The breakdown of omphacite plays an important role in the exhumation and retrogression of eclogites. Additionally, metamorphic reactions associated with grain size reduction have the potential to significantly impact deformation mechanisms and the rheology of crustal rocks. We analyze the breakdown reaction omphacite → diopsidic clinopyroxene + plagioclase ± amphibole and associated microstructures by electron backscatter diffraction. The reaction results in the formation of (diopsidic) clinopyroxene-plagioclase symplectites. Samples were chosen from localities on Holsnøy (western Norway) and Lofoten (northern Norway), that are representative of vermicular symplectites, partly recrystallized symplectites, and deformed symplectites. Interphase misorientation analysis based on the electron backscatter diffraction results reveals that the nucleation of (diopsidic) clinopyroxene-plagioclase symplectites was crystallographically controlled, with the diopside copying the lattice orientation of the omphacite, and the plagioclase growing along diopside planes with favorable, i.e., similar, interplanar spacing. Deformation of the (diopsidic) clinopyroxene-plagioclase symplectites occurred by fracturing, transitioning into grain boundary sliding accommodated by diffusion creep. The results indicate that the formation of vermicular symplectites is not associated with enhanced permeability and fluid flow. Subsequent recrystallisation and grain-size sensitive deformation of the symplectites facilitates fluid redistribution and weakening of the retrogressed eclogites.

绿辉石的分解对榴辉岩的折返和退缩起着重要作用。此外，与晶粒尺寸减小相关的变质反应有可能显着影响地壳岩石的变形机制和流变学。我们通过电子背散射衍射分析了绿辉石→闪辉石+斜长石±角闪石的分解反应以及相关的微观结构。该反应导致形成（透辉石）单斜辉石-斜长石。样品是从霍尔斯诺伊（挪威西部）和罗弗敦群岛（挪威北部）地区选取的，它们是蠕虫状辛矿、部分再结晶辛矿和变形辛矿的代表。基于电子背散射衍射结果的相间取向差分析表明，（透辉石）单斜辉石-斜长石斜长石的成核受到晶体学控制，透辉石复制了绿辉石的晶格方向，并且斜长石沿着透辉石平面生长，具有有利的，即相似的，晶面间距。 （透辉石）单斜辉石-斜长石的变形是通过破裂发生的，转变为扩散蠕变所适应的晶界滑动。结果表明，蠕虫状辛岩的形成与渗透性和流体流动的增强无关。随后的重结晶和辛辉石的晶粒尺寸敏感变形促进了流体的重新分布和退化榴辉岩的弱化。