Geological Characteristics And Research Significance Of Maficdykes In Longshoushan,Gansu Province

The Longshoushan area is located in the southwestern margin of the North China Craton,which on the southern margin of the Alashan block.It is an important metallogenic belt of copper and nickel and other polymetallic deposits in China.The predecessors carried out a series of geological research and prospecting work in the area,but the study of maficdykes in this area is not systematic.The maficdykes of Longshoushan are selected as research objects,through field survey,petrography,elemental geochemistry,Sr-Nd-Pb isotope and Ar-Ar dating.Based on the systematic study of lithology,the geochemistr,chronology,and the relationship between tectonic setting,source area,magma evolution and metal deposits were discussed.Some understanding and results as follows,the maficdykes in the Longshoushan area were produced in veins and distributed in three sections,that is East,Central and West.The main minerals of the rock are plagioclase,hornblende,pyroxene and biotite,and the phenocrysts are mainly composed of amphibole and pyroxene.The matrix is mainly plagioclase,and a small amount of biotite is visible.The accessory minerals are magnetite,apatite and zircon.All of the maficdykes have the characteristics of evolutionary basalts.The contents of ALK(K2O+Na2O)= 3.26%~8.43% are higher.From east to west,the content of Na2 O are decreasing and the content of K2 O are increasing.The U content in the rock is significantly higher than the average value of the continental crust and the continental lithospheric value rock,and it gradually increases from east to west,Relatively enriched with large ion lithophile elements such as Rb,Pb,Ba,and Ce,relatively depleted HFSE such as Ta,Th,Nb,Yb,and Zr.The total REE of the rocks is higher than that of the original mantle.The fractionation of LREE and HREE were obvious,which is enriched with LREE,and Eu shows weak anomaly.The diagenetic ages of basic rocks in the eastern part of Longshou Mountains are 400.6 Ma~423.5 Ma,those in the middle part are 362 Ma~485.6 Ma,and those in the western part are 237.2 +2.6 Ma.The study suggests that the tectonic setting of the maficdykes in the Longshoushan area is in an extensional plate environment,and the magma originated from the mixed zone of EMI-type enriched mantle and EMII-type enriched mantle.The magma evolution process experienced crystallization differentiation and partial melting,and the magma was contaminated by crustal materials during its ascending emplacement.There may be no direct genetic relationship between basic rock and uranium mineralization,but it may be activated and modified for the enriched ore of uranium.