Microfabric of folded quartz veins in metagreywackes: dislocation creep and subgrain rotation at high stress
Microfabric of folded quartz veins in metagreywackes: dislocation creep and subgrain rotation at high stress
复制标题
变灰砂岩中折叠石英脉的微结构：高应力下的位错蠕变和亚晶旋转
DOI：
10.1111/j.1525-1314.2009.00842.x
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发表时间：
2009
影响因子：
3.4
通讯作者：
B. Stöckhert
中科院分区：
文献类型：
--
作者：
C. Trepmann;B. Stöckhert
利用光学显微镜、扫描电镜（包括电子背散射衍射）和透射电镜研究了加利福尼亚Pacheco山口Franciscan俯冲杂岩细粒高压低温变质灰岩中褶皱石英脉的微结构。从云母和碎屑石英的形状优选取向可以看出，片理型主辉长岩是通过溶解-沉淀蠕变而变形的，没有任何晶体塑性变形的迹象。石英碎屑没有发生晶体塑性变形，说明在250 ~ 300℃的温度下，主晶宽中的流动应力保持在几十MPa以下。相反，褶皱石英脉的微组织表现为位错蠕变和亚晶旋转再结晶。对于再结晶晶粒尺寸为~ 8±6 μm的小晶粒，古应力计显示应力差为几百MPa。单相石英脉中的应力集中被解释为由于其比通过溶解-沉淀蠕变变形的细晶偏晶石英脉具有更高的有效粘度。褶皱的形状表明粘度的对比为一到两个数量级。溶蚀-沉淀蠕变变形是一个连续的过程。同样的道理也适用于脉状石英的褶皱和位错蠕变引起的脉状石英变形。微组织的动态再结晶主要是由亚晶旋转引起的，只有少量的应变引起的晶界迁移，这需要在高差应力下爬升控制蠕变过程中保持高角度晶界上低的位错密度对比。这些连续变形脉体中的石英记录是有特征的，与地震活跃地区的变质岩记录不同，在地震活跃地区，类似温度下的高应力变形是偶发的，与地震旋回有关。
The microfabrics of folded quartz veins in fine‐grained high pressure–low temperature metamorphic greywackes of the Franciscan Subduction Complex at Pacheco Pass, California, were investigated by optical microscopy, scanning electron microscopy including electron backscatter diffraction, and transmission electron microscopy. The foliated host metagreywacke is deformed by dissolution–precipitation creep, as indicated by the shape preferred orientation of mica and clastic quartz without any signs of crystal‐plastic deformation. The absence of crystal‐plastic deformation of clastic quartz suggests that the flow stress in the host metagreywacke remained below a few tens of MPa at temperatures of 250–300 °C. In contrast, the microfabric of the folded quartz veins indicates deformation by dislocation creep accompanied by subgrain rotation recrystallization. For the small recrystallized grain size of ∼8 ± 6 μm, paleopiezometers indicate differential stresses of a few hundred MPa. The stress concentration in the single phase quartz vein is interpreted to be due to its higher effective viscosity compared to the fine‐grained host metagreywacke deforming by dissolution–precipitation creep. The fold shape suggests a viscosity contrast of one to two orders of magnitude. Deformation by dissolution–precipitation creep is expected to be a continuous process. The same must hold for folding of the vein and deformation of the vein quartz by dislocation creep. The microfabric suggests dynamic recrystallization predominantly by subgrain rotation and only minor strain‐induced grain boundary migration, which requires low contrasts in dislocation density across high‐angle grain boundaries to be maintained during climb‐controlled creep at high differential stress. The record of quartz in these continuously deformed veins is characteristic and different from the record in metamorphic rocks exhumed in seismically active regions, where high‐stress deformation at similar temperatures is episodic and related to the seismic cycle.