Deformation and metamorphic reactions in polyphase rocks
Figure caption
99
Anorthosite deformed at granulite grade; plagioclase deformed by regime 2 dislocation creep, and recrystallized grains have same composition as host.
This image and image #100 illustrate a similar mechanism for the formation of mixed phase aggregates as shown above in a granitoid (images 93-98) but at higher temperatures in an anorthosite (700°C, same rock as images 49-54). These images of largely recrystallized anorthosite from Norway illustrate a difference in the recrystallization process for plagioclase and hornblende. Plagioclase porphyroclasts have tails consisting of monophase plagioclase domains. The new grains in the tails have the same composition as the porphyroclasts, indicating that they formed by dynamic recrystallization. Conversely, the tails extending from hornblende porphyroclasts are mixed phase aggregates of hornblende and plagioclase. The phase mixture may be due to heterogeneous nucleation of hornblende between plagioclase grains. This mechanism is consistent with the observed small change in composition between old and new hornblende grains.