大别-苏鲁超高压和高压变质带构造演化

大别—苏鲁是世界上超高压 (UHP) ( >2 .7GPa)和高压 (HP)变质岩石出露最为广泛的地区。通过区域研究 ,尤其是在选择的 30多个关键位置上不同尺度构造记录的深入观察 ,结合已有的可利用的变质、热事件及同位素年代学资料分析 ,揭示出它们曾遭受过一个复杂的从深俯冲到折返构造演化历程 ,识别出 5个主要的构造变质事件 :( 1)由块状榴辉岩中发育的微弱面理和线理所代表的第 1期变形变质事件 (D1) ;( 2 )面状榴辉岩中发育的含拉伸线理的透入性主面理、中小型鞘状褶皱及网络状韧性剪切带 ,代表第 2期构造变质事件 (D2 ) ;( 3)第 3期变形事件主体发生于麻粒岩 /角闪岩相后成合晶形成之后 ,主要构造记录是区域性陡倾斜面理及不均一置换的成分层、榴辉岩透镜体及布丁群、面理内褶皱、网状韧性剪切带系统以及减压部分熔融作用形成的混合岩和含榴花岗质岩石组构 ;( 4)区域性的碰撞期后地壳韧性薄化及剪张作用 (D4)形成缓倾斜角闪岩相主面理及线理、穹状及弧形构造和多层韧性拆离带 ,它们主导了现今观察到的大别—苏鲁超高压和高压变质带的区域构造几何图像 ;( 5 )第 5期构造热事件 (D5)表现为不均一断块抬升、红色沉积盆地发育及大规模的岩体和岩脉就位 ,代表造山晚期的构造揭顶及坍陷作用 ,该期构造控制着造山带

Tectonic evolution of Dabie-Sulu UHP and HP metamorphic belts, East-Central China

The Dabie-Sulu region in east-central China represents the most extensive ultrahigh-pressure (UHP) and high-pressure (HP) metamorphic belts in the world. A complex tectonic evolution is deduced from the structural records, at various scales, observed at more than 30 selected key localities in the UHP and HP metamorphic belts, underlining the episodic or progressive deformation partitioning, together with their metamorphic and thermal event features. The following sequence of main, distinct tectonometamorphic events is suggested: (1) the first event (D_1) is recognized from a weak foliation developed in the coesite- and microdiamond-bearing massive eclogite with a simple UHP metamorphic mineral assemblage; (2) the D_2 event is mainly inferred from a penetrative UHP eclogite facies main foliation bearing a stretching mineral lineation, mesoscale sheath-like folds and a network of ductile shear zones in the foliated coesite-bearing eclogite; (3) D_3 structures and fabrics which developed closely after the formation of granulite / amphibolite facies symplectites are characterized by a regional, penetrative, steeply dipping foliation and heterogeneous compositional layering, eclogite boudinages of various dimensions, intrafolial folds, and ductile shear zones forming an anastomosing pattern and leading to tectonic juxtaposition of the shear zone-bounded crustal slices; the D_3 deformation event was associated with decompressional partial melting and retrogressive metamorphism; (4) a post-collisional crustal thinning and transtension event (D_4) affecting the D_3 foliation and compositional layering is revealed by a regionally, gently dipping D_4 main foliation and stretching lineation, dome- and arc-shaped structures, low-angle detachment systems with different sliding directions; all of the structures took place prior to the intrusion of Late Mesozoic plutons, faulting and tectonic unroofing at shallow crustal levels (D_5); deformation/ metamorphism relationships indicate that the first two stages of ductile structure (D_1 and D_2), the early deformation, were related to Triassic subduction and collision between the Sino-Korean and Yangtze cratons at UHP (>2.7 GPa) and HP metamorphic conditions, whereas the D_3 and D_4 stages of ductile deformation were responsible for the exhumation of UHP and HP metamorphic rocks into the middle-upper crust driven by compression and later by extension, respectively; the D_4 structures dominate to some degree the map pattern of UHP and HP metamorphic rocks exposed in the orogen; the final stage of event (D_5) largely controls the present-day geomorphology of the Dabie-Sulu region. A new speculative tectonic evolution model for the UHP and HP metamorphic belts is proposed in the framework of the orogenic scale domain in terms of beyond plate tectonics—rheology and orogenesis of the continents. The model involves continental deep subduction and collision (250～230 Ma), and subsequent polyphase exhumation of the UHP and HP metamorphic rocks, that must be taken into account in future geodynamic and thermal modeling of the Dabie-Sulu orogen.