Study On The Genetic Relationship Between The Aegirine-Nepheline Syenite And The Fe-Nb-REE Ore In Bayan Obo

Aegirine Nepheline syenite is a type of syenite from the deep mantle,rich in potassium and sodium,silicon-depleted,calcium-rich and magnesium,over-alkaline and alkaline and strongly oversaturated.It is an important medium for us to study the genesis of deposits and regional tectonic events.In this paper,the aegirine nepheline syenite and the Bayan Obo Fe-Nb-REE ore in the Bayan Obo alkaline complex are selected for comparative study and analysis.The main purposes of this research are to study the nature of the source area of the aegirine nepheline syenite and the age of thermal events,to discuss the role of complex rocks in the process of mineralization of carbonate rocks,as well as to explore the genetic relationship between them and the Bayan Obo deposit which can provide support for the genetic mechanism of the Bayan Obo deposit.The Aegirine Nepheline syenite in the Bayan Obo alkaline complex is mainly composed of minerals such as potash feldspar,magnesia,neonite,nepheline,and biotite.From the perspective of rock geochemical composition,the SiO2 content is mainly concentrated between 62.37-62.82%.And the rock geochemical composition also shows alkali-rich content(content between 13.41-14.02%)and high potassium content(K2O / Na2 O = 2.69-5.08),Silicon-depleted,calcium-rich magnesium(CaO content between 1.78-2.66%,MgO content between 2.42-3.89%)and low FeO / MgO ratio(0.39-0.42),with M-type granite rock characteristics;Compared with the original mantle,the trace element data of the Aegirine Nepheline syenite shows a strong loss of such elements as U,Th,Sr,Ti,and a strong enrichment of such elements as Ba,Nb,Hf,Sm,Yb.What's more the standardized cobweb map of rare earth element chondrites shows the right declination of heavy rare earth elements and extreme enrichment of light rare earth elements and the source of magma is related to the depleted mantle,which may be in the process of ascent or emplacement Suffered from strong contamination of the earth's crust.Combined with the regional characteristics of the area and the previous research results,the paper believes that the 40Ar-39 Ar age of nepheline represents the thermal reconstruction time of the Late Paleozoic Permian magmatic rock intrusion event,rather than the diagenetic age of the aegirine nepheline.Based on the zircon U-Pb age of the Nixia syenite,it is preliminarily concluded that the age of the aegirine nepheline is about ~1.3Ga,and its tectonic-magmatic events may be related to the cracking of the Columbia supercontinent.