Appendix Nomenclature of Common Metamorphic Rocks Magmatic rocks are usually named after some locality. Only in rare cases does the rock 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 minerals is indicated by placing their names in front of the group name. For instance, there is the group of marbles, all of which contain wellcrystallized carbonates as their main constituent. A particular marble may be designated as dolomite marble, diopside-grossularite marble, tremolite 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 nomenclature 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 slates but finer than in mica schists. In phyllites the amount of phyllosilicates (phengite + some chlorite ± biotite) 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 quartz. If the amoun t of quartz exceeds the amoun t of phyllosilicates, the rock is called a quartz phyllite. 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 mineral present in the smallest amount. Minerals constituting less than 5% of the rock are generally not taken into consideration. Example: chloritoid-chlorite-albite phyllite, phlogopite-calcite phyllite. If amounts smaller than 5% are considered significant this can be designated by using an adjective form such as "graphite-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, talc, etc., constitute more than 50% of a rock, the corresponding rock is called a mica schist, chlorite schist, tremolite schist, talc schist, etc. Phengite-epidote-chlorite-albite schists are known as greenschists. If a schist contains more quartz relative to the sum of the phyllosilicates, the rock is called quartz-mica schist. A further subdivision 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 feldspar in a schist. If rocks contain more feldspar, they are designated as gneisses 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 composition 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. Gneiss. Medium- to coarse-grained rock having a gneissic fabric, i.e., it "splits parallel to's' generally along mica or hornblende 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 interlocking 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 commonly 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, shales, etc. The particular mineralogical composition is indicated according to the same rule as in the case of phyllites, e.g., kyanite-staurolite-garnet-biotite gneiss. Amphibolite. A rock consisting predominately of hornblende and plagioclase, which is produced by metamorphism 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. Amphibolites contain only small amounts of quartz or none at all. Marble. A rock consisting predominately of fine- to coarsegrained recrystallized calcite and/or dolomite. Other minerals present are indicated in the usual manner, e.g., muscovite-biotite marble. Quartzite. A rock composed of more than 80% quartz. The interlocking boundaries of the quartz grains impart a great strength to the rock. Metamorphic quartzites must be distinguished from unmetamorphosed, diagenetically formed quartzites. Fels. Fels is a term referring to massive metamorphic rocks lacking 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. Hornfels. Nonschistose and fine-grained rock, splintery on impact. 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. Hornfelses are typically produced by contact metamorphism of clays, fine-grained graywackes, etc. and occasionally by regional metamorphism. Granulite, Granolite, and Granoblastite. See p. 252ff. Eclogite. See p. 271. Prefixes Meta-. This prefix designates metamorphosed igneous or sedimentary 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 metamorphic rock 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 distinction 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, feldspars, 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 temperature. 