Mesozoic Contractional-Extensional Structures Of Yunmeng Mountain,Northern Beijing:Constraints From Tectonic Deformation And Magmatism

Large-scale lithospheric thinning and regional crustal extension take place in the North China craton during late Mesozoic.Under this condition of regional extension,a series of extension structure have developed in the north China craton,Yunmeng Mountain metamorphic core complex is a typical example in the Yanshan area.The study of the extension structure is helpful to explain the lithospheric thinning of the North China craton and the further recognition of the Yanshan orogenic belt from the crustal extension and detachment.The structural elements of Yunmeng area mainly contains Sihetang anticline nappe,Sihetang ductile shear zone,Yunmeng Mountain antiform,Hefangkou normal fault and Shuiyu ductile shear zone.Through the study of structural characteristics of different dikes based on the flow mechanism of the middle-lower crust and the intracrystalline deformation of the minerals.Dikes can be divided into three categories:Pre-shearing dykes,syn-shearing dykes and post-shearing dykes.The north part of Sihetang area,Yunmeng Mountain rock mass and Miyun diorite are all subjected to shear.They all develop a mass of A fold and boudin structure.the southeastern part of the rock mass are granitic mylonite,the orientation of the regional lineation plunge SE-NW.A lot of ductile deformation fabric is developed and show progressive deformation.Shicheng area develop large amount of strike-slip fabric caused by uplift of the dome.Sampled five samples of syntectonic granite dikes in this Sihetang shear zone,th e zircon dating ages are 161,8±1.8Ma,156.3±1.6Ma,150.0±1.6Ma,161.7±1.4Ma and 151.0 ±3.0 Ma,respectively.Sampled two samples of syntectonic dikes in the Shuiyu d uctile shear zone,the zircon dating ages are 160.3 ±3 Ma and 148.7 ±3.5 Ma.The bioti te40 Ar-39Ar isotope ages of Yunmeng Mountain granodiorite range from 119.1Ma t o 126.6 Ma,reflecting the cooling ages of Yunmeng Mountain rock mass range from 119.1 Mato 126.6 Ma.Comprehensive the above tectonic and magmatic results,Sihetang shear zone has already started to thrust in the 160-150Ma of the late Jurassic,until about 150Ma;Yunmeng Mountain rock mass ross in 150-140Ma,there appears dykes in Sihetang area and Shuiyu area,which are not deformed;After 127 Ma,the Yunmeng Mountain rock mass begins to exhume and the Shuiyu ductile shear zone develops,the dikes of Shuiyu and Sihetang have different degree of deformation,part of the dikes fold and forming the sausages.