Petrogenesis Of Mafic Dyke And Tectonic Implications Within Xiabaiyao,shangyi- Mashikou, Huai'an County, Northern Of Hebei Province

Mafic dyke is a kind of tectonic-rock assemblage formed by the emplacement of mafic magma into pre-existing fissures.Mafic dyke swarms are important signs of crustal extension and fragmentation.There are a large number of mafic dykes emplaced in Khondalite of Xiabaiyao and TTG gneiss of Mashikou,within northern of Hebei province.The two areas are separated by fault zones.The northwest part of the fault zone is Khondalite,and the southeast part of the fault zone is TTG gneiss.The surrounding rock of mafic dyke swarms in Xiabaiyao area are Khondalite,the dykes are mainly distributed in Friendship Reservoir,Changshengwan,Wanghuayao,Beicaonian,Dongshang,Paoshouyao and Xuanfuyao,which consist of4²0m-wide NNW?335�³40�?and SEE?105�¹10�?-trendinging dykes,the dyke swarms are most NNW-trending.The dyke swarms are mainly diabase with typical diabase structure,fine-grained to medium-grained,mineral assemblages are plagioclases?55%?+clinopyroxenes?25%?+Fe-Ti-oxides?10%?+apatites?7%?+biotites?3%?.Plagioclases are mainly andesine-labradorite with normal ring structure,clinopyroxenes are ordinary pyroxenes,and Fe-Ti-oxides have the reaction rim texture of biotites..The contentes of dykes have 2.23wt.%TiO₂,14.7wt.%TFe₂O₃,and 4.1wt.%MgO.The magnesium index?Mg^#?is about 35⁴1,and the solidification index?SI?is about 15²1.The LREE of rock are relatively high(163.57�10^-6258.26�10^-6),the LILE?Rb?Ba?K?are relatively enriched,and the HFSE?Nb?Ta?are relatively deficient.The Zircon LA-ICP-MS U-Pb age of the dyke shows that its average age is 1771�24Ma,which represents the emplacement age of the dyke.The surrounding rock of mafic dyke swarms in Mashikou area are TTG gneisses,the dyke swarms are mainly distributed in Xiajiaying,Ershidun,Shagou,Liugou,Ximalin Reservoir,Xinyaozi,Chelinggou and Songgou,which consist of 3²0m-wide NNW?325�³40�?-trendinging dykes and extends far.The dyke swarms are mainly diabase with typical diabase structure,medium-grained,mineral assemblages are plagioclases?50%?+clinopyroxenes?25%?+Fe-Ti-oxides?10%?+apatites?7%?+amphibo les?5%?+biotites?3%?.Plagioclases are mainly labradorite-andesine with normal ring structure,clinopyroxenes are ordinary pyroxenes,clinopyroxenes have the reaction rim texture of amphiboles,and Fe-Ti-oxides have the reaction rim texture of biotites.The contentes of dyke swarms have 2.62wt.%TiO₂,15.24wt.%TFe₂O₃,and 3.64wt.%MgO.The magnesium index?Mg^#?are about 29³2,and the solidification index?SI?are about 14²1.The LREE of rock are relatively high(128.64�10^-6293.16�10^-6),the LILE?Rb?Ba?K?are relatively enriched,and the HFSE?Nb?Ta?are relatively deficient.The Zircon LA-ICP-MS U-Pb age of the dyke shows that its average age is1768�22Ma,which represents the emplacement age of the dyke.Comprehensive comparative analysis shows that the mafic dyke swarms in Xiabaiyao and Mashikou areas have many similar or identical properties and should be the same mafic dyke swarms.The Th/Ta and La/Yb of mafic dykes are between3¹0 and 10¹5 respectively.The dyke swarms may originate from the enriched mantle and could be assimilated and contaminated by the lower crust.The Mg^#?SI vs.MgO in the mafic dyke swarms are positively correlated,indicating that the magma underwent significant crystallization fractionation.The TFe₂O₃?TiO₂ vs SiO₂ of the mafic dyke swarms show significant negative correlation.The magma has undergone the evolution trend of Fenner and generated Fe-rich mafic magma.According to the field characteristics of the dyke swarms and the analysis of geochemical data,the dyke swarms should be formed with the breakup of tectonic intraplates.The emplacement epoch of the mafic dyke swarms are between ca.1780 and ca.1760 Ma.Combined with the magmatic events of the North China Craton,the mafic dyke swarms should belong to the ca.1780 Ma Taihang dyke swarms event,forming a tectonic environment for intraplate extension and breakup,which may be the product of the initial breakup of the Colombian supercontinent.