Magnetic Fabric Characteristics And It Tectonic Implications Of Zheduoshan Pluton,Eastern Tibetan Plateau

The Zhedoshan pluton exposed along the Xianshuihe fault zone is a rare Cenozoic granite in the eastern margin of the Qinghai-Tibet Plateau.Previous studies mainly focued on the geochronology and geochemistry of the granites to provid important evidence for the interpretation of tectonic attributes,evolution history and geodynamic setting of eastern Tibet.However,the emplacement mechanism of granite and related structures have been rarely reported,which makes many research viewpoints unconsistent.It is also regard as a key area to explore the relationship between emplacement and the regional tectonics.Therefore,in this paper,combined with previous studies,a multidisciplinary structural study was conducted on Zhedoshan pluton.On the basis of field geological survey,the petrography and geochronology of the Zhedoshan pluton,a magnetic fabric study of the pluton and the structural rocks has been applied to determine the emplacement mechanism and formation setting of the Zhedoshan granite.According to petrography and the contact relationship between different pluton,the Cenozoic Zhedoshan granite pluton can be divided into three main sequences: Zhedoshan unit,Longbugou unit and Mugecuo unit.Meanwhile,thermochronological data show that the emplacement ages of Zhedoshan unit,Longbugou unit and Mugecuo unit are 21.52±0.30 Ma,19.59±0.16 Ma and 16.50±0.18 Ma,respectively.The magmatism in the Zhedoshan region lasted to 4.88±0.88 Ma.In addition,the earlier protolith ages of 168.6±1.8 Ma and 30.9±1.7Ma are obtained from the granitic mylonite.The magnetic susceptibility anisotropy(PJ)of the granites in Mugecuo unit and Longbugou unit ranges from 1.014 to 1.514 and 1.034 to 1.205,respectively,and the PJ values of most samples are lower than 1.2.The average PJ value of granite in Mugecuo unit is 1.096(<1.2)but the PJ value of the mylonite and strongly deformed Triassic country rock is generally higher than 1.2.The magnetic susceptibility anisotropy is roughly high near the fault zone and lower inside the pluton.The susceptibility ellipsoids is mainly oblate,some of which have the characteristics of simple shear strain,suggesting the extrusion environment.Based on the study of magnetic fabric,in conjunction of spatiotemporal coupling between the Zhedoshan pluton and the Xianshuihe fault and microstructure characteristics,it is clear that the Zhedoshan pluton is a syntectonic granite pluton in the Xianshuihe active fracture zone.Although the flow-related magnetic fabric was affected by deformation,it can roughly identify the emplacement information of the pluton.The magnetic foliation near the fault of the pluton is parallel to the fault or intersect at a small angle,but it is more disordered inside the pluton,suggesting that the magma may linearly ascend along the fault zone,and fill the inner space disorderly at a certain depth.The orientation of the magnetic lineation indicates that the magma moved roughly from south to north.The the emplacement of the granite in Zhedoshan unit occurred in the transition stage from dextral strike-slip to sinistral strike-slip and local tension zone formed provides space for the magmatic emplacement in this period.The granites of the Longbugou unit and Mugecuo unit were formed under condition of the nearly E-W strong compression.In this stage,due to the sinistral strike-slip movement and clockwise tectonic rotation of the Xianshuihe fault zone,the tension zone was further developed,and magma emplaced in this area on a larger scale.Since the Himalayan period,the crust partially melted and flowed eastward to accumulate in the Chuandian block on the west side of the Xianshuihe fault zone because of the uplift of the plateau and the thickening of the crust.The eastward expansion of the Tibet Plateau promoted the activation of the Xianshuihe fault zone,forming a local tension zone in the Zheduoshan area,which not only provides a window for the upwelling of high-temperature crustal fluid,but also facilitates the upward conduction of heat,causing the in-situ-melting of the crust,which also upwelling along the fault.Therefore,the Zheduoshan pluton is the result of the combined effect of surface geology and deep geology of the Qinghai-Tibet Plateau,and there are at least two mechanisms during ~21.5-4.88 Ma.