Metamorphism and microstructures along a high‐temperature metamorphic field gradient: the north–eastern boundary of the Královský hvozd unit (Bohemian Massif, Czech Republic)
Metamorphism and microstructures along a high‐temperature metamorphic field gradient: the north–eastern boundary of the Královský hvozd unit (Bohemian Massif, Czech Republic)
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沿高温变质场梯度的变质作用和微观结构：Královský hvozd 单元的东北边界（捷克共和国波希米亚地块）
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发表时间：
2002
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通讯作者：
D. Scheuvens
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作者：
D. Scheuvens
在中欧Variscides的Moldanubian带中研究了变质场梯度，从底部到顶部，分别包括十字石-蓝晶石带，白云母-硅线石带，K-辉晶石-硅线石带和K-辉晶石-堇青石带。在较高级别区域的深熔变沙坡岩中观察到的反应结构与基于无流体熔融反应的实验数据和成岩网格完全一致。根据结构和显微结构观察，可以得出结论，蓝晶石-十字石带与白云母-和钾长石-硅线石带之间的边界与变形机制的重要转换一致。除了轻微的同向重熔剪切（熔体增强变形）外，显微结构标准还指出：（a）变形机制从旋转重结晶（爬升调节位错蠕变）转换为石英中的棱柱滑移和高温（快速）晶界迁移（B），转换为石英-长石层中的扩散蠕变活动，以及（c）纤维岩-黑云母层中的强烈剪切调节应变。这表明，这些变形机制的任何组合都将深刻影响高级变质岩的流变特性，并显着降低岩石强度。因此，这些区域之间的边界标志着所研究横截面中的主要流变障碍，并且可能也在其他低至中压/高温区域中。在更高的变质等级（钾长石-堇青石区），达到流变临界熔体百分比，岩石流变学主要由熔体控制，其他变形机制次要。在研究区域，变形机制的转换是在顶向南逆冲推覆过程中高温上盘内大规模应变分配和变形集中的原因，从而在下盘岩石内保存了更完整的岩石结构记录，包括a？泥盆纪高压-中压-中温变质事件。逆冲作用伴随着钾长石-堇青石带的闪长岩的底辟上升和下盘的内褶，这表明在摩尔达努比亚带的这一部分发生了局部地壳翻转。
A metamorphic field gradient has been investigated in the Moldanubian zone of the central European Variscides encompassing, from base to the top, a staurolite–kyanite zone, a muscovite–sillimanite zone, a K‐feldspar–sillimanite zone, and a K‐feldspar–cordierite zone, respectively. The observed reaction textures in the anatectic metapsammopelites of the higher grade zones are fully compatible with experimental data and petrogenetic grids that are based on fluid‐absent melting reactions. From structural and microstructural observations it can be concluded that the boundary between the kyanite–staurolite zone and the muscovite‐ and K‐feldspar–sillimanite zones coincides with an important switch in deformation mechanism(s). Besides minor syn‐anatectic shearing (melt‐enhanced deformation), microstructural criteria point (a) to a switch in deformation mechanism from rotation recrystallization (climb‐accommodated dislocation creep) to prism slip and high‐temperature (fast) grain boundary migration in quartz (b) to the activity of diffusion creep in quartz–feldspar layers, and (c) to accommodation of strain by intense shearing in fibrolite–biotite layers. It is suggested that any combination of these deformation mechanisms will profoundly affect the rheological characteristics of high‐grade metamorphic rocks and significantly lower rock strength. Hence, the boundary between these zones marks a major rheological barrier in the investigated cross section and probably also in other low‐ to medium‐pressure/high‐temperature areas. At still higher metamorphic grades (K‐feldspar‐cordierite zone), where the rheologically critical melt percentage is reached, rock rheology is mainly governed by the melt and other deformation mechanisms are of minor importance. In the study area, the switch in deformation mechanism(s) is responsible for large‐scale strain partitioning and concentration of deformation within the higher‐temperature hanging wall during top‐to‐the‐S thrusting, thus preserving a more complete petrostructural record within the rocks of the footwall including indications for a ?Devonian high‐ to medium‐pressure/medium‐temperature metamorphic event. Thrusting is accompanied by diapiric ascent of diatexites of the K‐feldspar‐cordierite zone and infolding of the footwall, suggesting local crustal overturn in this part of the Moldanubian zone.