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RheologyofOrthopyroxeneConstrainedbyFoldingofSingleCrystals

, PP. 194-205

Keywords: 斜方辉石,流变学,单晶褶皱,显微构造,天山高压变质地体

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Abstract:

迄今为止,上地幔流变学的研究主要集中于橄榄石单晶与多晶集合体的实验和野外观察,但对辉石的流变学特征知之甚少,虽然辉石亦是上地幔岩石的主要组成矿物。因此,查清斜方辉石的流变学行为、塑性变形机制、重结晶作用以及与其共生的橄榄石之间的流变强度差等,现已成为国际地学界研究的新的热点课题。新疆中天山南缘库米什地区的榆树沟高压变质地体中出现地幔超糜棱岩,其中带状拉伸的斜方辉石(En90)碎斑晶发生了强烈地弯滑褶皱,周围橄榄石(Fo90)重结晶成细粒(~10μm)多晶基质。显微构造研究表明,单晶斜方辉石的褶皱主要通过单一的(100)[001]滑移进行的,而细粒重结晶橄榄石基质的变形机制则最可能以超塑性(颗粒边界滑移与扩散)为主。单晶斜方辉石发生弯滑褶皱,而不形成常见的膝折(Kinks),说明所处物理化学条件下斜方辉石晶体位错攀移与原子扩散并不足够活跃。此外,即使其晶格发生高达140°的旋转,斜方辉石依然没有发生光学显微镜下足以识别的重结晶结构,说明启动重结晶作用的临界剪切应变应不少于5.5。根据现有的层状材料的褶皱理论,推测榆树沟高压变质地体中地幔岩发生塑性变形时位错蠕变的斜方辉石的流动强度至少比相同条件下超塑性变形的细粒橄榄石多晶基质高近2个数量级。

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