Garnet is an exceptionally useful mineral for reconstructing the evolution of metamorphic rocks that have experienced multiple tectonic or thermal events. Understanding how garnet crystallizes and its mechanical behaviour is important for establishing a petrological and temporal record of metamorphism and deformation and for recognizing multiple geologic stages within the growth history of an individual crystal. Here, we integrate fine-scale microstructural (electron backscatter diffraction [EBSD]) and microchemical (Laser Ablation Inductively Coupled Plasma Mass Spectrometry [LA-ICP-MS] mapping) data obtained on a polycyclic garnet-bearing micaschist from the Alpine belt. Results suggest that fragmentation of pre-Alpine garnet porphyroblasts occurred during the late pre-Alpine exhumation and/or the onset of the Alpine burial, such that the older pre-Alpine garnet fragments were transported/redistributed during Alpine deformation and acted as nucleation sites for Alpine garnet growth. These processes produced a bimodal garnet size distribution (millimetre- and micrometre-sized grains). Thermodynamic modelling indicates that Alpine garnet grew during the final stage of burial (from 1.9 GPa 480°C to 2.0 GPa 520°C) and early exhumation (down to 1.6 GPa 540°C) forming continuous idioblastic rims on and sealing fractures in pre-Alpine garnet grains. We propose that fragmentation–overgrowth processes in polycyclic rocks, coupled with ductile deformation, may produce a bimodal garnet size distribution in response to fragmentation and re-distribution of pre-existing grains; these clasts can act as new nucleation sites during a subsequent orogenic cycle.

中文翻译：

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通过微化学和微观结构映射限制的破碎和过度生长形成多级石榴石晶粒

石榴石是一种非常有用的矿物，可用于重建经历过多次构造或热事件的变质岩的演化过程。了解石榴石如何结晶及其机械行为对于建立变质作用和变形的岩石学和时间记录以及识别单个晶体生长历史中的多个地质阶段非常重要。在这里，我们整合了从阿尔卑斯带多环石榴石云母片岩上获得的精细微观结构（电子背散射衍射 [EBSD]）和微量化学（激光烧蚀电感耦合等离子体质谱 [LA-ICP-MS] 绘图）数据。结果表明，前高山石榴石斑母细胞的碎裂发生在前高山折返晚期和/或高山埋藏开始期间，使得较老的前高山石榴石碎片在高山变形过程中被运输/重新分布，并作为成核位点。高山石榴石生长。这些过程产生了双峰石榴石尺寸分布（毫米和微米尺寸的颗粒）。热力学模型表明，高山石榴石在埋藏的最后阶段（从 1.9 GPa 480°C 到 2.0 GPa 520°C）和早期折返阶段（降至 1.6 GPa 540°C）生长，在前形成连续的成岩边缘并密封裂缝。高山石榴石颗粒。我们提出，多旋回岩石中的破碎-过度生长过程，加上延性变形，可能会产生双峰石榴石尺寸分布，以响应预先存在的颗粒的破碎和重新分布；这些碎屑可以在随后的造山周期中充当新的成核位点。