Structure and Rheological Evolution of UHP and HP Metamorphic Belts in the Tongbai-Dabie-Sulu Region, China
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摘要: 在岩石圈流变学基本原理指导下, 运用现代构造解析学方法, 在不同尺度上判别和分析了桐柏—大别—苏鲁UHP和HP变质带内深俯冲、同碰撞构造及UHP和HP岩石折返过程中的变形特征, 重点讨论同碰撞形成的高角度网结状榴辉岩相剪切带阵列、高角闪岩相剪切及有关变形组合以及碰撞期后伸展韧性薄化变形样式, 强调指出不同地壳层次和物理条件下变形分解作用的重要性, 而且, 在UHP和HP变质带内最有效的应变体制是剪切作用, 并在三维空间上形成不同格式的剪切带网状系统.以构造学记录为主线, 结合已有可利用的岩石学、变质作用pT轨迹和同位素年代学资料, 提出一个UHP和HP变质带尺度上的流变学演化模式, 其中, UHP和HP变质岩石由地幔深度折返到地壳表层, 经历了楔状挤出、碰撞期后地壳韧性薄化及晚造山伸展塌陷、揭顶作用等多个阶段的动力学过程.Abstract: The basic principles of lithospheric rheology and the modern analytical tectonics are used to examine deformational features of the rocks and the processes responsible for their formation and exhumation in the UHP and HP metamorphic belts within the Tongbai-Dabie-Sulu region, China. The discussion is concentrated on high-angle, anastomosing shear zone arrays which were formed under UHP eclogite facies conditions, and related to the Triassic (250-220 Ma) collision between the Sino-Korean and Yangtze cratons, and shear zone assemblages accompanied, following the collision, by decompressional partial melting, formed by a compressional regime on upper amphibolite facies conditions, as well as deformation pictures formed by ductile extension flow and thinning on amphibolite facies conditions, mostly between 200 Ma and 170 Ma. It is suggested that the deformation partitioning plays an important role in the deformational processes for the rocks within the UHP and HP belts, at different crustal levels and on different physical conditions. The efficient strain regime in the UHP and HP metamorphic belts is shear, which is generally localized in networks of ductile shear zones on all scales. On the basis of structural records mainly and combined with the previous data on petrography, pT paths of UHP metamorphic rocks and geochronology, a speculative rheological evolution model for the UHP and HP metamorphic belts is proposed. In this model, 250 to 220 Ma is considered as the most reliable estimated age for the occurrences of both the UHP and HP metamorphism and the continental collision between the Sino-Korean and Yangtze cratons. The major consequence of this collision was the development of a crustal-stacking wedge. The UHP and HP metamorphic rocks were exhumed from the mantle depth to the surface during a multistage history, including wedge intrusion (220-200 Ma), crustal ductile thinning and extension flow (200-170 Ma) and late orogenic collapse and unroofing.
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图 2 露头尺度的横剖面, 表示(a) 具小的石榴橄榄岩透镜体的块状榴辉岩与面理化榴辉岩的几何关系, 碧溪岭; (b) 面理化榴辉岩、高压片麻岩及退变质榴辉岩与面理化花岗岩的几何关系, 荣城市大疃
a: 1.石榴橄榄岩; 2.块状榴辉岩; 3. 面理化榴辉岩.b: ①面理化榴辉岩(Ec); ②退变质榴辉岩; ③高压片麻岩; ④面理化含榴花岗岩; ⑤角闪岩相剪切带
Fig. 2. Schematic cross-sections on the outcrop scale showing the geometry between massive eclogite with minor garnet peridotite lenses and foliated eclogite (a), and the geometry between foliated eclogite, retrograded eclogite and high pressure gneiss, and foliated garnet-bearing granite (b)
图 3 超高压单位内部的透入性D3面理(石马)
长椭圆形灰色体为榴辉岩或退变质榴辉岩, 该处D3面理产状为NE40°∠78°
Fig. 3. Sketch showing the penetrative D3 foliation in the UHP unit, Shima, Anhui Province
图 4 大别山超高压和高压变质带内碰撞期后拉伸线理
BMXF.八里畈-磨子潭-晓天断裂带; TLF.郯城-庐江断裂带; XGF.襄樊-广济断裂带; CC.核杂岩单位; UHP.超高压单位; HP+EB.高压及绿帘-蓝片岩单位
Fig. 4. Map of stretching lineations formed mainly under amphibolite facies conditions in the UHP and HP metamorphic belts, Dabie Mountains
图 5 桐柏—大别—苏鲁超高压和高压变质带流变学演化模式
(a) 深俯冲和碰撞阶段, 超高压和高压变质岩形成, 出现地壳堆垛楔; (b) 超高压和高压变质岩初期折返阶段, 形成挤出楔, 伴随减压部分熔融作用; (c) 第二阶段折返, 以碰撞期后地壳伸展和薄化作用为特征; (d) 第三折返阶段, 晚造山塌陷伴随大规模岩浆岩活动.GMF.龟梅断裂带; BMXF.八里畈-磨子潭-晓天断裂带; UHP.超高压单位; HP.高压单位; EB.绿帘-蓝片岩单位; SC.沉积盖层; Er.二郎坪群; C.石炭系; D+N.定远组及南湾组
Fig. 5. Schematic diagrams showing a model for the rheological evolution of the UHP and HP metamorphic belts in the Tongbai-Dabie-Sulu region
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