A comment by Stephen M. Richard about
Hi team--well, back to work. I've been collating the responses to the questions posed in the message "SLTT metamorphic, review of comments, and questions based on those", posted 03/14/01. Only 5 responses have been received, and here are the results thus far. ****If you have any objections to the emerging majority/consensus decision, please get your 2 cents in before the end of the week (04/27/01).**** Summary of votes received at this point: 1. Should texture terms (gneiss, schist, granofels, hornfels) be used in place of ‘rock’ in root names whenever possible? Yes: 4 ONE vote for give texture-based and rock terms equal weight 2. Should Robertson’s definition of amphibolite be restricted to: Amphibolite: rock that contains >50% amphibole YES: 2 votes NO: 2 votes; objection seems to be use of 50% as cut off; amphibolites might contain less am-phibole, and abundance of plagioclase should also be criteria pass: 1 3. Should fault rock terminology (including Pseudotachylite) be revised to be consistent with Scholtz (1990) as endorsed by Snoke and Tullis (1998)? (we need specific definitions here) YES 3 pass: 2 4. Should we endorse the Robertson [1999] distinction between schist and gneiss based on the presence of layering >0.5 centimeter thick in gneiss? NO: 3; major objects seems to be that schist-gneiss distinction is based on schistosity/fissility in schist, not in gneiss. See comment on Gneiss-Schist classification at bottom. Yes (conditional): 1 Pass: 1 5. Are leucogneiss, leucogranofels, mafic gneiss (or melanogneiss?), mafic granofels, mafic schist allowable root names? NO: 3. suggestion to use leucocratic, melanocratic and mafic as modifiers [but note this begs the question of whether there should be specific classes for leucocratic gneiss/leucogneiss etc. in the classification—smr] Yes (sort of): 2. terms are probably useful in a terminology hierarchy for completeness 6. Should the definitions of marble and metacarbonate rock read: Marble: metamorphic rock with sedimentary protolith, composed of >50% calcsilicate or car-bonate minerals, with carbonate minerals more abundant than calcsilicate minerals Metacarbonate rock: metamorphic rock with sedimentary protolith, in which modal mineral proportions can not be estimated, but rock is apparently largely composed of carbonate or calc-silicate minerals YES: 3 NO: 2. objection to calling a rock with (51% calcite+49% diopside) or (14% calcite, 12% dolo-mite, 25% calcsilicate mineral, and 49% of anything) else to be called a calcite marble 7. Should eclogite be considered a subdivision of meta-ultramafic rock based on modal mineralogy? NO: 5 (see discussion at end) 8. Should the terminology for generic metaigneous rocks be revised to change metafelsic-rock, metamafic rock, and meta-ultramafic rock to one the textural terms with felsic, mafic, and ul-tramafic as modifier to generate a root name? YES: 1 NO: 4. General dislike of use of ortho and para as root name prefixes: redundant in some cases (pelitic paraschist), not commonly used in North America, compositional adjectives (felsic, granitic, mafic…) more informative and appropriate 9. Should the term meta-ultramafite, with modifiers for grain size (e.g., coarse-grained meta-ultramafite) replace meta-ultramafite and meta-ultramafic-rock? YES: 1 NO: 2. see discussion at end. pass: 2 10. Should migmatite be added as a root name, replacing migmatitic gneiss and migmatitic gran-ofels (and migmatitic schist??). Use both migmatite and magmatitic gneiss/granofels/schist: 2 YES: 1 NO: 1, Don’t Know:1. both based on difficulty of defining migmatitic gneiss distinct from gneiss or schist Use Both: 2 11. Should whiteschist be included as a root name? If so, we need a definition for the term, and to place it in the key. YES: 3 OK with me: 1 Don’t Know: 1 12. Should metaquartzite be included as a root name? NO: 4 yes: 1 13. Should soapstone be included as a root name? Soapstone: non-schistose, massive talc rock NO: 4 Don’t Know: 1 14. Should monomineralic rocks be included with different names based on the mineral constitu-ent, or should there be a root name based on texture, e.g. monomineralic granofels, monomin-eralic hornfels, monomineralic schist. Keep ‘traditional’ names only: 3 Name with mineral Name: 1 Terms are necessary for complete hierarchy: 1 15. Should schist and granofels be subdivided into paraschist, orthoschist, paragranofels and or-thogranofels based on identification in sedimentary or igneous protolith? NO: 4 sort of No: 1 16. Should the root name pelite be retained as used by Robertson [1999]? If not, what is a better term and definition? YES: 1 NO: 4 (see #19) 17. Should the root name phyllite be added? phyllite: Metasedimentary (or just metamorphic?) rock possessing a silky or lustrous sheen on foliation surfaces imparted by fine-grained (< 0.1 mm) white mica (including muscovite, para-gonite and phengite) orientated parallel to the foliation in the rock. Individual mica flakes can be seen with the naked eye in contrast to slates where they cannot be distinguished, and to schists in which the mica crystals are >1 millimeter in diameter. YES: 5 18. Which of the following terms should be included in the controlled word list: Rodingite Fenite Skarn Greisen