Characterization of tectono-metamorphic events using crystal size distribution (CSD) diagrams. A case study from the Acebuches metabasites (SW Spain)

Crystal size is one of the main characteristics of a metamorphic rock since it is the result of several processes, including prograde metamorphism as well as retrometamorphism and static and dynamic recrystallisation. In this paper crystal size histograms, mean crystal size, crystal size distribution diagrams, mean aspect ratio and proportion of prismatic sections of amphiboles have been used to describe two tectono-metamorphic events that affected the Acebuches metabasites in the southwestern part of the Iberian Massif. The diagrams depicting the crystal size distribution (CSD) of metamorphic rocks show typically a straight segment that reflects the kinetic conditions during prograde metamorphism as well as retrometamorphic and dynamic recrystallisation processes, and a bell-shaped curve formed during the last stages of growth. The geometrical characteristics of the straight lines of CSD diagrams from different samples of the Acebuches metabasites vary consistently with the structural and metamorphic features indicative of the tectono-metamorphic history suffered by these rocks. This suggests that CSD diagrams can be a useful tool of microstructural analysis in metamorphic rocks.