Abstract Garnet porphyroblasts with spiral shaped inclusion trails of two micaschist samples have been investigated with X-ray computed tomography. For each garnet crystal, the orientation of its internal Foliation Intersection/Inflexion Axes (FIA) was measured, its volume and shape orientation, the latter via calculation of best-fit ellipsoids. Additionally, image analysis of inclusion trails was performed in slices passing through garnet centres oriented normal to FIA. Our data demonstrate subvertical and subhorizontal preferred orientations of the inclusion trails and of the porphyroblast crystals themselves in both samples. This supports a genetic model in which garnets periodically overgrew a succession of vertical and horizontal foliations during alternating crustal shortening and gravitational collapse without significantly rotating. The fact that both samples come from areas previously identified as regional shear zones supports the idea that porphyroblasts are able to maintain stable positions even in such environments. Our study exemplifies the strength of X-ray computed micro-tomography compared to previous techniques based on serial thin sections for distinguishing multiple FIA sets present in single samples.

中文翻译：

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螺旋石榴石的 X 射线计算机显微断层扫描：对它们如何形成的新测试

摘要 已经用 X 射线计算机断层扫描研究了具有两个云母片样品的螺旋形包裹体痕迹的石榴石卟啉母细胞。对于每个石榴石晶体，测量其内部叶面交点/弯曲轴 (FIA) 的方向、体积和形状方向，后者通过计算最佳拟合椭球。此外，在穿过垂直于 FIA 的石榴石中心的切片中对夹杂物轨迹进行了图像分析。我们的数据表明，两个样品中的内含物轨迹和卟啉晶体本身的近垂直和亚水平优选取向。这支持了一个遗传模型，其中在交替的地壳缩短和重力坍塌期间，石榴石周期性地过度生长一系列垂直和水平叶理，而没有明显旋转。两个样本都来自先前被确定为区域剪切带的区域，这一事实支持了卟啉细胞即使在这样的环境中也能够保持稳定位置的观点。我们的研究举例说明了 X 射线计算机显微断层扫描与以前基于连续薄切片的技术相比的强度，用于区分单个样本中存在的多个 FIA 集。