At 11:49 AM 16/10/2003 +0900, Shohei (Banno) wrote:
>To whom interested:
> I never thought the Turner's system of defining hornfels series
>is useful, But I will use the pyroxene hornfels facies in honour of
>Goldscmidt and Eskola. We need Eskola(1939) 's faceis and modified
>Coombs facies, which are zeolite, prehnite-actinolite and
>pumpellyite-actinolite facies. I think prhenite-pumpelllyite facies is
>confusing and better not to use.
> ...
I agree except "prehnite-actinolite facies" should be renamed
prehnite-chlorite facies. The subassemblage prehnite+actinolite is too
Ca-rich to define a facies in metabasite bulk composition. This can be
appreciated on a conventional ACF diagram, where the prehnite-actinolite
tie-line does not cross the ACF tie-line diagnostic of any other facies.
With few exceptions, the same is true of the subassembage
prehnite+pumpellyite. But the prehnite-chlorite ACF tie-line cuts the
tie-line from actinolite (or so-called "relict igneous clinopyroxene") to
epidote, pumpellyite, lawsonite, plagioclase, or any metamorphic zeolite.
Hence the subassemblage prehnite+chlorite has a reaction relationship to
metabasite assemblages diagnostic of any other Eskola/Coombs
facies. Clearly therefore, "prehnite-chlorite facies" should have facies
status in any set of facies based on mineral assemblages in metabasites,
but "prehnite-actinolite facies" and "prehnite-pumpellyite facies" should not.
Cheers, Dugald
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