from Dr. Christian Röhr, Bachgasse 13, D-61169 Friedberg-Ockstadt , Germany, E-Mail: christian@christian-roehr.de
developed 1988-1991, published in the Web in 1997
The determination of protoliths of metamorphic rocks are often done by plotting chemical analyses in discrimination diagrams, but the results rarely satisfy. The paper proposes to compare the analysis of a metamorphic rocks with individual analyses of non-metamorphic rocks and to find out the most similiar ones. This is done with the MS-DOS computer program Ancomp and the associated database. The database consists of 2146 analyses of igneous, sedimentary and pyroclastic rocks. Special attention is given to siltstones, widely ignored as protoliths until now, but forming an essential part of sedimentary basins. The methode is thought to give better insight in possible protolith of metamorphic rocks.
One aim of metamorphic petrology is the determination of the protoliths of metamorphic rocks. To the experienced student field relations and macroscopic appereance gives a good guess of the respective protolith. Chemical bulk rock analysis of main constituents and trace elements is nowadays a widespread tool to get better information. Often these analyses are evaluated by plotting oxides, elements or various parameters calculated out of the analyses in a discriminating diagram, but disappointingly the results are rarely unequivocal for various reasons.
Often the main assumption in the determination of the protolith by chemical analysis is that the composition of the rock remained essentially constant except for the well known mobility of the volatile constituents H2O and CO2. It is a matter of current research to better characterize the mobility of the various constituents (the so called mobile and immobile elements) and it will be necessary to establish the conditions under which these assigned mobilities are valid. Because of lack of better knowledge often a certain set of supposed immobile elements is accepted. These are used to discriminate between different rock species, where the borderlines were established from the investigation of rocks not affected by metamorphism.
A major problem of the application of most of the discriminating diagrams is that one must know in advance that the protolith of the metamorphic rock really is one of those in the diagram else the result is meaningless. For example, if you want to use a diagram that discriminates between claystone and graywacke, you must be sure that your gneiss has been a claystone or a graywacke before and you must exclude that it has been an arkose.
Generally you have to discriminate in a tree-like fashion starting from the unequivocal knowledge that your protolith was a rock and then discriminating down to the desired level. The structure of such a tree is examplified in Fig 1. Surely, chemical analyses will not be the first choice at every step and e.g. field relation or texture will be superior. It is a challenge for the future to develope such a tree-structure and to find out what the discriminating power of the various methodes of investigation is at each discrimination step.
Fig. 1: Schematic tree structure that indicates the sequence of discrimination to find out the protolith of an unknown metamorphic rock.
Another major problem of the application of discriminating diagrams is that you know little about the probability whether the given answer is correct or not. This probability will depend on that a really comprehensive set of data was used to construct the discriminating diagram and that the degree of overlap can be quantified.
For example, when constructing a discrimination diagram the rare Na2O-rich arkoses from plagioclase domininated source areas could be classified as outliers far away from the composition of the much more common K2O-rich arkoses from K-feldspar dominated source areas. The information that there are such Na2O-rich arkoses may be lost. But as a metamorphic petrologist one rarely knows something about source areas of metasediments. You want to have a discrimination procedure that treats all source areas equally, no matter whether this source area is rare or not.
My proposal in this direction is not only to use the analyses of metamorphic rocks with respect of compact discrimination diagrams or equations but as a supplement also to make access to as much as possible individual analyses of non-metamorphic rocks. Sorted after similiarity in relation to the analysis of the metamorphic rock under investigation, the resulting list of analyses of non-metamorphic rocks enables the user to get better insight in the possibilities of protoliths.
Such a database and a Pascal program to find out the most similiar analyses is presented here.
Main object of quality of such a procedure is the comprehensiveness of the used database of analyses of non-metamorphic rocks. It should cover the whole spread of possible protoliths. These are fresh as well as altered igneous rocks, both often analysed by igneous petrologists. More worse is the situation with analyses of sedimentary and pyroclastic rocks. Here no equivalent number of analyses exists or is hidden in unknown sources, especially when compared to the great variety of rocks and the great variety of depositional environments. Low grade metamorphic rocks, whose protoliths is unequivocal (slate, spilite etc.) are likely to be included to the database. In polymetamorphic areas metamorphic rocks themselves serve as protoliths. At the present stage it is not thought to be useful to include those analyses to the database.
At the present level no trace element data have been included. The database contains the ten oxides SiO2, TiO2, Al2O3, FeOtotal, MnO, MgO, CaO, Na2O, K2O and P2O5. These were recalculated to 100% leaving out H2O and CO2 and normalizing the state of oxidation by calculating all Fe as FeO. The analyses are described by the original rock name, the location and the source. The original names of the rocks are heterogeneous, because they are based on the preferred nomenclature of the respective author, but some are generalized for clarity.
