Characteristics And Origin Of Hongshan Syenite,Wuan Area,Hebei Province

The Hongshan rock outcrops on the south wing of Taihang Mountain,south of Wuan area,Handan City,Hebei Province.There are 8 important intrusive rock bodies in the district.There are many similarities such as formation ages and mineral compositions among these intrusive rock bodies.Most of which are basic,intermediate and alkaline igneous rocks.The intrusive rocks presents as a magmatic cycle.From west to east,the petrological changes from intermediate-basic to basic to intermediate-alkaline as zonation,which demonstrateds the magma differentiates with time evolution.Hongshan rock is mainly composed by alkaline syenite,diorite and monzonite,as well as early gabbro.The rock body emplaced in Carboniferous-Permian strata.TheSiO2 content of in syenite of Hongshan are among 63.5%to 65.01%,averagely 63.68%.Content of total iron is 34.90%in average,while the MgO,K2O,and Na2O K2O put together contents are 0.628%,5.926%and 12.118%in average.The ALK content is at medium level.All samples of syenite in Hongshan rock are belong to K alkaline and calc-alkaline series,shoshonite(SHO)series.The Log[CaO/(Na2O+K2O)]-SiO2 diagram shows that the syenite formed through an extension environment.Zircon U-Pb dating datas through LA-ICP-MS confirm that,intrusive magnatism time horizon from syenite of Hongshan rock was 120.1±4.6Ma.Total rare earth elements(REE)contents of syenite of Hongshan rock varies from 74.84 ppm to 103.983 ppm,which are 29.21 to 40.59 times of chondrites.Values of N(La/Yb)are from 21.19 to 25.93,which comfirms the fractionation differences of hing and light REE are low.Furthermore,rich in LREE and no significant negative Eu anomalies REE patterns demonstrate that magmatism is characterized by equilibrium crystallization and no dominant crystal fractionation.It can also infer that there are no plagioclase in the residual mineral assemblages of source rock since there are no obvious Eu negative abnormality.Deduced by amphibolite dehydration melting phase diagram,it can also infer that the residual mineral assemblages of source rock are clinopyroxene,garnet and/or hornblende.The pressure environment should among 1.5GPa to 2.OGPa,which means the depth of magma source regions should be greater than 48 kilometers.Combined with the age of the Hongshan rock,which is in the third stage of Yanshanian period,it suggested that Hongshan syenite should be formed in the post-orogenic extension environment and a stretching stage,and through the partly melting of crust-thickening process.