Micostructural Characteristics And Constraints On Deformation Temperature Of Ductile Shear Zone In Lianyungang,North Sulu Area

The Lianyun'gang area,lying at the western part of the Sulu High-Ultrahigh Pressure Metamorphic Belt,is an essential part of the Dabie-Sulu HP-UHP Metamorphic Belt which was formed during the subduction between the Yangtze Plate and the North China Plate.The Wulian-Yantai fault,which is the north boundary of Sulu HP-UHP Metamorphic Belt,is adjacent to the North China plates,while Jiashan-Xiangshui fault,the south boundary of this belt,is adjacent to the Yangtze plate.Two generations of deformation were recorded in the ductile shear zone are compared in multiple aspects in this paper according to the analysis of the macroscopic scale deformations in the field,the microscopic structure as well as the EBSD measurement of the quartz crystallographic preferred orientation in mylonite.Furthermore we represent structural framework and the evolution in the early Cretaceous of the Lianyun'gang area.The ductile shear zone in the Lianyun'gang area,where the geometrical structure is a dome,whose strike being NE-SW currently was generated by the alteration of the shear strength in the late stage of the exhumation.The direction of the foliation is NW and the stretching lineation is tend to NWW,while the asymmetric layer folds indicate the direction of decline is NW in the north of Shilianghe reservoir.But in the south of the reservoir,the direction of the foliation is SE and the stretching lineation is tend to SE,while the asymmetric lens indicate the direction of thrust is NW.Two generations of deformation were recorded in the ductile shear zone: the early high-temperature shearing deformation(D1),in which characteristics of ductile deformation is exhibited lithologically,which include dynamic recrystallizationdominated by the migration of the margin of quartz as well as the ductile extension of feldspar accompanied by mineral assemblages of the amphibolite facies;the late low-temperature shearing deformation(D2),where lithological characteristics of ductile-brittle deformation is presented,including the brittle deformations of rigid ruptures in feldspar,the oriented distribution of phengite accompanied by greenschist-facies metamorphism.From the core in this dome to the alar parts,the behavior of metamorphic faces is from amphibolite facies turn into greenschist-facies and the environment of deformation is from high-temperature turn into low-temperature.Meanwhile,the characteristics of the deformations of two generation in the shear zone are manifested in this paper and the deformation temperature is limited by the Ti-in-muscovite geothermometer and Ti-in-biotite geothermometer: the temperature of the generation D1 can be limited to be t1=505~523?,p1=0.50~0.70 GPa.The upper bound temperature of generation D2 can be limited to be t2=448 ~ 493 ?,p2=0.30~0.35 GPa.Furthermore,by comparing with the shear zone in Wulian area,we believe that the deformation in these 2 generations could be relavant to the extensional structure in Mesozoic.At the same time,we establish the extensional model of Lianyungang area,which is accordant with the extension of Wulian area.