The database includes 836 analyses of igneous rocks that should cover the whole range of igneous rocks but basaltic rocks are most abundant. Special attention was given to include altered igneous rocks as well (spilites, keratophyres). Details are given in Tab. 1.
The sources of the 303 analyses of pyroclastic rocks are documented in Tab. 2. They cannot be considered to be comprehensive yet especially because of their limited stratigraphic and areal spread.
The 1007 analyses of sedimentary rocks were compiled from the sources presented in Tab. 3. The analyses cover especially the widespread clastic sediments claystone, siltstone and graywacke. Limestones, dolomites and marls are subordinate. Some sediments as phosphorites, iron ores or organic sediments are still missing but these could be easily identified as protoliths.
The siltstones need a special comment. This group of sediments forms an essential part of the clastic input of shelf areas but siltstones are not considered in common discrimination diagrams. In the valuable study of Schulz-Dobrick (1975) which revealed a comprehensive set of nearly 600 analyses of sediments from the Devonian to Carboniferous Rhenish Slate Mountains, the siltstones were discriminated by having an Al2O3-content in the range 8 - 15 wt.-%. Further fieldnames in the siltstone group were fine-grained, clayey sandstones but also black and alum shale as well as sandstone-claystone intercalations. Schulz-Dobrick (1975) calls the siltstone-group 'Tonsandstein'. The inclusion of siltstones is thought to be an important feature of the database.
The MS-DOS program Ancomp compares analyses with those from the database and finds out the most similiar ones. To do this it needs the database-files as well as an ASCII-file that contains the analyses of the user. The user generates this files by a text-editor of his choice. Each line carries the ten oxides plus a text description (max. 70 characters, Fig. 2). The program will normalize the oxides to 100%. Ancomp asks for the input filename, as well as for the name of a file where the output should be sent to.
47.98 1.77 15.44 11.81 0.17
7.98 9.37 2.48 0.92 0.44 Metagabbro, KTB VB
69.49 0.73 13.84 4.84 0.12 2.44 2.38 2.62 2.15 0.07 Paragneis
33a, Nordrach, Schwarzwald
74.38 0.10 13.45 1.74 0.04 0.37 0.85 3.31 4.32 0.03
Biotit-Leptinit 182, Nordrach
4.16 0.01 0.89 0.99 0.32 0.66 52.58 0.07 0.05 0.01
Silikatmarmor, Val Strona
41.7 0.29 3.12 8.11 0.10 33.5 1.83 0.20 0.13 0.05
Serpentinit, Föhrenbühl, EGZ
28.0 1.86 17.5 10.54 0.34 28.5 0.34 0.04 0.01 0.03
Chloritschiefer Stbr. Marienstollen
Fig. 2:
Example of the ASCII input file for Ancomp that carries the analyses of
the metamorphic rocks of the user. Format: real numbers of weight-% of the
ten oxides SiO2, TiO2, Al2O3,
FeOtotal, MnO, MgO, CaO, Na2O, K2O, and
P2O5 seperated by at least one blank and in addition
max. 70 characters of text for sample discription.
The comparison is done by calculating the sum of absolute differences between the actual analysis and those of all database-analyses over ten oxides . The comparison can be manipulated in two ways. The user can exclude selected oxides from comparison, e.g. Na2O and K2O if these two are regarded mobile. A further choice is whether the NORMAL or the LOG oxides shall be used. The use of NORMAL oxides will give the oxides with large quantity (e.g. SiO2) a much larger weight than those with minor quantity (e.g. MnO). The use of LOG oxides will greatly reduce this behaviour.
After comparison the sum of differences is sorted in ascending order to find out the most similiar analyses in the database. The user is asked for howmany of the most similiar analyses shall be displayed. The program works succesively through all analyses in the file provided by the user, displays a shortened output to the screen and sends the full output to the file specified by the user. The output file can be viewed and printed after termination of the program using a text-editor. The output file has 132 characters per line, so the printer has eventually to be adjusted accordingly, e.g. 20 character per inch on standard 8 inch wide (A4) paper.
For the execution of program the following files are necessary:
Ancomp.EXE - the executable MS-DOS program
BASE_OXI.NOR - database containing the normal oxide values
BASE_OXI.LOG - database containing the logarithms of the normal oxide
values
BASE_TXT.STR - database containing the text string descriptions of the samples
In addition the follwing files are given for information
READ.ME - a description, who to prepare an input file
EXAMPLE.DAT - an example of an input file
EXAMPLE.OUT - example of an output file
In subdirectory PAS you can find
Ancomp.PAS - the Turbo Pascal source code of Ancomp.exe
ANACONV.PAS - the Turbo Pascal source code of anaconv.exe
In subdirectory DATABASE you can find all information about the database and how to modify or append it.
