Strain localization associated with channelized melt migration in upper mantle lithosphere: Insights from the Twin Sisters ultramafic complex, Washington, USA
Strain localization associated with channelized melt migration in upper mantle lithosphere: Insights from the Twin Sisters ultramafic complex, Washington, USA
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DOI：
10.1016/j.jsg.2012.10.009
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发表时间：
2013-05-01
影响因子：
3.1
通讯作者：
Young, Laura I.
中科院分区：
文献类型：
--
作者：
Kruckenberg, Seth C.;Tikoff, Basil;Young, Laura I.
我们介绍了双胞胎姐妹超镁铁质杂岩(华盛顿州)的野外、显微结构和结构研究结果，证明了与自然变形的上地幔中纯橄榄岩通道的形成有关的局部变形。孪生姐妹杂岩是上地幔岩石圈暴露良好、几乎没有改变的部分，主要由纯橄榄岩和方辉橄榄岩(在厘米到米尺度的原始成分层中)以及可变变形的斜方辉石和单斜辉石岩脉组成。研究区内分布着一系列类似于N-S走向的m级纯橄榄岩带(典型的斑岩结构)，横贯原始成分层和古老的斜方辉石岩脉。构造关系表明，这些纯橄榄岩带代表了以前的沟道化熔体迁移带(即纯橄榄岩通道)，应变局部化与熔体迁移有关。早期形成的斜方辉石岩脉要么不存在于横贯纯橄榄岩通道内，要么相对于它们在相邻围岩中的位置在通道内发生了置换。这些先前存在的斜方辉石岩脉提供了应变标记，说明位移主要沿着具有相反剪切意义的河道边缘进行。在所有注意到偏移量的情况下，海峡的中心相对于其边缘向南移动。因此，在熔体迁移过程中，物质流动和应变在通道内被分割，而纯橄榄岩通道不适应相邻寄主橄榄岩的净剪切位移。与纯橄榄岩河道相邻的原生成分层记录了河道边缘两侧橄榄石[100]结晶轴的相反旋转，这与露头尺度的偏移标志所推断的运动学反转是一致的，表明纯橄榄岩河道的形成也导致了水道近端的主岩变形。集中在条带边缘的应变局部化很可能是由熔融诱导的弱化促进的。纯橄榄岩带内的沟道化运动可能是由于沟道内的基质压实、熔体迁移过程中的压力梯度或同轴变形过程中这些过程的组合造成的。(C)2012爱思唯尔有限公司。保留所有权利。
We present results of field, microstructural, and textural studies in the Twin Sisters ultramafic complex (Washington State) that document localized deformation associated with the formation of dunite channels in naturally deformed upper mantle. The Twin Sisters complex is a well-exposed, virtually unaltered section of upper mantle lithosphere comprised largely of dunite and harzburgite (in cm- to m-scale primary compositional layers), and variably deformed orthopyroxenite and clinopyroxenite dikes. A series of similar to N-S striking, m-scale dunite bands (typically with porphyroclastic texture) occur throughout the study area and crosscut both the primary compositional layers and older orthopyroxenite dikes. Structural relationships suggest that these dunite bands represent former zones of channelized melt migration (i.e., dunite channels), and that strain localization was associated with melt migration. Early formed orthopyroxenite dikes are either absent within cross-cutting dunite channels, or have been displaced within channels relative to their position in the adjacent host rocks. These pre-existing orthopyroxenite dikes provide strain markers illustrating that displacement was localized primarily along channel margins, which have opposite senses of shear. In all cases where offsets were noted, the center of the channel was moved southward relative to its margins. Material flow and strain was, therefore, partitioned within channels during melt migration, and dunite channels did not accommodate net shear displacement of the adjacent host peridotites. Primary compositional layers adjacent to dunite channels document opposite rotation of olivine [100] crystallographic axes on either side of channel margins, consistent with the kinematic reversal inferred from offset markers at the outcrop scale, suggesting that the formation of dunite channels also induced host rock deformation proximal to channels. Strain localization that was focused at the margin of the bands was likely facilitated by melt-induced weakening. Channelized movement within the dunite bands may have resulted from matrix compaction within channels, pressure gradients during melt migration, or a combination of these processes during coaxial deformation. (C) 2012 Elsevier Ltd. All rights reserved.