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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: >Dugald >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. Cheers, Dugald