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 (silicate 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 Acmite, 173, 190, 201, 271 Actinolite, 40, 71 ff, 170 ff, 175, 178 f, 182, 186 ff, 192 f, 195, 197, 207, 235, 239 Adularia, 184 f Aegirine, See Acmite AFK Diagram, 41 ff AFM Diagram, 48 ff Akermanite, 136 ff Albite, 70, 84f, 88,149, 167f, 170ff, 175 ff, 179 ff, 201, 203, 206, 239 Alkali feldspar, See K feldspar Allochemical, See Metasomatic Almandine, 41, 51 f, 76, 78,82,85, 89 ff, 167, 169 ff, 171,212 f, 214 ff, 218,221 ff, 224 ff, 228 ff, 238 f, 256, 260 ff, 272, 275 f, 279, 305, 309 f Amphibolite, 165 ff, 170, 327 Amphibolite facies, 64 ff, 75 ff Analcime, 9, 12, 181, 184 ff Anatexis, 8, 65, 84 f, 246, 278 ff, 302 ff, 308 ff, 318 ff Andalusite, 39, 77,82, 90, 91 ff, 146, 205, 221, 225, 249 Andesite, metamorphism of, 165 ff Andradite, 40, 260, 275 f Ankerite, 214 Annite, 51 Anorthite, 40, 139ff, 143ff, 146, 147 ff, 243. See also Plagioclase Anthophyllite, 41,151 ff, 158,246, 249 Antigorite, 151 ff, 238. See also Serpentine Aragonite, 173f, 176f, 186ff, 191, 194 f, 201, 235 f Basalt, metamorphism of, 165 ff Biotite, 41,43 f, 51 ff, 85f, 98,170, 206 ff, 210 ff, 213 ff, 221 ff, 224 ff, 229, 237 f, 256, 260 f, 264 ff, 279, 305, 308 ff Bronzite, 260 f Brucite, 126 f, 132 f, 152 ff, 161 f, 238 Burial metamorphism, 4 f, 6 Calcite, 34,70, 112ff, 126 ff, 128 ff, 139 ff, 147 ff, 166, 170, 172 f, 183 f, 187f, 196,214, 243ff, 260 Carbonate rocks, metamorphism of, 19 f, 110 ff Cataclastic metamorphism, 2 Chabazite, 185 Charnockitic granolite, 258 f Chlorite, 9,11,41,51 f, 70 ff, 75 ff, 81,98,159,168, 170ff, 175 ff, 182ff, 186ff, 192ff,203,206, 210 ff, 213 ff, 218, 221,235 ff, 240 Chloritoid, 39, 51 f, 73, 75 ff, 106 f, 210ff, 213 ff, 216 ff, 218, 240, 275 Chondrodite, 127 Chrysotile, 151 ff Clays, metamorphism of, 202 ff Clinoch10re, 126 f Clinohumite, 127 Clinoptilolite, 9, 184, 193. See also Heu1andite
332 Petrogenesis of Metamorphic Rocks Clinopyroxene, See Diopside Clinozoisite, See Zoisite Coalification, 2, 13 Contact metamorphism, 2 ff, 96 ff, 133 ff, 248 f Cordierite, 39, 76ff, 81 ff, 85, 90ff, 98,206,216,218 ff, 224ff, 228 ff, 240, 242, 265 ff, 279, 305, 309, 314, 316 Corundum, 142 Crossite, 41, 173 ff, 194 Cummingtonite, 40, 158 Depth zones, metamorphic, 55 f Diagenesis, 1,5,8,9 ff, 185 f, 202f Diaphtoresis, 17 Diatexite, 317 f Differentiation, metamorphic, 16 f Diopside, 40, 97, 106 f, 112 ff, 131, 136 f, 160 ff, 168 f, 186, 245, 260, 262 ff, 271 Dolomite, 40, 112 ff, 126 ff, 151, 160, 214, 244, 260 Dynamothermal metamorphism, 3 ff Eastonite-Siderophyllite, 51 Eclogite, 87, 165, 263, 271 ff Enderbitic granolite, 258 f Enstatite, 41, 151 ff Epidote, 40, 70 f, 166, 168 ff, 172, 175, 180, 192 ff, 207,214 f, 239, 271. See also Zoisite Eutectic, 285 Facies, general, 55 f Fluid phase, 15 ff, 19 ff, 110 ff Forsterite, 112ff, 132, 136f, 151 ff, 161 ff, 238, 244, 246, 260 Fugacity, oxygen fugacity, 22 ff Gedrite, 41, 311 Geobarometer, 241,243, 246ff, 267 Geothermometer, 241, 246 ff, 267, 276 Gismondine, 185 Glauconite, 9, 206 Glaucophane, 5, 11,41,65,73,88 f, 173ff, 190f, 194f,201,235f, 271, 275 Gmelinite, 185 Gneiss, nomenclature, 326 f Grade of metamorphism, 5 f, 7 f, 62 f, 64 ff, 234 ff Granite, 279 ff, 301 f, 309 ff Granitization, 280, 301, 314, 319, 322 Granoblastite, 254 f, 264 ff Granoblastic texture, 253 f, 255 f Granolite, Definition, Nomenclature, 252 ff, 258 f Granulite, 54, 87 f, 165,252 ff. See also Granolite Graphite, 22 f Graywacke, metamorphism of, 200f, 264 Greenschist, Greenstone, 170 ff Greenschist facies, 64 f, 74 f Grossularite, 40, 139 ff, 143 ff, 214, 243,256,260, 263,272,275 f Grunerite, 