Development of garnet porphyroblasts by multiple nucleation, coalescence and boundary misorientation-driven rotations

Two types of garnet porphyroblast occur in the Schneeberg Complex of the It alian Alps. Type I porphyroblasts form ellipsoidal pods with a centre consi sting of unstrained quartz, decussate mica and small garnet grains, and a m argin containing large garnet grains. Orientation contrast imaging using th e scanning electron microscope shows that the larger marginal garnet, grain s comprise a number of orientation subdomains. Individual garnet grains wit hout subdomains are small (< 50 <mu>m), faceted and idioblastic, and have s imple zoning profiles with Ca-rich cores and Ca-poor rims. Subdomains of la rger garnet grains are similar in size to the individual, small garnet grai ns. Type 2 porphyroblasts comprise only ellipsoidal garnet, with small subd omains in the centre and larger subdomains at the margin. Each subdomain ha s its own Ca high, Ca dropping towards subdomain boundaries. Garnet grains, with or without subdomains, all have the same Ca-poor composition at rims in contact with other minerals. The compositional zonation patterns are bes t explained by simultaneous, multiple nucleation, followed by growth and am algamation of individual garnet grains. The range of individual garnet and garnet subdomain sizes can be explained by a faster growth rate at the porp hyroblast margin than in the centre. The difference between Type I and Type 2 porphyroblasts is probably related to the growth rate differential acros s the porphyroblast. Electron backscatter diffraction shows that small, individual garnet grains are randomly oriented. Large marginal garnet grains and subdomain-bearing garnet grains have a strong preferred orientation, clustering around a sing le garnet orientation. Misorientations across subdomain boundaries are smal l and misorientation axes are randomly oriented with respect to crystallogr aphic orientations. The only explanation that fits the observational data i s that individual garnet grains rotated towards coincident orientations onc e they came into contact with each other. This process was driven by the re duction of subdomain boundary energy associated with misorientation loss. R otation of garnet grains was accommodated by diffusion in the subdomain bou ndary and diffusional creep and rigid body rotation of other minerals (quar tz and mica) around the garnet. An analytical model, in which the kinetics of garnet rotation are controlled by the rheology of surrounding quartz, su ggests that, at the conditions of metamorphism, the rotation required to gi ve a strong preferred orientation can occur on a similar time-scale to that of porphyroblast growth.