Comparative grain-boundary studies of some basic and ultrabasic granulites, nodules and cumulates

Average interfacial angles and their standard deviations for single-phase aggregates involving calcic plagioclase and clinopyroxene in basic Lewisian granulites occurring near Scourie, Sutherland, are similar to those in granulites occurring near Broken Hill, Australia. However, the Scourie rocks suggest that the relative proportions and/or grain-size of phases present may affect interfacial angles in two-phase aggregates. The micro-structures indicate that the Scourie granulites have undergone extensive grain adjustment in the solid state, no evidence of residual igneous micro-structures having been observed. The compositions of olivine and orthopyroxene in ultramafic granulites can be reconciled with this suggestion. Olivine aggregates in some ultramafic nodules enclosed in basaltic rocks and olivine and plagioclase aggregates in some layered adcumulates have interfacial angles similar to those of granulites and have polygonal grains with curved boundaries. Available evidence suggests that adcumulates have undergone at least some adjustment of grain shapes in the solid state.