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Home , Melilite, Phengite, Phyllite, Schist, Sericite

Appendix Nomenclature of Common Metamorphic Rocks

Magmatic rocks are usually named after some locality. Only in rare cases does the name give any indication about the fabric and mineralogical composition of the rock. The names of magmatic rocks have to be memorized like words of a foreign language. Fortunately, this difficulty is not encountered in the nomenclature of most metamorphic rocks. It is only necessary to learn a few names of rock groups, which are characterized by a certain fabric and/or mineralogical composition. Furthermore, the presence of the main or critical is indicated by placing their names in front of the group name. For instance, there is the group of marbles, all of which contain well• crystallized carbonates as their main constituent. A particular marble may be designated as dolomite marble, -grossularite marble, marble, etc. Thus the nomenclature of most metamorphic rocks is clear and easily understood. A more elaborate nomenclature based on quantitative mineralogical composition was proposed by Austrian petrographers after a discussion with colleagues from other countries.1 This no• menclature is recommendable and is to a large extent adopted here.

Names of Important Rock Groups

Phyllite. Fine-grained and very finely schistose rock, the platy minerals of which consist mainly of phengite. Phengite sericite gives an overall silky sheen to the schistosity planes. The grain size is coarser than in but finer than in . In the amount of phyllosilicates (phengite + some chlorite ± ) exceeds 50%. The other most abundant constituent

l"Ein Vorschlag zur quantitativen und qualitativen Klassifikation der kristallinen Schiefer" (a symposium). Neues lahrb. Minerals Monatsh. : i63-172 (1962). 326 Petrogenesis of Metamorphic Rocks is . If the amoun t of quartz exceeds the amoun t of phyllosilicates, the rock is called a quartz . In both phyllites and quartz phyllites, albite may amount to as much as 20%. An exact designation of the rock is achieved by placing the name of subordinate constituents in front of the rock name, beginning with that present in the smallest amount. Minerals constituting less than 5% of the rock are generally not taken into consideration. Exam• ple: -chlorite-albite phyllite, phlogopite-calcite phyllite. If amounts smaller than 5% are considered significant this can be desig• nated by using an adjective form such as "-bearing." Schists. Medium- to coarse-grained rock, the fabric of which is characterized by an excellent parallelism or planar and/or linear fabric elements (schistosity). The individual mineral grains can be recognized megascopically (in contrast to phyllites). If mica, chlorite, tremolite, , etc., constitute more than 50% of a rock, the corresponding rock is called a mica , chlorite schist, tremolite schist, talc schist, etc. Phengite--chlorite-albite schists are known as . If a schist contains more quartz relative to the sum of the phyllosilicates, the rock is called quartz-mica schist. A further subdivi• sion of schists is effected according to the same rules as in the case of phyllites. The cited symposium gives 20% as the maximum amount of in a schist. If rocks contain more feldspar, they are designated as rather than schists. It is true that schists commonly contain less than 20% and gneisses more than 20% feldspar, but this Qistinction is generally not valid. The most characteristic difference between schists (or quartz schists) and gneisses is not the mineralogical com• position but the fabric. This distinction between schistose and gneissic fabric was clearly stated by Wenk (1963): "When hit with a hammer, rocks having a schistose fabric (schists) split perfectly parallel to's' into plates, 1-10 mm in thickness, or parallel to the lineation into thin pencil-like columns." Schists split into thinner plates than gneisses. . Medium- to coarse-grained rock having a gneissic fabric, i.e., it "splits parallel to's' generally along mica or layers, into plates and angular blocks, a few centimeters to tens of centimeters in thickness, or parallel to B into cylindrical bodies (pencil gneisses). The prevalent light-colored constituents (feldspar + quartz) have in• terlocking boundaries and provide, as compared to schists, a better coherence and a coarser fissility to the rock; nevertheless, the fissility in many cases creates an almost perfect plane" (Wenk, 1963). Some prefer a definition of gneiss based not only on fabric but also on mineralogical features. Thus Fritsch et al. (1967) advocated the use of the term gneiss for a rock with recognizable parallel structure consist- Appendix 327 ing predominentiy of quartz and feldspar- feldspar amounting com• monly to more than 20% and mica to at least 10%. Two groups of gneisses are recognized. Orthogneisses are formed from magmatic rocks, such as granites, syenites, diorites, etc. On the other hand, paragneisses are derived from sediments, such as graywackes, , etc. The particular mineralogical composition is in• dicated according to the same rule as in the case of phyllites, e.g., ---biotite gneiss. . A rock consisting predominately of hornblende and plagioclase, which is produced by of basaltic magmatic rocks, tuffs, or marls. The hornblende prisms lie within the plane of schistosity if this is developed. The fissility generally is not as well developed as in schists. contain only small amounts of quartz or none at all. Marble. A rock consisting predominately of fine- to coarse• grained recrystallized calcite and/or dolomite. Other minerals present are indicated in the usual manner, e.g., -biotite marble. . A rock composed of more than 80% quartz. The in• terlocking boundaries of the quartz grains impart a great strength to the rock. Metamorphic must be distinguished from un• metamorphosed, diagenetically formed quartzites. Fels. Fels is a term referring to massive metamorphic rocks lack• ing schistosity, e.g., quartz-albite fels, plagioclase fels, calcsilicate fels. Generally, in English books, the term "rock" is used for such metamorphic rocks, e.g., lime-silicate rock (Harker, 1932, 1939). It is suggested that "fels" be used instead. . Nonschistose and fine-grained rock, splintery on im• pact. The edges of thin rock chips occasionally are translucent like horn. The rock has a granoblastic fabric, i.e., it is a mosaic of equidimensional small mineral grains, in which frequentyl larger porphyroblastic minerals (or relics) are embedded. are typically produced by contact metamorphism of clays, fine-grained graywackes, etc. and occasionally by regional metamorphism. , Granolite, and Granoblastite. See p. 252ff. Eclogite. See p. 271.

