北祁连加里东期俯冲-增生楔结构及动力学

北祁连加里东朝俯冲-增生楔可分为浅部和深部两个单元。浅部单元主要由蛇绿岩、蛇绿混杂岩及深海复理石所组成,极浅变质或没有变质。深部单元主要由HP/LT蓝片岩、透镜状的蛇纹岩、变辉长岩及绿片岩(主要为退变质产物)所组成。普遍遭受HP/LT变质作用和绿片岩相的退变质作用。两个单元同时形成于不同的构造层次,具有类似的原岩特征。在加里东期,俯冲-增生楔共经历4期变形作用(D₁,D₂,D₃,D₄)和3期变质作用(M₁,M₂,M₃).从D₁→D₄反映了俯冲-增生楔从俯冲作用→深部构造板底垫托作用→折返(构造顶蚀)→剥蚀的动力学演化过程。

FRAMEWORK OF NORTH QILIAN CALEDONIAN SUBDUCTION ACCRETIONARY WEDGE AND ITS DEFORMATION DYNAMICS

The North Qilian Caledonnian subduction accretionary wedge may be subdivided into deep and shallow unit.The shallow unit,which mainly consists of ophiolite,ophiolitic melange and marine flysch distributed in the front part of wedge undera very low metamorphic conditon,was formed by off - scrapping of marine sediments and mafic ocean crust.On the other hand,the deep unit,which mainly composed of HP/LT Blueschists,lenticuler serpentinites,meta-gabbros and greenschist rocks developed in the deep part of the subduction zone,is associated with tectonic underplating.The two units are coeval and produced at different levels with similar protoliths.During Caledonian period (460-410Ma) the subduction-accretionary wedge experienced four stage deformation(D₁,D₂,D₃,D₄) and three stage metamorphism (M₁,M₂,M₃).D₁ and D₂ are characterized by formation of thrust shear zone with a top-to-the-north(of NNE) sense of shear under HP/LT condition (M₁,M₂) and are associated with tectonic underplating in the subduction zone as a result of Qilian ocean subducting under Alashan Block.D₃ resuled in extension ductile shear zone concetrated on greenschist rocks (M₃,formed buy retro metamoorphism of HP/LT rocks) and indicated a top to the south normal sense of shear.During D₃ early ductile thrust shear zone was reactivated as a normal shear zone.Moreover,D₃ is linked to preservation of HP/LT rocks.In the region with less D₃ deformation,HP/LT mineral assemblges are well preserved,whereas in the extension ductile shear zone indicating strong D₃ deformation,D₁₊₂ deformation structure and HP/LT assmblages are only found locally as relicts,S₁ and S₂ are transposed by penetrative S₃,and HP/LT minerals are almost replaced by greenschist face mineral assemblges.However,the extension ductile shear zone related to D₃ led to exhumation(tectonic unroofing) of HP/LT rocks of its basewall.D₄ is defined by gentale folds without crenulation cleavages and may represent a higher level deformation during late exhumation.D₁→D₄ indicate dynamic envolution of the subduction-accretionary wedge from subduction to tectonic underplating at deep level,tectonic unroofing related to extension and thrusting and erosion.