Evolution of microstructure and melt topology in partially molten granitic mylonite: Implications for rheology of felsic middle crust
Evolution of microstructure and melt topology in partially molten granitic mylonite: Implications for rheology of felsic middle crust
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DOI：
10.1029/2007jb005508
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发表时间：
2008-10
影响因子：
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通讯作者：
K. Schulmann;J. Martelat;S. Ulrich;Ondrej Lexa;P. Štípská;J. Becker
中科院分区：
文献类型：
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作者：
K. Schulmann;J. Martelat;S. Ulrich;Ondrej Lexa;P. Štípská;J. Becker
[1]中地壳斑状花岗岩的变形研究表明，与较强的石英相比，长石集合体的应变强度异常高。与变形花岗岩演化阶段相对应的显微构造有三种类型：(1)以斜长石粘性流动为特征的变质花岗岩，(2)以重结晶斜长石和钾长石条带状糜棱岩结构发育为特征的石英金长片麻岩，(3)以石英带状交替和长石与石英混合集合体为特征的带状糜棱岩。碱性长石的原始弱化是通过非均相成核过程分解成钠长石链和钾长石而实现的。随后碱性长石的崩解和单矿物层理的发育可归因于与∼2%熔体有关的Mu-Bi层的同形变脱水熔融的开始。最后的变形阶段是以长石的混合为标志的，这是由于外部水的引入而产生了更高的熔体产量。由于熔体连接性阈值效应触发了晶界滑动变形机制，已经很少的熔体是导致长石极度弱化的原因。晶界滑移在小熔体分数时控制扩散蠕变，在高熔体分数时演化为颗粒流动。强烈的石英在整个变形历史中表现为位错蠕变变形机制，其特征是滑移系的活动变化，这归因于变形长石的流变性改变导致应力和应变速率分配的变化。
[1] The deformation study of midcrustal porphyritic granite reveals exceptionally high strain intensities of feldspar aggregates compared to stronger quartz. Three types of microstructures corresponding to evolutionary stages of deformed granite were recognized: (1) the metagranite marked by viscous flow of plagioclase around strong alkali feldspar and quartz, (2) quartz augen orthogneiss characterized by development of banded mylonitic structure of recrystallized plagioclase and K-feldspar surrounding augens of quartz, and (3) banded mylonite characterized by alternation of quartz ribbons and mixed aggregates of feldspars and quartz. The original weakening of alkali feldspar is achieved by decomposition into albite chains and K-feldspar resulting from a heterogeneous nucleation process. The subsequent collapse of alkaline feldspar and development of monomineralic layering is attributed to the onset of syn-deformational dehydration melting of Mu-Bi layers associated with production of ∼2% melt. The final deformation stage is marked by mixing of feldspars which is explained by higher melt production due to introduction of external water. An already small amount of melt is responsible for extreme weakening of the feldspar because of Melt Connectivity Threshold effect triggering grain boundary sliding deformation mechanisms. The grain boundary sliding controls diffusion creep at small melt fraction and evolves to particulate flow at high melt fractions. Strong quartz shows a dislocation creep deformation mechanism throughout the whole deformation history marked by variations in the activity of the slip systems, which are attributed to variations in stress and strain rate partitioning with regard to changing rheological properties of the deforming feldspars.