Prefixes Meta-. This prefix designates metamorphosed igneous or sedi• mentary rocks in which the original fabric still can be recognized; e.g., metabasalts, metagraywackes. Others use the prefix "meta-" in a more general sense to designate metamorphic rocks according to the type of original rock from which they are derived. Example: Meta-graywacke or metadiorite = rock derived from graywacke or diorite. 328 Petrogenesis of Metamorphic Rocks Ortho-. This prefix indicates that the originated from a magmatic rock; e.g., orthogneiss, orthoamphibolite. Para-. This prefix indicates that the metamorphic rock originated from a sedimentary rock; e.g., paragneiss, para-amphibolite.

Classification

A quantitative classification of common metamorphic rocks is shown in Figures A-I and A-2 taken with slight modification from the cited symposium (1962). The objections of Wenk regarding the dis• tinction between gneiss and schist or phyllite should not be ignored; therefore, the boundary between the two groups, shown as a broken line in the two figures at 20% feldspar, should not be taken as critical in assigning a name to a rock. The distinction between gneiss and schist or phyllite is not based on mineralogical composition but on the character of fissility. This distinction is particularly significant if the mineralogical composition is the same. The classification shown in Figures A-I and A-2 applies to rocks predominately composed of either quartz, , and phyllosilicates, or quartz, phyllosilicates, and carbonates. In many metamorphic rocks, these minerals are the main constituents. Figure A-2 is valid for rocks of lower temperature and Figure A-2 for rocks formed at higher tem• perature. In higher-grade metamorphic rocks, schists take the place of phyllites and calcsilicates such as diopside and grossularite, which are not found in rocks of low temperature are present, e.g., in marbles (sili• cate marble).

~:~~~~il~· Carbonate r-;-~r------~------~r-I

Albite (Microcline) Sericite (Biotite. Chlorite) Fig. A-l Composition of metamorphic rocks of lower temperature ranges in terms of certain main constituents as indicated in the diagram. Appendix 329

Muscovite. Biotite Fig. A-2 Composition of metamorphic rocks of higher temperature ranges in terms of certain main constituents as indicated in the diagnim.

The names of most metamorphic rocks consist of compound terms:

a. A combination of the names of constituent minerals; b. A name for the category of rock according to its fabric, such as phyllite, gneiss, schist, fels.

