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

As pointed out by Gilboy ( 1982), lack of conformity in naming metamorphic rocks derived from elastic sediments commonly causes problems with clear communication of geological facts and hinders attempts at lithologic correlation .

Most geologists working in unmetamorphosed sedimentary bas ins use elastic sedimentary rock classification systems that are based on a combination of grain size, rock fragment content, and mineral composition (e.g. Dott, 1964; Young, 1967; Folk, 1968; Pettijohn et al .. 1973). Recrystallization during metamorphism changes these parameters, especially grain size and rock fragment content, and therefore renders such classification diagrams of limited use for metamorphosed sedimentary rocks.

Mineralogy and grain size are inter-related in some elastic rock types. Thus mudstone is extremely fine grained and contains abundant clay minerals, whereas 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 known (e.g. Winkler, 1979; Yardley, 1989), the mineral composition of a metamorphosed sedimentary rock can be used to infer broadly the orig inal g rain size of a rock. Mineralogy therefore becomes the main criterion for classifying metamorphosed elastic rocks.

Field geologists in the 1950s and 1960s generally defined metamorphic lithological units on the basis of mineral content, and so used terms such as garnet­biotite-quartz-feldspar gneiss for metamorphosed sedimentary rocks; and amphibolite, hornblende gneiss and fe lsic gneiss for rocks of volcan ic or unknown derivation. These names, however, conveyed no information about the genesis of the rocks.

In Saskatchewan' s current I :20 000 scale mapping program, rock names such as metagreywacke, meta­arkose, or meta-arenite are widely used. Although these terms are commonly chosen to provide insight into the depositional setting of the rocks, they are not used in a consistent way (e.g. Summary of Investigations 1997). The term greywacke, for example, has had so many different definitions over the last 200 years (Bates and Jackson, 1990 and references therein) that its use has become very ambiguous. Arkose is also a term that has various de finitions (e.g. Huckenholz, 1963; Pettijohn et al. , 1973). As pointed out above, one component of existing elastic

Saskatchewan Geological Survey

classification diagrams is the rock fragment content, which is unlikely to be preserved at higher metamorphic grades. At such grades, composition alone provides no guide to the original rock fragment content. Consequently, at medium to high metamorphic grades, an arkose as defined by Pettijohn et al. ( 1973) is indistinguishable from an ' arkosic arenite ' or a ' lithic' arenite. Similarly, a metamorphosed 'arkosic wacke' cannot be distinguished from a ' lithic greywacke'.

The problem of classifying metamorphosed sedimentary rocks is not new. Tyrell ( 1921) used the term pelite as the metamorphic derivative of fine­grained sedimentary rock such as si ltstone or mudstone, and psammite as the metamorphic derivative of a sandstone. This nomenclature is still commonly employed, however, when used on the same map in conjunction with names such as meta-arkose, metagreywacke, or meta-arenite (e.g. Summary of Investigations 1997), confused communication of geological facts may ensue.

2. Proposal

We suggest use of two separate diagrams to classify most metamorphosed si liciclastic sedimentary rocks of northern Saskatchewan. At very low metamorphic grades, where most primary textures, and in particular labile (i.e. mechanica lly or chemically unstable) fragments of sand-size are discernible, we propose use of a set of ternary discrimination diagrams based on those of Dott ( 1964) and Pettijohn et al. ( 1973, Figure I). We suggest that:

I) The term 'argillite' be substituted for 'mudstones ' as this is a we ll-defined and much used term for slightly metamorphosed mudstones (Bates and Jackson, 1990).

2) The term ' wacke' (following Dott, 1964) be used in preference to ' greywacke' (Pettijohn et al., 1973) because of the varying definitions and connotations of greywacke (see Bates and Jackson, 1990).

3) The term ' feldspathic arenite' (following Dott, 1964) be used in preference to 'arkose' or 'arkosic arenite' (Pettijohn el al., 1973) because of its varying definitions (see Pettijohn et al., 1973; Bates and Jackson, 1990).

