Patchy Zoning in Plagioclase: A Discussion

In a paper on patchy zoning in igneous plagioclase which has published in this Journal, Dr. Joseph Vance (1965) has made another valuable contribution to our knowledge of the plagioclase feldspars, and he has directed attention toward an important and much neglected feature of plagioclase. While studying igneous rocks from the Antarctic Peninsula, I have observed many examples of patchy zoning in plagioclase and, although this zoning is generally very similar to that described by Vance, it differs in several important respects, which suggests that patchy zoning may develop by more than one mechanism. Patchy zoning in plagioclase in igneous rocks of widely differing age from the Antarctic Peninsula has been described by Fraser (1964, 1965), and a summary of these papers is given here to provide a suitable basis for discussion. The main characteristics of the zoning are best displayed in the rocks of a small, intrusive complex which occurs on the Anagram Islands (lat. 65012 S., long. 64o20' W.) off the west coast of the Antarctic Peninsula. The rocks comprising this complex range in composition from olivine gabbro to tonalite and quartz-diorite. In the olive and hypersthene gabbros, the plagioclase has an average composition of Abi6Ans4, and it is either unzoned or