Classification of Metamorphosed Clastic Sedimentary Rocks: a Proposal
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B. Contributed Papers - Northern Saskatchewan Geological Survey 87 88 Summary of Investigations 1999, Volume l Classification of Metamorphosed Clastic Sedimentary Rocks: A Proposal Ralf 0. Maxeiner. Chris F. Gilboy. and Gary M Yeo Maxeiner. R.0. Gilboy, C.f., and Yeo. G.M. ( 1999): Classification.of m.etamorp~osed elastic sc?iment~I)' rocks: A proposal; in Summary of Invt:stigations J 999. Volume I. Saskatchewan Geological Survey, Sask. Energy Mmes, Misc. Rep. 99-4. 1. l . Introduction classification diagrams is the rock fragment content, which is unlikely to be preserved at higher As pointed out by Gilboy ( 1982), lack of conformity in metamorphic grades. At such grades, composition naming metamorphic rocks derived from elastic alone provides no guide to the original rock fragment sediments commonly causes problems with clear content. Consequently, at medium to high communication of geological facts and hinders metamorphic grades, an arkose as defined by Pettijohn attempts at lithologic correlation. et al. ( 1973) is indistinguishable from an 'arkosic arenite' or a 'lithic' arenite. Similarly, a Most geologists working in unmetamorphosed metamorphosed 'arkosic wacke' cannot be sedimentary basins use elastic sedimentary rock distinguished from a ' lithic greywacke'. classification systems that are based on a combination of grain size, rock fragment content, and mineral The problem of classifying metamorphosed composition (e.g. Dott, 1964; Young, 1967; Folk, sedimentary rocks is not new. Tyrell ( 1921) used the 1968; Pettijohn et al.. 1973). Recrystallization during term pelite as the metamorphic derivative of fine metamorphism changes these parameters, especially grained sedimentary rock such as siltstone or grain size and rock fragment content, and therefore mudstone, and psammite as the metamorphic renders such classification diagrams of limited use for derivative of a sandstone. This nomenclature is still metamorphosed sedimentary rocks. commonly employed, however, when used on the same map in conjunction with names such as meta-arkose, Mineralogy and grain size are inter-related in some metagreywacke, or meta-arenite (e.g. Summary of elastic rock types. Thus mudstone is extremely fine Investigations 1997), confused communication of grained and contains abundant clay minerals, whereas geological facts may ensue. arkose is generally coarser grained and composed largely of quartz and feldspar (e.g. Garrels and Mackenzie, 197 1) . Because mineralogical reactions at different metamorphic grades are relatively well 2. Proposal known (e.g. Winkler, 1979; Yardley, 1989), the We suggest use of two separate diagrams to classify mineral composition of a metamorphosed sedimentary most metamorphosed siliciclastic sedimentary rocks of rock can be used to infer broadly the original grain size northern Saskatchewan. At very low metamorphic of a rock. Mineralogy therefore becomes the main grades, where most primary textures, and in particular criterion for classifying metamorphosed elastic rocks. labile (i.e. mechanically or chemically unstable) fragments of sand-size are discernible, we propose use Field geologists in the 1950s and 1960s generally of a set of ternary discrimination diagrams based on defined metamorphic lithological units on the basis of those of Dott ( 1964) and Pettijohn et al. ( 1973, Figure mineral content, and so used terms such as garnet I). We suggest that: biotite-quartz-feldspar gneiss for metamorphosed sedimentary rocks; and amphibolite, hornblende gneiss I) The term 'argillite' be substituted for 'mudstones' and fe lsic gneiss for rocks of volcanic or unknown as this is a well-defined and much used term for derivation. These names, however, conveyed no slightly metamorphosed mudstones (Bates and information about the genesis of the rocks. Jackson, 1990). In Saskatchewan's current I :20 000 scale mapping 2) The term ' wacke' (following Dott, 1964) be used program, rock names such as metagreywacke, meta in preference to 'greywacke' (Pettijohn et al., arkose, or meta-arenite are widely used. Although 1973) because of the varying definitions and these terms are commonly chosen to provide insight connotations of greywacke (see Bates and Jackson, into the depositional setting of the rocks, they are not 1990). used in a consistent way (e.g. Summary of 3) The term 'feldspathic arenite' (following Dott, In vestigations 1997). The term greywacke, for 1964) be used in preference to 'arkose' or 'arkosic example, has had so many different definitions over the arenite' (Pettijohn el al., 1973) because of its last 200 years (Bates and Jackson, 1990 and references varying definitions (see Pettijohn et al., 1973; therein) that its use has become very