Eric Essene wrote:
"There is no such thing as "shallow contact metamorphic facies", as
facies simply delineate a portion of P-T space, not the process by which
I did not make this up, nor approve of it (in fact, I don't like it); I was just citing terms from petrology textbooks.
"There are hornblende hornfels facies and pyroxene hornfels facies rocks that usually but not always formed during contact metamorphism. They could be defined by the absence of ferromagnesian garnet and by assemblages and composition fields that have been lucidly described by Pattison, for instance."
Absence of almandine is a valid point, but defining a facies by the absence of a mineral is bit odd. Apart from that, spessartine-rich almandine may still be stable, and even a thin section won't reveal the garnet composition unless it is put under the microprobe. However, once the sample is fully analysed and P-T is determined, the facies aspect becomes somewhat obsolete.
Bruce Bathurst wrote:
"Facies, as opposed to most isograds, are environmentally sensitive but independent of composition. They are identified in the field, using a lens, from an association of rocks containing typical assemblages, or the occasional single rock containing a critical assemblage."
This underlines the practical use of facies as I understand it. I wouldn't restrict this entirely to the field though, and even experienced petrographers may not be able to identify every critical mineral in the field.