BASE.DBF - dBase file, containing all analyses
ANACONV.EXE - executable MS-DOS program, that converts database files for
use with Ancomp
READ.ME2 - description, how to modify or append the database
To download the compressed file Ancomp.ZIP (165 KB) containing all the mentioned
files above click here (exe-file newly compiled in 2002):
Ancomp.zip
You need an UNZIP program (e.g. pkunzip.exe) to decompress this file.
The KTB Feldlabor (Windischeschenbach, Bavaria) gave valuable technical assistance by the compilation of the database.
Amstutz GC (ed) (1974) Spilites and Spilitic Rocks, 482 p.,
Springer-Verlag, Berlin
Best MG (1982) Igneous and Metamorphic Petrology. Freeman, New York
Chab J and Pelc Z (1973) Proterozoische Grauwacken im NW-Teil des
Barrandiums.- Sbor. Geol. Ved. 25: 7-84
Condie KC (1967) Geochemistry of early Precambrian graywackes from
Wyoming.- Geochim. Cosmochim. Acta 31: 2135-2149
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Early Precambrian Graywackes from the Fig Tree Group, South Africa. Geol.
Soc. Amer. Bull. 81: 2759-2775
Davoine P (1968) La Geochemie des Leptynites (Distintion Ortho-Para).-
Doc. Labo. Geol. Fac. Sci. Lyon 26: 175 p.
Flick, Nesbor (1988) Der Vulkanismus in der Lahnmulde. Jahresber.
Mitt. Oberrhein. Geol. Ver., 70: 411-475
Floyd PA, Leveridge BE, Franke W, Shail R, Dörr W (1990) Provenance
and depositional environment of Rhenohercynian synorogenic greywackes from
the Giessen Nappe, Germany. Geol Rdsch 79: 611-626
Gill JB (1981) Orogenic Andesites and Plate Tectonics. Springer-Verlag,
Berlin
Hentschel H (1970) Vulkanische Gesteine. In: Lippert HJ, Hentschel
H, Rabien A, Erl geol Kte Hessen 1:25000 Bl. 5215 Dillenburg, p. 314-374,
Wiesbaden
Johannsen A (1952) A descriptive petrology of the igneous rocks.
University of Chicago press, Chicago
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des Spilites a Partir de Donnees Nouvelles sur le Cortege Spilito-Keratopyrique
Hercynotype. In: Amstutz GC (ed), Spilites and Spilitic Rocks, pp. 253-330,
Springer-Verlag, Berlin
Kamp P van de, Leake BE, Senior A (1976) The Petrography and Geochemistry
of some Californian Arkoses with Application to Identifying Gneisses of
Metasedimentary origin. J. Geol. 84: 195-212
LeMaitre RW (1976) The chemical variability of some common igneous
rocks. J Petrol 17:
LeMaitre RW (1982) Numerical Petrology, Elsevier Amsterdam
Lippert HJ, Hentschel H, Rabien A (1970) Erl geol Kte Hessen 1:25000
Bl. 5215 Dillenburg, 550 p, Wiesbaden
Niggli P (1952) Gesteine und Minerallagerstätten, Bd. II,
Birkhäuser, Basel
Pettijohn FJ (1963) Data of Geochemistry, S. Chemical Composition
of Sandstones - Excluding Carbonate and Volcanic Sands. U.S. geol. Surv.
Prof. Paper 440-S: 19 p.
Roaldset E (1972) Mineralogy and Geochemistry of Quarternary Clays
in the Numedal Area, Southern Norway. Norsk Geol. Tidsskrift 52: 335-369
Schmincke HU, Viereck LG, Griffin BJ, Pritchard RG (1982) Volcaniclastic
Rocks of the Reydarfj”rdur Drill Hole, Eastern Iceland 1. Primary Features.
