大别-苏鲁区超高压(UHP)变质岩的多阶段构造折返过程

Multi-Stage Tectonic Exhumation Processes of Ultrahigh-Pressure(UHP) Metamorphic Rocks in the Dabie-Sulu Area,East-Central China

追溯和重塑超高压变质岩由100多千米地幔深度折返至上地壳及地表的过程,对理解会聚板块边缘及大陆碰撞带的运动学和动力学是极为重要的.主要依据构造学、岩石学、地球化学和可利用的地质年代学资料,结合区域多期变形分析,大别-苏鲁区超高压变质岩的折返过程至少可分解出4个大的阶段.块状榴辉岩记录了三叠纪(约250～230Ma)大陆壳岩石的深俯冲/碰撞作用.超高压变质岩早期迅速折返发生于超高压峰期变质作用(P>3.1～4.0GPa,T≈800±50℃)之后,处于地幔深度和柯石英稳定域,相当于区域D2变形期阶段.分别与区域变形期D3、D4和D5对应的折返过程,以及后成合晶、冠状体等卸载不平衡结构发育和减压部分熔融作用2个中间性构造热事件,均发生在地壳层次.网络状剪切带在折返过程的不同阶段和不同层次均有发育,标志着在超高压变质带内的变质和变形分解作用曾重复进行.着重指出,超高压变质岩的折返,主要是由大陆壳的深俯冲/碰撞和伸展作用控制的构造过程,且受到俯冲带内、带外诸多因素的约束,其中水流体就起关键作用.

Exhumation processes of ultrahigh-pressure（UHP） metamorphic rocks,by which these deep-seated rocks were rapidly returned from mantle depths of more than 100 km within deep subduction zones into the upper crust and to the surface,are of paramount importance for the understanding of the kinematics and dynamics of convergent plate margins and continental collision belts.At least four large successive stages of the exhumation process of UHP metamorphic rocks in the Dabie-Sulu area,east-central China,mainly based on structural,petrological,geochemical data and available geochronological data,combined with previously regional polyphased deformation analyses,can be distinguished.Massive eclogites recorded the deep continental subduction/collision between the Sino-Korean and Yangtze cratons during the Triassic（~250-230 Ma）.The early exhumation occurred after the peak of UHP metamorphism at conditions of up to 3.1-4.0 GPa and 800±50 ℃,which may correspond to the D2 regional deformation phase and took place at mantle depths,displaying the model of vertical ductile extrusion,within the coesite stability field.The subsequent exhumation stages,corresponding with D3,D4 and D5 regional deformation phase,respectively,and two intermediate tectono-thermal events,i.e.,the formation of the granulite/amphibolite facies symplectite or corona and the generation of extensive partial melting,occurred in crustal levels.Shear zones of an anastomosing rheological type were developed at diffterent stages and depths,indicating that the partitioning of deformation and metamorphism within the UHP metamorphic belt repeated.These imply that the exhumation of the UHP rocks is a complex tectonic process,in response to subduction/collision or extension of the continental crust,and is strongly constrained by a number of internal and external factors of the belt.In particular,fluids play important roles in the exhumation process of UHP rocks in the Dabie-Sulu area,east-central China.