At 12:35 PM 16/11/2001 +0000, Bruce Yardley wrote:
...the really high temperature effects caused by melting close to
>shallow igneous contacts are actually very distinctive, not the end of a
>continuous spectrum, and I would argue that they do merit their own facies
>name, if only to emphasise that they cannot fit into any other facies.
>However whether you can realistically justify both pyroxene hornfels and
>sanidinite I very much doubt.
At 10:11 PM 17/11/2001 -0500, Jorge Julian Restrepo wrote:
>I wonder to what facies you would assign assemblages with tridymite,
>sanidine, mullite and glass that are now assigned to the sanidinite facies.
SCMR's paper#3 lists clinopyroxene-orthoclase-plagioclase as the
characteristic mineral assemblage of both granulite and pyroxene-hornfels
facies, and notes that sanidinite facies is "distinguished from pyroxene
hornfels facies by the occurrence of especially high-temperature varieties
and polymorphs of minerals". As Miyashiro rightly points out, disordered
feldspars and pigeonite etc may be stable in granulite facies but invert
and/or unmix during slow cooling. If so, the presence of disordered
minerals and glass etc may merely indicate unusually rapid cooling. Re the
very low P imputed to sanidinite facies, it might be argued that
sufficiently rapid cooling can be achieved only in rocks at very low P, but
I think this would be stretching the facies concept beyond its point of
failure. In practice I think we impute very high T and low P to these rocks
from their distinctive field setting (xenoliths in fresh basaltic lava
flows, very narrow contact aureoles around unaltered mafic sills, etc), not
from distinctive mineral assemblages in metabasites.