I know this is somewhat removed from the original specific question, but:
One of the first things I try to get across in a Metamorphic Petrology course is that the compositional variety of metamorphic rock types is much larger than that of igneous and sedimentary rocks combined, due to metasomatic processes, melt extraction, etc. Isn't this wonderful? Perhaps not all students would agree, I guess.....
As some metamorphic rock terms also refer to structural characteristics and metamorphic grade, to name only two aspects, the variety of rock terms is indeed quite large. Why should this put off anybody? We cannot make things simpler than they actually are, and variety does reflect on nomenclature to some extent. We must be thankful that not every rock we see is a quartzite or a marble (or a simple amphibolite, for that matter). It would make Structural Geology easier, but Metamorphic Petrology redundant.
I agree with Christian and Anthi that metamorphic rock nomenclature is flexible enough to allow for more than one name for a lot of metamorphic rocks. There is nothing wrong with that. The basic rule to follow is that others must have an idea what you are talking about.
Christian: I am not so sure whether we can restrict the term granulite to basic rocks after it had been used for some 100 years in a different sense. In fact, the original term referred to a felsic rock.
School of Geological & Computer Sciences
University of Natal