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Jürgen Reinhardt wrote: >> >>> [log in to unmask] 11/19/01 06:32PM >>> >> Crossing vertically up-P from a hornfels to a regional facies does >> not represent a change in grade, so nothing dramatic should happen. >> >> An ignomineous petrologist > The change from amphibolite facies to eclogite facies is also not a change in grade, >but something does happen. Yes, but because no change in the grade of metamorphism has taken place, the change must just be an artifact of original differences in the protoliths. >I hope you didn't mean crossing from contact to regional faies, because you might get >stoned to death as things stand right now. Yes, perhaps temperature decreased when passing from the facies of contact metamorphism around a pluton to the older, deeper facies of regional metamorphism; if so, the metamorphism decreased in grade with depth. > Perhaps one way of keeping the distinction between different P-T gradients alive > is to use a fourth facies series term as in Blatt & Tracy (I haven't seen this > in any other book so far). I don't have hang-ups about "hornfels" or, alternatively, > maybe "very-low-pressure/high-temperature". > > A metaphoric petrologist The great value metamorphic petrologists place on nomenclature is most gratifying, and speaks well of all. The GEO-LEXICOGRAPHY list! I'm sure all will agree with Prof Yardley that classification & definition, being the sole of petrology, are worthy of their own geo-lexicographical list. Unfortunately, natural classifications cannot be separated from theory, and operational definitions cannot be separated from technique. So, there is no pure nomenclature to separate. Natural Classification 'The quartz-feldspathoid and quartz-olivine boundaries are natural dividing-lines, and as such they have a scientific importance which artificial boundaries can never have.' -S.J. Shand, Eruptive Rocks, p228. Cf. Yoder & Tilley. Operational Definition 'Thus, Eskola (1939) attempted to give an excessively positivistic definition which was based only on directly observable chemical and mineral compositions. This attempt stemmed probably from the positivistic spirit he had aquired in his youth under the influence of F.W. Ostward, Ernst Mach, and others.' -A.Miyashiro, Metamorphism & Metamorphic Belts, p294. (For 'excessively positivistic', today read 'operational'.) Lexicography (with etymology!) Facies, taken directly from the Latin for form or shape (especially of the face), share a conceptual meaning in several sciences. (1) It's more general use in Victorian England, as today, was generally synonymous with 'appearance'. (2) It's more restricted use was a more abstract 'appearance' that attempted to capture a phenomenon by ignoring distracting features. That property used to identify the phenomenon is usually prefixed. Examples of definition 2 Hence mineral facies capture metamorphism by ignoring compositional changes of the protolith. Lithofacies capture sedimentary environments by ignoring chronostratigraphic changes of the protolith. Faunal facies capture chronostratigraphic environments by ignoring lithostratigraphic changes of the protolith. (This was one of geology's earliest 'natural classifications', sought by Edward Forbes, a contemporary of Sedgwick & Murchison.) Metamorphic Facies Goldschmidt studied the hornfels facies of his contact metamorphic aureole (dfn 1). His classes neatly defined a mineral facies for Eskola (dfn 2), whose holotypic rock was a hornfels! (Eskola's contact facies near Orijarvi were used to define the amphibolite facies.) Some objected to this restricted use of facies, though it had old precedents. 'In petrology the expression "facies" had, until now, designated areas within a single geological region that varied either in "Struktur" (Strukturfacies) or in the rock's mineral assemblage & chemical composition (Konstitutionsfacies).' -F.Becke, 1921, Zur Facies-Klassifikation der metamorphen Gesteine, T.M.P.M., v35, p218. (My translation. Facies also brought to Becke's mind the 'tektonische Facies' of B.Sander.) 'The term "facies" as applied by Eskola must be considered as somewhat unfortunate, as Becke has already pointed out. Facies is a term long since established in geology, but its use is now bewilderingly lax. It has been adopted in paleontology, but is also used i tectonics and stratigraphy. Petrology has not escaped, the term being used with reference to a variant of a rock type, as in "marginal facies", &c.' -C.E.Tilley, 1924, Facies of metamorphic rocks, Geol Mag, v.61, p.168. Very low temperature & high temperature contact metamorphic facies So, if the context of discussion is not the mineral-facies classification of rocks, one should be able to use 'aphanitic facies' or 'schistose facies'. 'Hornfels facies' seems ambiguous, at best. If the appearance of the rock makes its petrogenesis very clear, one might use 'contact-metamorphic facies' or 'regional-metamorphic facies'. Or, one can bravely extend this definition of facies even further. Personally, I see no ambiguity and wouldn't object. Bruce Bathurst