Study of garnet adjacent to and within shear zones that underwent synkinematic eclogitization reveals microstructures and textures that, for the first time, indicate grain- boundary sliding as the main deformation mechanism in garnet. Monomineralic garnet layers exhibit characteristic features of superplastic behavior and show little strength contrast to the surrounding minerals. The low strength is caused by the synkinematic growth of garnet limiting the grain size to tens of micrometers. This behavior may significantly influence rheologic properties of high- and ultrahigh-pressure shear zones in subduction environments and may play a role during the formation of garnet in the transition zone of the mantle.

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