Deformation of granitoids at low metamorphic grade. II: Granular flow in albite-rich mylonites
Deformation of granitoids at low metamorphic grade. II: Granular flow in albite-rich mylonites
复制标题
DOI：
10.1016/0040-1951(93)90164-f
复制
发表时间：
1993-05
期刊：
影响因子：
2.9
通讯作者：
H. Stünitz;J. Gerald
中科院分区：
文献类型：
--
作者：
H. Stünitz;J. Gerald
三种研究的花岗岩在低变质等级下的高应变变形主要由富含钠长石的多相聚集体的颗粒流主导。这些聚集体是由中间斜长石和钾长石的逆行分解反应形成的。随着晶粒尺寸的减小，花岗岩中的主要变形机制发生变化：粗晶（低应变）实例通过晶内塑性（石英）和断裂（长石）的组合而变形。在糜棱岩中，石英的晶内塑性仅起次要作用，主要变形机制是主要由钠长石和石英组成的多相聚集体的非碎裂颗粒流。变形似乎是稳定的，可能是因为钠长石-石英混合物中的晶粒生长受到抑制。这些细粒骨料的机械性能比纯石英骨料弱。因此，主要由长石分解反应引起的变形机制的变化对于花岗岩和其他以长石为主的岩性在低变质等级下的高剪切应变变形的定位似乎很重要。以钠长石为主的矿物聚集体的流变行为可能对以长石为主的中上地壳变形有两个后果：（1）长石，在含水流体的存在下，可能是一种重要的变形。矿物控制低变质等级的流变性，即低于角闪岩相 P-T 条件； （2）低品位花岗岩中细粒多相聚集体的颗粒流的发生可能很常见，并且在基于石英等单一矿物的晶内塑性流动定律的中上地壳流变学建模中需要非常小心。
The high strain deformation at low metamorphic grades of three investigated granitoids is dominated by the granular flow of albite-rich polyphase aggregates. These aggregates formed by retrograde breakdown reactions from intermediate plagioclase and K-feldspar. The predominant deformation mechanism changes in granitoids as the grain size is reduced: coarse-grained (low strain) examples are deformed by a combination of intracrystalline plasticity (quartz) and fracturing (feldspar). In the mylonites, intracrystalline plasticity of quartz plays only a minor role and the dominant deformation mechanism is a non-cataclastic granular flow of polyphase aggregates, consisting largely of albite and quartz. Deformation appears to be stable, probably because grain growth in albite-quartz mixtures is inhibited. These fine-grained aggregates are mechanically weaker than pure quartz aggregates. Thus, the change in deformation mechanism, mainly due to feldspar breakdown reactions, appears to be important for the localization of high shear strain deformation at low metamorphic grades in granitoids and other lithologies modally dominated by feldspar.The rheological behaviour of albite-dominated mineral aggregates may have two consequences for middle to upper crustal deformation of modally feldspar-dominated lithologies: (1) feldspar, in the presence of aqueous fluids, can be an important mineral controlling the rheology at low metamorphic grades, that is, below amphibolite faciesP–Tconditions; (2) the occurrence of granular flow of fine-grained polyphase aggregates in low-grade granitoids is probably common and calls for great care in the modelling of middle to upper crustal rheology based on flow laws for intracrystalline plasticity of single minerals such as quartz.