A comment by Stephen M. Richard about
Dear SLTT Metamorphic team (and other potentially intereseted parties!!)-- At this point, we're zeroing in on a draft classification based on rock fabric and modal mineralogy. This classification system is not a system for naming metamorphic rocks for the purpose of geologic discourse. It is a descriptive classification system that attempts to provide objective criteria for grouping the domain of metamorphic rocks (including metasomatic rocks and strain-dominated metamorphic rocks) into kinds based on fabric and mineralogical criteria only. This philosophy departs from that of Robertson [1999], in that we are designing not a set of names to use for particular rocks, but a set of ‘bins’ into which rocks that are similar (based on the classification criteria) can be ‘placed’. The names we assign to the ‘bins’ are not the issue. Starting premises: 1. the scheme should be as descriptive as possible 2. a rock may be classified in more than one way 3 the scheme is for classifying rocks as metamorphic rocks, thus interpretation of the protolith of a rock should not be used to determine the classification of the rock 4 hand sample scale 5 rock mineralogy is described in other database fields 6. A geologist can give a rock any name they feel is most appropriate (a la Robertson, 1999, BGS scheme) using an uncontrolled ‘rock name’ field in the database. Thought experiment to check if classification is descriptive--imagine a civil engineer or materials scientist trying to use the system ********************************************************* we (the NADM SLTT metamorphic rock classification team http://geology.usgs.gov/dm/terms/) need to reach some sort of consensus or at least a majority decision on the following points. In the discussion of the questions, I'm presenting my preferred approach. I. 'meta' prefix Should the metamorphic rock lithology classification pick-list include terms such as meta-sandstone, meta-granite, etc? Lets dodge this bullet. If we allow such terms then we face two giant problems-- First--defining exactly what they mean in descriptive terms so that e.g. sandstone can be distinguished from meta-sandstone from quartzo-feldspathic granofels or semi-pelitic schist. Second--choosing which meta-this and meta-that terms are included in the controlled word list. Inclusion of meta-xxxx terms is not necessary because the geologist will be able to use the meta-xxxx name in the uncontrolled word list, and in the lithology classification classify the rock according to what kind of sandstone it is(was) **and** according to what kind of metamorphic rock it is (see premise #2). Terms such as meta-xxxx do not communicate the ***kind of metamorphic rock*** we're classifying, thus defeating the purpose of classification as a metamorphic rock. ************************************************************************ II. Basic classes of metamorphic rocks based on rock fabric. The question is do the following 8 basic classes partition metamorphic rocks based on descriptive criteria in a manner that different observers can agree upon? Note that all quantitative boundaries (%, ratios, dimensions) should be considered open to discussion—the qualitative distinctions are the issue here. 1. Rocks with fragmental fabric, in which >10% of rock is fragments bounded by fractures. Cutoff % for definition is debatable, but need to distinguish from type #3 below, and define at what point the fragmental fabric is significant enough to merit classification as a different rock. (remember that since a rock may belong to more than one class, it may meet this criteria and also belong to one of the following classes, as well as a protolith class....) 2. Rocks that are too fine-grained to determine mineralogy. Criteria is that average grain size <0.032 millimeter. This criteria is meant to separate rocks that can be classified based on modal mineralogy from rocks too fine grained to distinguish mineralogy. The cut-off diameter corresponds to the sand-silt boundary of sedimentary rock classification schemes, which varies between 0.032 mm [Robertson, 1999, BGS grain size scheme] and 0.074 mm (Engineering grain-size scale, ASTM standard D422-63; D643-78) 3. Phaneritic rocks with granoblastic fabric and very little or no foliation. Very little foliation means some foliation may be present, but does not meet the criteria for foliated. Foliated means that >= 10% of the mineral grains in the rock are fabric elements. To be a fabric element, a mineral grain must have an inequant crystal habit, or a mineral grain or grain aggregate must have an inequant shape due to deformation with and aspect ratios >= 1.5:1 between the long and short axis of the deformed grain. 4. Foliated phaneritic rocks in which at least one major constituent displays a foliation defined by the shape of deformed mineral grains or grain aggregates with an aspect ratio > 1.5:1, and >10% 'matrix' is present associated with the deformed mineral grains. Matrix is an aggregate of new mineral grains (not present in the protolith, but may be same mineral species) that are similar in size or smaller than the mineral grains affected by non-cataclastic deformation. Non-cataclastic deformation is deformation in which the material continuity of the deformed volume is maintained on the scale of observation, indicated by the absence of thoroughgoing fractures in the volume. The definition is meant to identify a fabric in the rock due to crystal plastic and/or other types of non-cataclastic deformation. This definition is somewhat problematic because some knowledge about the protolith must be known or assumed; a provision might be made that in the absence of other knowledge, the protolith is assumed to have consisted of equant mineral grains with a size distribution similar to the largest grains in the metamorphic rock. 5. Phaneritic rocks with a well developed, continuous schistosity. Here the sticking point is the definition of ‘well developed schistosity’. I favor a definition that >50% of rock consists of mineral grains with a tabular, lamellar, or prismatic crystallographic habit that are oriented in a continuous planar or linear fabric (following Bates & Jackson). Continuous is defined on a hand sample-scale, and in quantitative terms to mean that domains lacking the fabric are < 1 centimeter thick if they are layers, and <5 cm in diameter if they are irregular patches, and constitute < 25% of the rock. IUGS suggests using criteria that rock splits on scale <1 centimeter, but this criteria depends on the tool used to do the splitting, the skill of the operator doing the splitting , and the degree of weathering or alteration of the rock being split, and is thus not objective and universally applicable. The IUGS criteria could not be used to classify a rock in thin section. The IUGS criteria also defines schistosity as due to inequant mineral grains or grain aggregates, without specifying that their shape is due to crystallographic habit. 6. Foliated, phaneritic rocks without well developed, continuous schistosity that have continuous compositional layering, > 5 mm thick. Continuous means that layers defining the foliation can be traced for > 10 centimeter (length of lateral continuity), and are spaced at a distance <= the average length of lateral continuity. 7. Foliated, phaneritic rocks without well developed, continuous schistosity or con-tinuous compositional layering in which >50% of rock is part of fabric elements defining foliation. 8. Foliated, phaneritic rocks without well developed, continuous schistosity or continuous compositional layering in which 10-50% of rock is part of fabric elements defining foliation Please pick 10 of your favorite metamorphic rocks and see if they reasonably fit into this classification. Classes 7 and 8 are 'everything-else' type classes necessary for completeness, but I think lots of 'foliated granite' and 'slightly schistose marble' type rocks will end up in them. We need to come up with something to call these. Suggestions please?? cheers steve 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 Basic divisions in classification of metamorphic rocks (this page):
(No comments about this document have been posted.)