Reaction localization and softening of texturally hardened mylonites in a reactivated fault zone, central Argentina
Reaction localization and softening of texturally hardened mylonites in a reactivated fault zone, central Argentina
复制标题
阿根廷中部重新活动的断层带中结构硬化糜棱岩的反应局部化和软化
DOI：
--
复制
发表时间：
2005
期刊：
影响因子：
--
通讯作者：
R. Wintsch
中科院分区：
文献类型：
--
作者：
S. Whitmeyer;R. Wintsch
位于阿根廷中部东sierra Pampeanas的Tres Arboles韧性断裂带经历了多次韧性变形和断裂事件，涉及各种结构和反应硬化和软化过程。大部分断裂带的特征是（D2）超长岩，由细粒黑云母+斜长石组成，缺乏明确的优选方向。D2织物由交错生长和互锁的晶粒组成，几乎没有显示出位错或溶解蠕变的织构证据。这些超长岩中含有片麻岩碎屑和斜长石、硅线石和石榴石的斑岩碎屑，这些斑岩是由上盘的片麻岩和混染岩原岩（D1）继承而来。石榴石+硅线石+黑云母的组合表明D1相关组构发育于上部角闪岩相条件下，而黑云母+石榴石+硅线石+斜长石的持续存在表明D2超长岩发育于中部角闪岩相条件下。绿片岩相，糜棱质剪切带（D3）局部覆印D2超长岩。白云母+绿泥石±黑云母的细粒叶状结构截断了早期的黑云母+斜长石结构，粗粒白云母部分取代了残留的硅线石颗粒。剪切带（D3）斜长石钙长石含量为c. An30，与D1和D2斜长石钙长石含量（c. An35）不同。D3斜长石的钙长石含量与普遍存在的晶界流体相一致，有利于白云母部分取代斜长石。黑云母部分被白云母和/或绿泥石所取代，特别是在推断的高应变地区。石英沉淀在斑岩压力阴影和带状中，有助于确定糜棱岩结构。所有的D3反应都需要引入H+和/或H2O，这表明是一个开放的体系，通常会导致体积减小。同构造期D3白云母+石英+绿泥石优先向有利于应变局部化的方向生长，产生强烈的结构软化。应变局部化只发生在反应随着水流体的渗透而进展的地方，在几百微米的尺度上。局部压裂和微震活动可能导致了断裂带的重新激活和流体的初始引入。然而，沿离散剪切带的主要绿片岩相变形（D3）主要是部分开放体系中置换反应局部化的结果。
The Tres Arboles ductile fault zone in the Eastern Sierras Pampeanas, central Argentina, experienced multiple ductile deformation and faulting events that involved a variety of textural and reaction hardening and softening processes. Much of the fault zone is characterized by a (D2) ultramylonite, composed of fine‐grained biotite + plagioclase, that lacks a well‐defined preferred orientation. The D2 fabric consists of a strong network of intergrown and interlocking grains that show little textural evidence for dislocation or dissolution creep. These ultramylonites contain gneissic rock fragments and porphyroclasts of plagioclase, sillimanite and garnet inherited from the gneissic and migmatitic protolith (D1) of the hangingwall. The assemblage of garnet + sillimanite + biotite suggests that D1‐related fabrics developed under upper amphibolite facies conditions, and the persistence of biotite + garnet + sillimanite + plagioclase suggests that the ultramylonite of D2 developed under middle amphibolite facies conditions. Greenschist facies, mylonitic shear bands (D3) locally overprint D2 ultramylonites. Fine‐grained folia of muscovite + chlorite ± biotite truncate earlier biotite + plagioclase textures, and coarser‐grained muscovite partially replaces relic sillimanite grains. Anorthite content of shear band (D3) plagioclase is c. An30, distinct from D1 and D2 plagioclase (c. An35). The anorthite content of D3 plagioclase is consistent with a pervasive grain boundary fluid that facilitated partial replacement of plagioclase by muscovite. Biotite is partially replaced by muscovite and/or chlorite, particularly in areas of inferred high strain. Quartz precipitated in porphyroclast pressure shadows and ribbons that help define the mylonitic fabric. All D3 reactions require the introduction of H+ and/or H2O, indicating an open system, and typically result in a volume decrease. Syntectonic D3 muscovite + quartz + chlorite preferentially grew in an orientation favourable for strain localization, which produced a strong textural softening. Strain localization occurred only where reactions progressed with the infiltration of aqueous fluids, on a scale of hundreds of micrometre. Local fracturing and microseismicity may have induced reactivation of the fault zone and the initial introduction of fluids. However, the predominant greenschist facies deformation (D3) along discrete shear bands was primarily a consequence of the localization of replacement reactions in a partially open system.