With onset of groundmass recrystallization, and obliteration of original grain size and rock fragment

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content, a revised version of Gilboy' s ( 1982) ternary classification is more suitable (Figure 2). In this classification, a psammite is the metamorphosed equivalent of feldspathic arenite or lithic arenite, a psammopelite is the equivalent of wackes, and a petite derives from an argillite. The aluminum silicate mineral content allows for the limited presence of Ca-

bearing silicates such as hornblende. However, calc­silicate rocks with appreciable amounts of diopside, tremolite-actinolite, or carbonate are not covered by this classification diagram. Similarly, magnesium­aluminum-rich rocks such as cordierite-anthophyllite schists which clearly represent metamorphosed

alteration zones should not be referred to as pelites.

Low Grade

1. Ouartz Arenfte

2. $ubfeldsarenite

3. Sublitharen1te . · / ~artzWac:ke

95%/. • / • \

I Quartz' •,

\ ... 95%, ·· 1 \

75%_i _213 '\

" (

I \ ~,;. \ 'H \ .···

Feldspathic I A,enlte

Feldspar

\ "

\/ Lilhtc \ Atef'Mte ·\

Cl . Sand-sized Rock Fragments

\ \ . . --~~

_ .. ··

\ \ ...... ~~

...,<:. ,-s-e o\o

Figure 1 • A set of ternary diagrams, modified after Doti (1964) and Pettijohn et al. ( 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 understanding that all the rocks have been metamorphosed.

High Grade

1. Quartzite 2. Feldspathic

Quartzite 3. Impure

Quartzite

Quartz

'\

75%~~ ; \ % 25%

/ . 3 x /I/ !,/ '\

/ / \/' .

i / .if l \ . <b . ~ I ./]§ / ~ /

/ /-/ ff / \ l <tj / ~ / q, \

! tJ..0 / 4i _/ ~ '\ , ' tJ.. , 'U

/ /' ~ \ I '

i l _ ~ _ L ~- ___ . __ _ \,'.._ __ ~ _ _, _ __ _ ___ , \\ /

Feldspar ASM 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 been destroyed. ASM=aluminum silicate mineral content (exclusive of feldspar.t; inclusive of mica, garnet, Al1 Si05 polymorphs, hornblende, staurolite, etc.).

90

The two classification systems (Figures I and 2) should not be used together in the same paper or map legend unless signaling an increase in metamorphic grade.

To further discriminate between various types of pelites, psammopelites, or psammites, compositional qualifiers based on chemistry (e.g. aluminous, calcic, potassic) or mineralogy (e.g. homblendic, gametiferous, microcline-bearing) can be used . Textural metamorphic qualifiers such as gneissic, schistose, or migmatitic can also be applied. You could, for example, have a unit of 'gneissic psammite' , or an ' aluminous psammitic gneiss'. Alternatively, the latter may be referred to as a 'gneissic sillimanite psammite' . With the first appearance of a calc-silicate mineral, such as hornblende, the qualifier calcic should be used. It is therefore conceivable to have a ' homblendic psammopelite' or 'calcic psammopelite' , which may be a rock that derived from weathering of mafic volcanics ( e.g. Delaney, 1991 ; Ashton and Leclair, 1991 ; Maxeiner and Watters, 1991 ).

The tenn calcareous ought to be reserved for carbonate-bearing rocks, which are not dealt with in this attempt to classify metamorphosed elastic sedimentary rocks. The term aluminous ought to be reserved for rocks with particularly Al­rich minerals such as the Al2Si05

polymorphs, staurolite, and cordierite.

Coarser elastic sedimentary rocks. such as conglomerates, commonly retain their original textural integrity even during high-grade metamorphism, and use of the term ' metaconglomerate' is recommended for such rocks.