Gondite Yes to all: 2 Yes only to Skarn and greisen: 1 No to all: 1 Pass: 1 19. As an alternative to BGS usage of psammite, semipelite, and pelite as root names, use psam-mitic, semipelitic, and pelitic as adjective modifiers consistent with Robertson's triangular dia-gram. Yes: 4 Yes (tentative): 1 Discussion Proposed definitions for schist and gneiss The definitions for schist and gneiss must be considered in the continuum of rocks from quartzo-feldspathic granofels with no schistosity (foliation defined by alignment of tabular mineral grains, inde-pendent of the modal abundance of the tabular mineral) to pure mica schists, to slate, to gneiss and migma-titic gneiss, to pelitic hornfels or granofels containing large percentage of mica without schistosity. Based on a compilation of definitions of schist and gneiss, the various criteria used to distinguish these rock types are: Presence of compositional banding Thickness of compositional banding Homogeneity of the rock Modal abundance of mica Modal abundance of quartz+feldspar Degree of fissility (how thinly can the rock be parted along the ‘schistosity’) Grain size This discussion requires the definition of some key terms used in the definition of the rock types: Schistosity: foliation in a coarse-grained rock due to the parallel, planar arrangement of mineral grains of the platy, prismatic or ellipsoidal types, usually mica (Bates & Jackson, 3rd ed.); foliation defined by preferred orientation of tabular crystals, especially of micas (Turner&Weiss, 1963). Alternatively: secondary foliation defined by preferred orientation of inequant fabric elements in a medium- to coarse-grained rock; individual foliation-defining elements (e.g. mica) are visible with the naked eye (Passchier & Trouw , 1998) The alternative definition allows a schistosity to be defined by the grains that are inequant due to plastic deformation—thus a pure quartz tectonite with a grain shape fabric defined by tectonically flattened quartz grains would have a schistosity under this definition. I prefer the older definitions because the distinction between foliation related to crystallographic orientation of tabular or elongate crystals (schistosity) and foliation defined by grain shape of mineral aggregates or deformed mineral grains (grain-shape fabric) is important. Compositional layering: non-genetic term for an alternation of layers with different lithological composi-tion (Passchier & Trouw , 1998) Penetrative fabric element: a fabric element that occurs penetratively throughout a rock at the scale of observation (Passchier & Trouw , 1998). Penetrative: repeated at distances so small, compared with the scale of the whole, that they can be considered to pervade the whole uniformly and be present at every point (paraphrase from Turner&Weiss [1963]; see discussion in this text p. 21-24). It seems to me that the distinction of schist and gneiss is based on the homogeneous distribution of ori-ented tabular crystals (tabular due to crystal habit, not deformation of equant grains) in schist, resulting in a schistosity that is penetrative on a hand-sample scale. A schist may have compositional layering at any scale, and still be a schist. A gneiss does not have penetrative schistosity on a hand sample scale, but may have schistose layers separated by non-schistose layers. The boundary of gneiss and schist in this logic is placed where the schistose layers are close enough together that the schistosity is deemed penetrative. My interpretation of Robertson’s [1999] definition is that the schistosity becomes non-penetrative when at least some non-schistose layers are more than 5 millimeter thick, the rock thus being defined to be a gneiss. Problems: rocks in which a schistosity is present oblique to compositional layering. Marble: As I understand the term, marble should imply that a significant percentage (75%?) of the rock is car-bonate mineral. This follows the definition in Bates & Jackson (3rd edition): “a metamorphic rock consist-ing predominantly of fine- to coarse-grained recrystallized calcite or dolomite, usually with a granoblastic, saccharoidal texture” Generic mafic and ultramafic metamorphic rocks Meta-ultramafite conveys that the grain size is too small to determine mineralogy, and the name is based mostly on color, whereas meta-ultramafic rock conveys that the mineralogy is discernible, and the rock is indeed composed of 90% mafic minerals. However, I don’t like these terms because they have the connotation that the protolith was an ultramafic rock. Better to use ultramafic hornfels for ‘meta-ultramafite’ and ultramafic granofels for ‘meta-ultramafic-rock’. If pyrope and omphacite are both taken to be ‘mafic’ minerals, which it would appear they should be, then eclogite would be an ultramafic granofels. In a classification scheme based on texture then composition, eclogites would in general be a subtype of granofels, specifically an omphacite-pyrope granofels, or ultramafic granofels. Stephen M. Richard Arizona Geological Survey 416 W. Congress St., #100 Tucson, Arizona, 85701 USA phone: (520) 770-3500. FAX: (520) 770-3505 email: srichard@iname.com or Richard_Steve@pop.state.az.us
Further discussion of responses to SLTT metamorphic classification questions (this page):