This paper presents a detailed case study of decorated dislocations of olivine with C-type fabric in the Lüliangshan garnet peridotite from the North Qaidam ultrahigh-pressure (UHP) metamorphic belt in NW China. Electron backscatter diffraction (EBSD) measurements reveal a weak C-type fabric in olivine, i.e., the crystallographic [100] axes form a weak point maximum subnormal to the foliation while the [001] axes form a girdle in the foliation plane with a weak point maximum subparallel to the lineation. Oriented hydrous mineral of sodic amphibole and pyroxene were found in porphyroclastic garnet indicating considerable water content in the parental majoritic garnet at high pressure conditions. However, the water content of olivine and garnet is below the detection limit (less than a few ppm wt. H₂O) of the instrument (a Nicolet 6700 FTIR with Continuum FTIR Microscope). Two distinctive deformation features of decorated olivine grains were recognized: (1) linear morphologies of many dislocations with long straight segments parallel to the [001] orientation, which are likely screw dislocations with b = [001]; (2) olivine grains with planar dislocation, subgrain boundaries or undulose extinction are scarce in thin sections under optical microscopy. The dislocation structures and densities in olivine are heterogeneous at multi-scales. The mean 3D dislocation density and the estimated differential stress based on the dislocation density piezometer are 0.4–0.8 × 10⁻¹² m⁻² and 65–108 MPa, respectively. Decorated dislocations and EBSD analyses suggest the most likely dominant activation slip systems with b = [001]. Considering that most C-type fabric of olivine are reported from UHP garnet peridotites, we emphasize the important effects of high pressure and relatively low temperature on the formation of the C-type fabric in olivine.

本文详细介绍了柴北缘超高压变质带吕梁山石榴石橄榄岩中 C 型组构橄榄石装饰位错的详细案例研究。电子背散射衍射 (EBSD) 测量显示橄榄石中的 C 型结构较弱，即结晶 [100] 轴形成低于叶理的最大弱点，而 [001] 轴在叶理平面中形成带弱点与线的最大次平行。在斑碎屑石榴石中发现了钠闪石和辉石的定向含水矿物，表明在高压条件下母体多晶石榴石中的水含量相当可观。然而，橄榄石和石榴石的水含量低于检测限（小于几 ppm wt. H ₂O) 仪器（带有 Continuum FTIR 显微镜的 Nicolet 6700 FTIR）。认识到装饰橄榄石晶粒的两个明显变形特征：（1）许多具有平行于[001]取向的长直段的位错的线性形态，这可能是b  = [001]的螺旋位错；(2) 光学显微镜下薄片中稀有平面位错、亚晶界或波状消光的橄榄石晶粒。橄榄石中的位错结构和密度在多尺度上是不均匀的。平均 3D 位错密度和基于位错密度压力计的估计微分应力为 0.4–0.8 × 10 ^-12  m ^-2和 65-108 兆帕，分别。装饰位错和 EBSD 分析表明b  = [001]的最可能占主导地位的激活滑移系统。考虑到大多数橄榄石 C 型组构来自 UHP 石榴石橄榄岩，我们强调了高压和相对低温对橄榄石 C 型组构形成的重要影响。