the feature of earthcrust origination indicate that the magma source of the carbonatite in Dodarmo region is characterized by mixture of mantle source and crustal source.The zircon of the Carbonatite and the captured zircon of the basalt are all the magmatic zircon,and with the weighted mean age between 230Ma and 245Ma,this represent the Late Triassic magmatic event.There is one zircon of the basalt sample with U-Pb age of 66±1Ma,and the ⁴⁰Ar-³⁹Ar dating result of the basalt is 83?98Ma,and there is no obvious magmatic events since the magmatic event in Late Triassic in research area,so the basalt magmatic event should occur in Late Cretaceous.The U–Pb dating of the basalt and the Carbonatite in research area coincide with the age of the Bangong Lake-Nujiang suture belt's close that last from Late Jurassic to Early Cretaceous,the Dodarmo region was in the extensional condition in the back arc,and forms the continental rift basin.The extension and thinning of the lithosphere result in the upwelling of asthenosphere,and the relatively low pressure condition lead to the immiscible action of silicate magma and carbonate magma.The upwelling of asthenosphere release a lot of heat witch drive the carbonate strata in the crust melting or partial melting and mixed with the carbonatite magma,and then invaded into the strata or erupted to form the outcrop of the Carbonatite in the field.