Commonly, rocks with the fabric characteristics of gneiss, schist, etc., are formed in the appropriate field of mineralogical compositions as given in the preceding figures, but this is not invariably so. In any case, the name gneiss, schist, etc. be used only if the characteristic fabric is developed, irrespective of mineralogical composition.

References

Fritsch, W. Meinner, H., and Wieseneder, H. 1967. Neues lahrb. Mineral. Monatsh. 1967: 364-376. Harker, A. 1932, 1939. Metamorphism. Methuen, London. Wenk, C. 1963. Neues lahrb. Mineral. Monatsh. 1963: 97-107. Index

ACF Diagram, 35 ff, 167 ff Aragonite, 173f, 176f, 186ff, 191, Acmite, 173, 190, 201, 271 194 f, 201, 235 f Actinolite, 40, 71 ff, 170 ff, 175, 178 f, 182, 186 ff, 192 f, 195, 197, , metamorphism of, 165 ff 207, 235, 239 Biotite, 41 ,43 f, 51 ff, 85f, 98,170, Adularia, 184 f 206 ff, 210 ff, 213 ff, 221 ff, , See Acmite 224 ff, 229, 237 f, 256, 260 f, AFK Diagram, 41 ff 264 ff, 279, 305, 308 ff AFM Diagram, 48 ff Bronzite, 260 f Akermanite, 136 ff , 126 f, 132 f, 152 ff, 161 f, Albite, 70, 84f, 88,149, 167f, 170ff, 238 175 ff, 179 ff, 201, 203, 206, Burial metamorphism, 4 f, 6 239 Alkali feldspar, See K feldspar Calcite, 34,70, 112ff, 126 ff, 128 ff, Allochemical, See Metasomatic 139 ff, 147 ff, 166, 170, 172 f, , 41, 51 f, 76, 78,82,85, 183 f, 187f, 196,214, 243ff, 89 ff, 167, 169 ff, 171,212 f, 260 214 ff, 218,221 ff, 224 ff, 228 Carbonate rocks, metamorphism of, ff, 238 f, 256, 260 ff, 272, 275 19 f, 110 ff f, 279, 305, 309 f Cataclastic metamorphism, 2 Amphibolite, 165 ff, 170, 327 Chabazite, 185 Amphibolite facies, 64 ff, 75 ff Charnockitic granolite, 258 f Analcime, 9, 12, 181, 184 ff Chlorite, 9,11,41,51 f, 70 ff, 75 ff, Anatexis, 8, 65, 84 f, 246, 278 ff, 81,98,159,168, 170ff, 175 ff, 302 ff, 308 ff, 318 ff 182ff, 186ff, 192ff,203,206, Andalusite, 39, 77 ,82, 90, 91 ff, 146, 210 ff, 213 ff, 218, 221,235 ff, 205, 221, 225, 249 240 Andesite, metamorphism of, 165 ff Chloritoid, 39, 51 f, 73, 75 ff, 106 f, , 40, 260, 275 f 210ff, 213 ff, 216 ff, 218, 240, Ankerite, 214 275 Annite, 51 Chondrodite, 127 Anorthite, 40, 139ff, 143ff, 146, 147 Chrysotile, 151 ff ff, 243. See also Plagioclase Clays, metamorphism of, 202 ff , 41,151 ff, 158,246, Clinoch10re, 126 f 249 Clinohumite, 127 Antigorite, 151 ff, 238. See also Clinoptilolite, 9, 184, 193. See also Serpentine Heu1andite 332 Petrogenesis of Metamorphic Rocks Clinopyroxene, See Diopside Gmelinite, 185 Clinozoisite, See Zoisite Gneiss, nomenclature, 326 