Emplacement Mechanism And Tectonic Significance Of The Late Triassic Shahewan Rapakivi Texture Granite In East Qinling

Granite is one of the important components of continental orogenic belt,it appears in the whole process of orogenic belt evolution as well as the construction and destruction of crustal formation,which make it a clear clue to study continental orogenic events.As a special class of granite which usually indicates post-orogenic extensional or anorogenic environments,rapakivi granite had been widely studied in the last century.In Qinling orogenic,part of the Indosinian granites present rapakivi textures likewise,which influences the identification of regional tectonic nature during rock emplacement and causes many controversies.This paper consequently takes the Shahewan rapakivi texture granite in East Qinling as the research object and studies the mechanism of its emplacement as well as the tectonic background of the emplacement from the perspective of internal fabrics of the pluton combining with the rock magnetism researches and gravity investigations.The results of the study show that a total of 950 samples from 184 mining sites in the Shahewan pluton represent high average magnetic susceptibility values,which are basically greater than5000?SI and concentrated in 12000-16000?SI,indicating that the samples are ferromagnetic minerals dominated granites.The?-T curves,triaxial isothermal remanent magnetization and low-temperature magnetic fabric characteristics indicate that the samples are mainly controlled by ferromagnetic minerals which are mainly magnetite.The anisotropy(P_J)of the samples is mostly lower than 1.2,which reflects the characteristic of weak deformation,magma flow fabric and submagmatic flow fabric combining with the observation of microstructural,recording the process of magma emplacement.The Flynn diagram shows that the magnetic foliation of specimen is more developed than the magnetic lineation,the magnetic susceptibility ellipsoid parameter(T)is generally higher than 0,which indicates that the ellipsoids are mainly oblate;the magnetic fabric of the pluton is characterized by high-angle north-dip magnetic foliation and mid to low-angle,east-west and south-west dipping Magnetic lineation.The center of magma emplacement is determined by the centripetal direction of the magnetic foliation and the plunging direction of the magnetic lineation as well as the gravity simulation data,which indicates the magma rising from the middle of the pluton through dike and flowing to the other directions.The distribution of principal axis stereographic projections of a considerable part of the mining points is relatively scattered and there is no obvious stress distribution law.After discussing the reliability of the rock mass magnetic fabric and comprehensively analyzing the above experimental results,we get a further understanding of the emplacement mechanism and tectonic setting of Shahewan pluton,the emplacement center and emplacement flow direction of the pluton are identified.Moreover,we consider that the regional tectonic background of the emplacement is still a collision and convergence environment,and the process of emplacement is affected by the local tension caused by the Shangdan strike-slip fault and Shanyang-Fengzhen strike-slip fault in the same time.