It seems you did not quite get the point I am making. There are rocks that have (historically, yes) names, such as amphibolite, blueschist eclogite, etc. which imply metamorphism in the respective facies, because the facies has been named after them. Does that sound logical? We all know that most greenschist facies rocks are neither green nor schists. So what, indeed. The best guess for an eclogite is still that it formed in the eclogite facies J J J.
And, apart from the sensible use of facies for large-scale maps (as Dugald pointed out), we do ask students about metamorphic facies in outcrops where we generally do not carry around microscopes or a microprobe. Are your students so clever that they remember the stability limits for every mineral assemblage they encounter in the field?
If yes, maybe you can send us a few.
You say that "low-pressure facies are also identified by assemblages". Fair enough. Can you tell me then where the pressure limit for the so-called "shallow contact metamorphic facies" lies, and how you define it? If we cannot constrain the facies within at least broad P-T limits, they may just as well be dumped.
School of Geological & Computer Sciences
University of Natal