f Coalification, 2, 13 Grade of metamorphism, 5 f, 7 f, 62 f, Contact metamorphism, 2 ff, 96 ff, 64 ff, 234 ff 133 ff, 248 f Granite, 279 ff, 301 f, 309 ff Cordierite, 39, 76ff, 81 ff, 85, 90ff, Granitization, 280, 301, 314, 319, 98,206,216,218 ff, 224ff, 228 322 ff, 240, 242, 265 ff, 279, 305, Granoblastite, 254 f, 264 ff 309, 314, 316 Granoblastic texture, 253 f, 255 f , 142 Granolite, Definition, Nomencla- Crossite, 41, 173 ff, 194 ture, 252 ff, 258 f Cummingtonite, 40, 158 Granulite, 54, 87 f, 165,252 ff. See also Granolite Depth zones, metamorphic, 55 f Graphite, 22 f Diagenesis, 1,5,8,9 ff, 185 f, 202f Graywacke, metamorphism of, 200f, Diaphtoresis, 17 264 Diatexite, 317 f , Greenstone, 170 ff Differentiation, metamorphic, 16 f Greenschist facies, 64 f, 74 f Diopside, 40, 97, 106 f, 112 ff, 131, Grossularite, 40, 139 ff, 143 ff, 214, 136 f, 160 ff, 168 f, 186, 245, 243,256,260, 263,272,275 f 260, 262 ff, 271 Grunerite, 40 Dolomite, 40, 112 ff, 126 ff, 151, 160, 214, 244, 260 Hedenbergite, 40, 271 Dynamothermal metamorphism, 3 ff , 24 f, 211 , 9,13, 181ff, 183ff, 186, Eastonite-Siderophyllite, 51 196 Eclogite, 87, 165, 263, 271 ff Hornblende, 40, 75 f, 159, 166 ff, Enderbitic granolite, 258 f 171,238f,256,260,271,274, , 41, 151 ff 305, 311 Epidote, 40, 70 f, 166, 168 ff, 172, Hornfels, Hornfelsfacies, 57 ff, 98, 175, 180, 192 ff, 207,214 f, 248, 327 239, 271. See also Zoisite Hydrothermal metamorphism, 2 f Eutectic, 285 Hypersthene, 41, 88, 248, 252 ff, Facies, general, 55 f 256, 258, 260 ff, 267 f, 271 Fluid phase, 15 ff, 19 ff, 110 ff Hypersthene zone, 87 f, 252 ff Forsterite, 112ff, 132, 136f, 151 ff, 161 ff, 238, 244, 246, 260 Idocrase, 40, 149 f Fugacity, fugacity, 22 ff , 9, 11, 72 f, 203, 206 Ilmenite, 311 Gedrite, 41, 311 Isochemical, 16 ff Geobarometer, 241,243, 246ff, 267 , 66 ff Geothermometer, 241, 246 ff, 267, Isoreactiongrad, 66 ff 276 Gismondine, 185 Jadeite, jadeitic , 41 , 65, 70, Glauconite, 9, 206 73,88f, 173f, 188, 190, 194f, Glaucophane, 5, 11,41,65,73,88 f, 200 f, 235 f, 271, 276 173ff, 190f, 194f,201,235f, 271, 275 Kaolinite, 11, 139 f, 196, 203 ff Index 333 K feldspar (including alkali Montmorillonite, 9, 11, 196, 303, feldspar), 43, 53, 82 ff, 98, 206 148,202 f, 206 f, 209 ff, 214, Mordenite, 182, 185 225 ff, 261 ff, 264 ff, 282 ff, Muscovite, 41, 43 f, 76 ff, 81, 82 ff, 308 ff 86 ff, 98, 148, 170,203,206, Kyanite, 39, 77,82 ff, 90, 91 ff, 146, 212ff, 222 ff,225 ,237 f, 241 f, 205,221,243,250f,256,264, 246, 308 ff 271,274 Nontronite, 185

