| The southern margin of the North China Craton,Taishan-Yanshan orogenic belt is one of the regions where alkaline rocks develop in China,and is dominated by syenite.Recently,there is a local obvious hydrothermal copper(gold)mineralization in the interior of the rock.Petrogenesis,magmatism,rare elements,rare earths elements,radioactive elements and the possibility of gold or other precious metal mineralization has attracted the attention of domestic and abroad scholars.In this paper,field geologic surveys,indoor petrology,the major-trace elements of rocks and single-particle zircon U-Pb isotopic dating and Hf isotopic dating of the Hongshan alkaline rock mass in Yongnian County were carried out,and the composition of the facies,the geochemical characteristics and the age of formation,and the constraints of the genesis were systematically demonstrated,and magmatic evolution on the mineralization were discussed.The main achievements and progresses achieved were as follows.(1)The zircon U-Pb isotope dating of Hongshan alkaline rock shows that the age of fine-grained hornblende biotite syenite in the deep igneous facies is 126±2.1 Ma.The age of fine-grained biotite hornblende monzonite is 128.6±2.2Ma;the age of altered syenite is 131.35±0.86 Ma.The age of the main syenite in the rock(126Ma)reveals that magma occurs in 126-128 Ma.(2)The main elements of Hongshan alkaline rocks are characterized by the evolution of partial-aluminum and calc-alkaline magmatism.The light rare earth elements are rich and there is a weak positive Eu anomalies;they show enrichment in LILEs such as Cs,Rb,Ba,K,Sr,Pb and strong depletion in HFSEs such as Nb,Ta,P,Ti etc.Biotite hornblende monzonite appear Th,U,Zr,Hf enrichment.(3)The zircon ?Hf(t)=-22.90~-12.80 in the black hornblendeite of the Hongshan alkaline rock mass,combined with the existing Sr-Nd isotopic composition,reveals a rich base-based magmatic intrusion originating from the type I enrichment mantle.As a result,the lower crust melted to form a magma,which was mixed with the Mesozoic lower crust and sedimentary rocks to form alkaline magma.After magmatic crystallization,multiple stages were in place and eventually formed Hongshan alkaline rock.(4)During the evolution process of Hongshan magma,W and Bi are higher in the middle and late stages of magmatic evolution process,and Au,Ag,Cu,Mo,and Sb are significantly higher in the late stage;the evolution is shown to produce W,Bi,etc.in the middle stage.Elemental mineralization or metallogenic characteristics,but in the late stage there are the elements of Au,Ag,Cu metallogenic features,especially the elements of Ag.(5)The Hongshan alkaline rock mass is located on the southern margin of the North China Craton.The early post-Cretaceous Paleo-Pacific plate subduction was followed by a post-arcing environment,a strong stretching effect induced mantle upwelling of the asthenosphere.Thus,the enriched lithosphere mantle melted partially and formed alkali-rich basaltic magma,then the magma invaded the crust during the magmatic invasion,and mixed with the lower crust material,and finally formed a series of magmatic rocks through crystallization differentiation in the southern section of Taihang Mountain. |
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