The effect of Dauphin, twinning on plastic strain in quartz
The effect of Dauphin, twinning on plastic strain in quartz
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DOI：
10.1007/s00410-010-0554-7
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发表时间：
2011-04-01
影响因子：
3.5
通讯作者：
Pennacchioni, Giorgio
中科院分区：
文献类型：
--
作者：
Menegon, Luca;Piazolo, Sandra;Pennacchioni, Giorgio
本文对阿尔卑斯西北部阿罗拉单元绿片岩相（T = 300-400A℃）花岗原长玄武岩中的5块石英斑岩进行了电子背散射衍射分析。在变形初期，两个斑岩碎屑在与压缩方向几乎正交的负菱形面（z}）取向（z孪晶取向）内普遍发育孪生。孪晶是由石英弹性柔度的各向异性驱动的，导致更具柔度的正菱形{r}平面的极点几乎平行于压缩方向（r-孪晶取向）。相比之下，我们报道了在两个已经与压缩方向正交的{r}平面定向的卟岩碎屑中缺乏孪生。对一种孪生卟啉细胞进行了更详细的研究。它显示了许多塑性应变局部化为离散的r-孪晶，因为r-孪晶比z-孪晶储存了更多的弹性应变能。在塑性变形过程中，多芬孪晶的存在导致了主导活动滑移系统的转换，从基底< a >（无孪晶区域）到{pi}和{pi'} < a >（普遍孪晶区域）。动态再结晶是沿r孪晶局部化的，主要发生在逐渐的亚晶旋转中，局部有胀形再结晶成分。部分再结晶晶粒发生了刚体旋转，大约沿体涡量轴旋转，这导致了大的取向偏差角的形成。石英的再结晶粒度计可产生100 MPa的差应力。与文献数据的比较表明，机械道芬孪生在含石英岩中有潜在的古地压计用途。
We present an electron backscatter diffraction analysis of five quartz porphyroclasts in a greenschist facies (T = 300-400A degrees C) granitoid protomylonite from the Arolla unit of the NW Alps. Mechanical Dauphin, twinning developed pervasively during the incipient stage of deformation within two porphyroclasts oriented with a negative rhomb plane {z} almost orthogonal to the compression direction (z-twin orientation). Twinning was driven by the anisotropy in the elastic compliance of quartz and resulted in the alignment of the poles of the planes of the more compliant positive rhomb {r} nearly parallel to the compression direction (r-twin orientation). In contrast, we report the lack of twinning in two porphyroclasts already oriented with one of the {r} planes orthogonal to the compression direction. One twinned porphyroclast has been investigated with more detail. It shows the localization of much of the plastic strain into discrete r-twins as a consequence of the higher amount of elastic strain energy stored by r-twins in comparison to z-twins. The presence of Dauphin, twins induced a switch in the dominant active slip systems during plastic deformation, from basal < a > (regions without twinning) to {pi} and {pi'} < a > (pervasively twinned regions). Dynamic recrystallization is localized along an r-twin and occurred dominantly by progressive subgrain rotation, with a local component of bulging recrystallization. Part of the recrystallized grains underwent rigid-body rotation, approximately about the bulk vorticity axis, which accounts for the development of large misorientation angles. The recrystallized grain size piezometer for quartz yields differential stress of 100 MPa. The comparison of this palaeostress estimate with literature data suggests that mechanical Dauphin, twinning could have a potential use as palaeopiezometer in quartz-bearing rocks.