Larnite, 134 ff . See Forsterite Laumontite, 2, 5 f, 11 f, 39, 65, 88 f, Omphacite, 271, 276 139, 172, 177 ff, 193 f, 196, Orthopyroxene. See Enstatite and 200 f, 234 ff Hypersthene , 5, 11 ff,40,65,68ff,88f, 139,172 ff, 175 ff, 191 f, 194, Paragenesis, general, 28 ff 196, 200 f, 234 ff Paragonite, 11, 41, 43 f, 70, 149, Levyne, 185 203, 205 f Lizardite, 151 , metamorphism of, 202 ff, 261, 264 ff Mafic rocks, metamorphism of, 165 Periclase, 132 f, 152 ff ff Phase Rule, 32, 113 Magnesite, 111 ff, 126 f, 132 f, 151 Phengite, 41, 43 f, 72, 203, 205 ff, 160 ff, 207 ff, 210 ff, 214. See Magnetite, 23 f, 151, 163,211,218, also Muscovite 311 Phillipsite, 185 Margarite, 39, 68 f, 140, 145 f, 149, Phlogopite, 41, 51, 127 f, 160 241 Phyllite, nomenclature, 325 f Mariolite, 40 Pistacite, 40 Marl, metamorphism of, 139 ff, 196 Plagioclase, 75 f, 82 ff, 85 ff, 106 f, Mejonite, 40 142, 147 ff, 166 ff, 239, 256, Melilite, 134 ff 260ff, 264 ff, 286ff, 308 ff. See Merwinite, 134 ff also Albite and Anorthite Mesolite, 185 Prehnite, 40, 65, 68, 72 ff, 89,139, Metamorphism 142, 172f, 176ff, 186ff, 192f, Beginning of, 11 ff, 180 196, 200 f, 203, 234 ff Definition, 1 f, 8 Pressure Dynamothermal, 3 ff Directed, 25 f Factors of, 15 ff Fluid, 18 f Hydrothermal, 2 f, 181 f Hydrostatic, 18 Retrograde, 17 Load, 18 f Types, 1 ff Overpressure, 18 f, 25 f Metamorphic grades. See Grades of Solid, 19 metamorphism Units, 18 Metasomatism, 16, 319 f, 322 Pressure divisions of metamorphic Metatexite, 317 f grades, 88 ff , 8, 82, 103,278 ff, 305, Pumpellyite, 40, 65, 68 ff, 89, 151, 311 f, 314 ff 172f, 174ff, 176ff, 186ff, 192 Monticellite, 134 ff ff, 196 f, 200 f, 203, 234 ff 334 Petrogenesis of Metamorphic Rocks Pyrope, 41, 214, 256, 272, 275 f Stilbite, 185, 193 , 11,39,73, 145f, 196f, Stilpnomelane, 41, 44, 51 f, 170 f, 203 ff, 210 f 173 ff, 178,201,206 ff, 210 f, 214, 237 f Quartz. Too many to be cited here Subfacies, metamorphic, 59 ff

Rankinite, 134 ff Regional metamorphism, 3 ff, 101 ff Talc, 41,110 f, 151 ff, 235 f, 244, , 176, 207 246, 250 f . 197, 271 Thomsonite, 185 Tilleyite, 134 ff Sagvandite, 151 Tremolite, 40,97, 106 f, 112 ff, 160 Sanidinite facies, 134 ff ff, 244 f Saponite, 182 Sapphirine, 159 f, 267 f Ultramafic rocks, metamorphism of, , 40, 128, 260 151 ff Schist, nomenclature, 326 Scolecite, 185 Serpentine, 132, 151 ff, 161 ff, 235 f Vesuvianite. See Idocrase , 39, 82 ff, 90,91 ff, 102 ff, 221,225 ff, 229 f, 249, 256, Wairakite, 88, 171 ff, 180, 181 ff, 261,264,267,279,305,308ff, 187 ff, 193, 196, 134 ff 314, 316 Wollastonite, 20, 40, 97,128 ff, 134 Spessartine, 41, 214, 256, 275 ff, 139 ff, 143 ff, 249 Sphene, 166ff, 175 ff, 180, 197,201, 311 Spilite, 193 f Zeolite facies, 5, 181, 182 ff Spinel, 127, 159 Zoisite, inclusive Orthozoisite and Spurrite, 134 ff Clinozoisite, 40, 68 ff, 81,89, Staurolite, 39, 51 f, 75 ff, 81 f, 98, 139ff, 145f, 148f, 166, 168ff, 102 ff, 106 f, 213, 216 ff, 221 172, 186 ff, 235 f, 239, 243, ff, 240, 242, 249 ff 274