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Herbornseelbach (Lahn-Dill area, Germany). Geol Rdsch 76: 709-734
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Geosynklinalablagerungen im Rhenoherzynikum. Ph.D.Thesis, University
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and General geochemistry.- Bull. geol. Soc. Amer. 67: 919-934
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Only the source were the analysis is taken from is cited, but not a possible original source.
| Tab. 1: Analyses of Igneous Rocks in the Ancomp Database | |||
|---|---|---|---|
| Number of Analyses |
Source | Rocks | Remarks, Locations etc. |
| 220 | Wimmenauer (1985) | the whole range | world wide |
| 151 | Wilson (1989) | redominantly basic and internediate volcanics | world wide |
| 12 | Gill (1981) | andesites | pacific realm |
| 66 | Davoine (1968) | Dacites and rhyolites | world wide, many after Johannsen (1952) |
| 29 | Best (1982) / LeMaitre (1976) |
the whole range | avarages |
| 53 | Hentschel (1970) | diabase, spilite, keratophyre, picrite, basalt | Devonian, Lahn-Dill-area, Germany |
| 18 | Schmincke and Sunkel (1987) |
scoria and pillow lava | Carboniferous,Lahn-Dill, Germany |
| 267 | Amstutz (ed., 1974) especially Juteau and Rocci (that volume) |
diabase, spilite, keratophyre, pillow lava, basalt | Germany, Australia, Japan, India, Finnland, France, England, Italy, former Soviet Union |
| 9 | Schulz-Dobrick (1975) | diabase, spilite, keratophyre, picrite | Rheinisches Schiefergebirge, Germany |
| 11 | Vinx (1982) | gabbro, norite | Harzburg massif, Germany |
| 836 | total number of igneous rock analyses | ||
| Tab. 2: Analyses of Pyroclastic Rocks in the Ancomp Database | |||
|---|---|---|---|
| Number of Analyses |
Source | Rocks | Remarks, Locations etc |
| 18 | Schmincke and Sunkel (1987) | rhyolit. tuff, basalt. volcaniclastics | Carboniferous, Dill-Basin, Germany |
| 1 | Hentschel (1970) | rhyolitic tuffite | Lower Devonian, Dill-Baisn, Germany |
| 54 | Amstutz (ed., 1974) especially Juteau and Rocci (that volume) |
tuffite (intertrappean), silic. volc. sed. breccia keratophyric tuff spilitic tuffite (Schalstein) spilitic tuffite (Schalstein) tuff |
Bombay, India Schirmeck, Vosges, France " " Lahn-Dill, Germany Cornwall, England |
| 119 | Kirnbauer (1986) | rhyolitic tuffite | Lower Devonian, Taunus, Germany |
| 13 | Schulz-Dobrick (1982) | keratophyric tuffite, spilitic tuffite | Rhein. Schiefergeb., Germany |
| 66 | Schmincke et al. (1982) | volcaniclastic rocks | Eastern Iceland, drill hole |
| 32 | Flick and Nesbor (1988) | basaltic pyroclastic rocks (Schalstein) | Devonian, Rhein. Schiefergebirge, Germany |
| 303 | total number of pyroclastic rock analyses | ||
| Tab. 3: Analyses of Sedimentary Rocks in the Ancomp Database | |||
|---|---|---|---|
| Number of Analyses |
Source | Rocks | Remarks, Locatioons etc. |
| 32 | Davoine (1968) | arkoses | France, Australia etc. |
| 22 | Tourtelot (1963) | claystone, marlstone | Cretac. Pierre Shale, USA |
| 34 | Pettijohn (1963) | arkose, lithic sandstone (subgraywacke), graywacke, greensand, phosphatic sandstone |
world wide |
| 7 | Shaw (1956) | slate | Littleton, New Hampshire |
| 73 | Roaldset (1972) | Quarternary clay | Numedal area, Norway |
| 49 | van de Kamp et al. (1976) | arkoses and interbedded shale, feldspar sands |
Tertiary, Holocene |
| 27 | Condie et al. (1976) | shale, graywacke | Precambrian, South Africa |
| 69 | Chab and Pelc (1973) | graywacke | Proterozoic, Bohemia |
| 61 | Niggli (1952) | clay, marl, dolomite | Switzerland, USA, Germany |
| 23 | Condie (1967) | graywacke | Precambrian, Wyoming, USA |
| 566 | Schulz-Dobrick (1975) | 204 claystones 134 siltsones 50 sandstones 42 graywackes 38 carbonaceous claystones 25 limestones 17 quartzitic sandstones 12 lydites 11 clayey limestones 7 slaty graywacke 5 carbonaceous sandstones 4 konglomerates 3 carbonaceous graywackes 2 dolomites 1 arkose |
Rheinisches Schiefergebirge, Germany, Lower Devonian to Upper Carboniferous |
| 10 | Floyd et al. (1990) | graywackes | Giessen nappe, Germany |
| 34 | Lippert et al. (1970) | quartzites, slate, shale, sandstone, limestone, lydite, alum shale |
Dill-Basin, Germany Devonian to Carboniferous |
| 1007 | total number of sedimentary rock analyses | ||