Jürgen, Dugald and all,
No one should identify a metamorphic facies in hand specimen at all.
Facies are distinguished by general associations in a variety of rocks
subjected to the same P-T. Low pressure facies are also identified by
assemblages, but not by their mechanism of formation. After all, many
blueschist facies rocks are neither blue nor schists, yet no one has a
problem with that term. If schists are not required for blueschist or
greenschist facies rocks, why does anyone boggle at hornfels facies rocks
without hornfelses? These are simply historical terms, well established by
Eskola and subsequent workers. Hornfelses occur without contact
metamorphism and vice versa, so what?
>I would fully support Dugald's statement. Can anybody tell me how to
>differentiate between hornblende-hornfels facies and amphibolite facies
>when looking at a hand specimen? What defines the upper pressure limit of
>the "shallow contact metamorphic facies"? If we can use these facies terms
>only in a field-related sense, where does "pure" contact metamorphism end
>and where does low-pressure, regional-style thermal metamorphism start?
>The idea that aureoles generally contain hornfelses is clearly wrong. Do
>we then explain to students that a foliated hornblende-plagioclase rock
>cannot be called a hornblende-hornfels, but rather an amphibolite that
>originated in the hornblende-hornfels facies? What is lost if we abandon
>these contact-metamorphic facies terms?
>School of Geological & Computer Sciences
>University of Natal
Professor of Geology
Department of Geological Sciences
2534 C.C. Little Bldg.
425 E. University Ave.
University of Michigan
Ann Arbor MI 48109-1063 USA