Summary of Investigations 1999, Volume I

The matrix of poorly sorted conglomerates will undergo the same changes as arenites or mudstones, and can be described as psammitic or psammopelitic. Pebble and larger-sized fragments in such conglomerates will usually remain recognizable; however, in some o ligomictic conglomerates, where clasts and matrix are of the same composition, it may be difficult to recognize the protolith. Parts of the Janice Lake fanglomerate in the Wollaston Domain (e.g. Delaney el al., 1995) are an example.

3. Acknowledgments

We would like to thank our colleagues for critical review of the original manuscript. A special thanks to Ken Ashton for numerous lively discussions on the subject.

4. References Ashton, K.E. and Leclair, A.O. ( 1991): Revis ion

bedrock geological mapping, Wildnest-Attitti lakes area (parts of NTS 63M- I and -2); in Summary of Investigations, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 9 1-4, p29-40.

Bates, R.L. and Jackson, J.A. (1990): Glossary of Geology; th ird edition, Amer. Geo!. Inst. , Alexandria, 788p.

Delaney, G.D. ( 1991): Revision bedrock geological mapping, Descharnbault-Oskikebuk lakes area (parts ofNTS 731-16 , 63L-1 l , - 12, -13, and-1 4); in Summary of Investigations 199 1, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 91-4, p47-56.

Delaney, G.D., Maxeiner, R.M. , Rawsthome, M.L. , Reid, J ., Hartlaub, R., and Schwann, P. ( 1995): Geological se tting of sediment-hosted Copper mineralization in the Janice Lake area, Wollaston Domain; in Summary of Investigations 1995, Saskatchewan Geological Survey, Sask. Energy Mines. Misc. Rep. 95-4, p30-48.

Dott, R.H. ( 1964): Wacke, greywacke, and matrix­what approach to immature sandstone classification?; J. Sed. Petrol., v34, p625-632.

Folk, R.L. ( 1968): Petrology of Sed imentary Rocks; Hemphill 's Book Store, Austin, I 70p.

Folk, R.L. , Andrews, P.B., and Lew is, D.W. (1970): Detrital sedimentary rock classification and nomenclature for use in New Zealand; N.Z. J. Geo!. Geophys., v 13, p937-968.

Garrels, R.M. and Mackenzie, F.T. ( 197 1): Evolution of Sedimentary Rocks; Norton and Co., New York, 397p.

Saskatchewan Geological Survey

G ilboy, C. F. ( 1982): Classification of Clastic Metasediments; Sask. Dep. Miner. Resour., Open File Rep., 82-3, 33p.

Huckenholz., H.G. ( 1963): Mineral composition and texture in greywackes from the Harz Mountains (Germany) and in arkoses from the Auvergne (France); J. Sed. Petrol., v33, p9 14-918.

Maxeiner, R.O. and Watters, B.R. (199 1 ): Mineralization and associated alteration in the western Hanson Lake area; in Summary of Investigations 1991 , Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 9 1-4, pl09- I I 7.

Pettijohn, F.J., Potter, P.E., and Siever, R. ( 1973): Sand and Sandstone; Springer Verlag, Berlin, 6 I 8p.

Summary of Investigations ( 1997): Sibbald, T.I.I ., Harper, C.T., and Paterson, D.F. (eds.), Saskatchewan Geo log ical Survey, Sask. Energy Mines, Misc. Rep. 97-4, 229p.

Tyrell, G.W. (1921): Some points in petrographic nomenclature; Geol. Mag., v58, p494-502.

Winkler, H.G.F. (1 979): Petrogenesis of Metamorphic Rocks; fifth edition, Springer Verlag, New York, 348p.

Yardley, B. W.D. ( 1989): An Introduction to Metamorphic Petrology; Longman, Singapore, 248p.

Young, G.M. ( 1967): Sedimento logy of Lower Visean? rocks in the western part of the Ballina and Donegal synclines, northwestern Ireland; unpubl. Ph.D. thesis, Univ. G lasgow, 204p.

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