ambiguous. Bates and Jackson, 1990). Arkose is also a term that has various definitions (e.g. Huckenholz, 1963; Pettijohn et al., 1973). As pointed With onset of groundmass recrystallization, and out above, one component of existing elastic obliteration of original grain size and rock fragment Saskatchewan Geological Survey 89 content, a revised version of Gilboy' s ( 1982) ternary bearing silicates such as hornblende. However, calc classification is more suitable (Figure 2). In this silicate rocks with appreciable amounts of diopside, classification, a psammite is the metamorphosed tremolite-actinolite, or carbonate are not covered by equivalent of feldspathic arenite or lithic arenite, a this classification diagram. Similarly, magnesium psammopelite is the equivalent of wackes, and a petite aluminum-rich rocks such as cordierite-anthophyllite derives from an argillite. The aluminum silicate schists which clearly represent metamorphosed mineral content allows for the limited presence of Ca- alteration zones should not be referred to as pelites. The two classification systems Low Grade (Figures I and 2) should not be used together in the same paper or map legend unless signaling an increase in metamorphic grade. 1. Ouartz Arenfte 2. $ubfeldsarenite . · / ~artzWac:ke To further discriminate between 3. Sublitharen1te 95%/. • / • \ various types of pelites, Quartz' I ... •, psammopelites, or psammites, 95%, ·· 1 \ \ compositional qualifiers based on 1 I \ 75%_i _2 3 '\ ~,;. \ chemistry (e.g. aluminous, calcic, " ( 'H \ .··· potassic) or mineralogy (e.g. \ \ " \ . .- -~~ \ homblendic, gametiferous, \/ \ ...... ~~ microcline-bearing) can be used . ...,<:. ,-s-e Textural metamorphic qualifiers _. .·· o\o such as gneissic, schistose, or Feldspathic I Lilhtc \ A,enlte Atef'Mte ·\ migmatitic can also be applied. You could, for example, have a Cl . unit of 'gneissic psammite' , or an Feldspar Sand-sized ' aluminous psammitic gneiss'. Rock Fragments Alternatively, the latter may be referred to as a 'gneissic Figure 1 • A set ofternary diagrams, modified after Doti (1964) and Pettijohn et al. sillimanite psammite' . With the ( 1973), for use in describi11g /ow-grade metamorphic rocks i11 which rock fragments have been preserved; the prefix meta· has been omitted in this diagram, with the first appearance of a calc-silicate understanding that all the rocks have been metamorphosed. mineral, such as hornblende, the qualifier calcic should be used. It is therefore conceivable to have a High Grade 'homblendic psammopelite' or Quartz 'calcic psammopelite', which may be a rock that derived from 1. Quartzite '\ weathering of mafic volcanics (e .g. Delaney, 1991 ; Ashton and 2. Feldspathic Leclair, 1991 ; Maxeiner and Quartzite 75%~~ ; \ % 25% Watters, 1991 ). 3. Impure / . 3 x The tenn calcareous ought to be Quartzite reserved for carbonate-bearing /I/ !,/ '\ rocks, which are not dealt with in this attempt to classify \/' . / / metamorphosed elastic .i <b ./ ~.if I l \ sedimentary rocks. The term ./]§ / ~ / aluminous ought to be reserved / /-/ ff / \ for rocks with particularly Al l <tj / ~ / q, \ rich minerals such as the Al 2Si05 ,! tJ..0 '/ tJ..4i _,/ ~'U '\ polymorphs, staurolite, and / /' ~ \ cordierite. I ' Coarser elastic sedimentary i rocks. such as conglomerates, / l _ ~ _ L ~- ___ . __ _\, '.._ __ ~ _ _, _ __ _ ___, \\ commonly retain their original Feldspar ASM textural integrity even during high-grade metamorphism, and Figure 2 - A ternary diagram, modified after Gilboy ( 1982), for use ill describi11g metliu~ to high-grade metamorphic rocks in which sand-size rock fragments have use of the term been destroyed. ASM=aluminum silicate mineral content (exclusive offeldspar .t; 'metaconglomerate' is rec ommended for such rocks. inclusive ofmica, garnet, Al1 Si05 polymorphs, hornblende, staurolite, etc.). Summary of Investigations 1999, Volume I 90 The matrix of poorly sorted conglomerates will G ilboy, C.F. ( 1982): Classification of Clastic undergo the same changes as arenites or mudstones, Metasediments; Sask. Dep. Miner. Resour., Open and can be described as psammitic or psammopelitic. File Rep., 82-3, 33p. Pebble and larger-sized fragments in such conglomerates will usually remain recognizable; Huckenholz., H.G. ( 1963): Mineral composition and however, in some oligomictic conglomerates, where texture in greywackes from the Harz Mountains clasts and matri x are of the same composition, it may (Germany) and in arkoses from the Auvergne be difficult to recogni ze the protolith. Parts of the (France); J. Sed. Petrol., v33, p914-918. Janice Lake fanglomerate in the Wollaston Domain (e.g. Delaney el al., 1995) are an example. Maxeiner, R.O. and Watters, B.R. (199 1): Minerali zation and associated alteration in the western Hanson Lake area; in Summary of 3. Acknowledgments Investigations