Influence of modal composition on deformation in garnet Iherzolite nodules

Abstract

A textural distinction has been drawn, in peridotites derived from kimberlite pipes, between relatively undeformed granular types and strongly deformed sheared types¹, that corresponds roughly with the absence and presence of large strained olivine grains (porphyroclasts) surrounded by smaller, recrystallized grains (neoblasts). More detailed classifications have also been proposed^2–6. The textural variety is probably caused by both widespread mantle creep and more localized phenomena connected with kimberlite eruption^7–12. From consideration of dynamically recrystallized grain size¹¹ and dislocation densities¹², it seems that nodules that contain neoblasts have been deformed in high stress and strain rate conditions, over short periods of time, before eruption. The formation of the sheared textures and the eruption of the kimberlite are approximately simultaneous. I report here detailed textural observations on 10 garnet Iherzolite nodules collected from kimberlite pipes in South Africa and Lesotho. They all have sheared textures, as defined above, showing a range from small amounts of grain boundary recrystallization to almost complete consumption of olivine porphyroclasts. Sections were cut parallel to the foliation and modal analyses were accurately determined by point counting one slide from each sample. The number of olivine porphyroclasts on each slide was then independently counted. The fewer the grains, the greater is the accuracy of estimating their number. Average porphyroclast areas were then calculated (Table 1). The data suggest that modal composition controls the extent of shearing.