Petrofabrics and microstructures of omphacites in a high temperature eclogite from the Swedish Caledonides

Abstract

Eclogites from the Eastern Belt of the Seve Nappes, Jämtland, Central Scandinavian Caledonides, have an LS fabric in which the foliation S and lineation L are both defined by the shape preferred orientation of omphacite. The orientation distribution diagram of omphacite reveals a crystallographic fabric defined by [001] parallel to L and [010] in a great circle distribution perpendicular to S and L. Optical and electron microscopy studies indicate that the eclogites have been deformed primarily by dislocation creep mechanisms operating in omphacite i.e. whilst garnets contain very low densities of dislocations and can be treated effectively as rigid bodies, omphacites contain defect structures consisting of well-ordered dislocation boundaries which delineate subgrains. The majority of dislocations in omphacite have Burgers vectors 1/2 <110> and [001]. In the absence of extensive recrystallization, the omphacite deformation fabric is explained by multiple crystallographic slip along {110} [001], (100) [001] and {110} <110> assisted by dislocation climb.