40 Hedenbergite, 40, 271 Hematite, 24 f, 211 Heulandite, 9,13, 181ff, 183ff, 186, 196 Hornblende, 40, 75 f, 159, 166 ff, 171,238f,256,260,271,274, 305, 311 Hornfels, Hornfelsfacies, 57 ff, 98, 248, 327 Hydrothermal metamorphism, 2 f Hypersthene, 41, 88, 248, 252 ff, 256, 258, 260 ff, 267 f, 271 Hypersthene zone, 87 f, 252 ff Idocrase, 40, 149 f Illite, 9, 11, 72 f, 203, 206 Ilmenite, 311 Isochemical, 16 ff Isograd, 66 ff Isoreactiongrad, 66 ff Jadeite, jadeitic pyroxene, 41, 65, 70, 73,88f, 173f, 188, 190, 194f, 200 f, 235 f, 271, 276 Kaolinite, 11, 139 f, 196, 203 ff
K feldspar (including alkali feldspar), 43, 53, 82 ff, 98, 148,202 f, 206 f, 209 ff, 214, 225 ff, 261 ff, 264 ff, 282 ff, 308 ff Kyanite, 39, 77,82 ff, 90, 91 ff, 146, 205,221,243,250f,256,264, 271,274 Larnite, 134 ff Laumontite, 2, 5 f, 11 f, 39, 65, 88 f, 139, 172, 177 ff, 193 f, 196, 200 f, 234 ff Lawsonite, 5, 11 ff,40,65,68ff,88f, 139,172 ff, 175 ff, 191 f, 194, 196, 200 f, 234 ff Levyne, 185 Lizardite, 151 Mafic rocks, metamorphism of, 165 ff Magnesite, 111 ff, 126 f, 132 f, 151 ff, 160 Magnetite, 23 f, 151, 163,211,218, 311 Margarite, 39, 68 f, 140, 145 f, 149, 241 Mariolite, 40 Marl, metamorphism of, 139 ff, 196 Mejonite, 40 Melilite, 134 ff Merwinite, 134 ff Mesolite, 185 Metamorphism Beginning of, 11 ff, 180 Definition, 1 f, 8 Dynamothermal, 3 ff Factors of, 15 ff Hydrothermal, 2 f, 181 f Retrograde, 17 Types, 1 ff Metamorphic grades. See Grades of metamorphism Metasomatism, 16, 319 f, 322 Metatexite, 317 f Migmatite, 8, 82, 103,278 ff, 305, 311 f, 314 ff Monticellite, 134 ff Index 333 Montmorillonite, 9, 11, 196, 303, 206 Mordenite, 182, 185 Muscovite, 41, 43 f, 76 ff, 81, 82 ff, 86 ff, 98, 148, 170,203,206, 212ff, 222 ff,225,237 f, 241 f, 246, 308 ff Nontronite, 185 Olivine. See Forsterite Omphacite, 271, 276 Orthopyroxene. See Enstatite and Hypersthene Paragenesis, general, 28 ff Paragonite, 11, 41, 43 f, 70, 149, 203, 205 f Pelites, metamorphism of, 202 ff, 261, 264 ff Periclase, 132 f, 152 ff Phase Rule, 32, 113 Phengite, 41, 43 f, 72, 203, 205 ff, 207 ff, 210 ff, 214. See also Muscovite Phillipsite, 185 Phlogopite, 41, 51, 127 f, 160 Phyllite, nomenclature, 325 f Pistacite, 40 Plagioclase, 75 f, 82 ff, 85 ff, 106 f, 142, 147 ff, 166 ff, 239, 256, 260ff, 264 ff, 286ff, 308 ff. See also Albite and Anorthite Prehnite, 40, 65, 68, 72 ff, 89,139, 142, 172f, 176ff, 186ff, 192f, 196, 200 f, 203, 234 ff Pressure Directed, 25 f Fluid, 18 f Hydrostatic, 18 Load, 18 f Overpressure, 18 f, 25 f Solid, 19 Units, 18 Pressure divisions of metamorphic grades, 88 ff Pumpellyite, 40, 65, 68 ff, 89, 151, 172f, 174ff, 176ff, 186ff, 192 ff, 196 f, 200 f, 203, 234 ff
334 Petrogenesis of Metamorphic Rocks Pyrope, 41, 214, 256, 272, 275 f Pyrophyllite, 11,39,73, 145f, 196f, 203 ff, 210 f Quartz. Too many to be cited here Stilbite, 185, 193 Stilpnomelane, 41, 44, 51 f, 170 f, 173 ff, 178,201,206 ff, 210 f, 214, 237 f Subfacies, metamorphic, 59 ff Rankinite, 134 ff Regional metamorphism, 3 ff, 101 ff Riebeckite, 176, 207 Rutile. 197, 271 Sagvandite, 151 Sanidinite facies, 134 ff Saponite, 182 Sapphirine, 159 f, 267 f Scapolite, 40, 128, 260 Schist, nomenclature, 326 Scolecite, 185 Serpentine, 132, 151 ff, 161 ff, 235 f Sillimanite, 39, 82 ff, 90,91 ff, 102 ff, 221,225 ff, 229 f, 249, 256, 261,264,267,279,305,308ff, 314, 316 Spessartine, 41, 214, 256, 275 Sphene, 166ff, 175 ff, 180, 197,201, 311 Spilite, 193 f Spinel, 127, 159 Spurrite, 134 ff Staurolite, 39, 51 f, 75 ff, 81 f, 98, 102 ff, 106 f, 213, 216 ff, 221 ff, 240, 242, 249 ff Talc, 41,110 f, 151 ff, 235 f, 244, 246, 250 f Thomsonite, 185 Tilleyite, 134 ff Tremolite, 40,97, 106 f, 112 ff, 160 ff, 244 f Ultramafic rocks, metamorphism of, 151 ff Vesuvianite. See Idocrase Wairakite, 88, 171 ff, 180, 181 ff, 187 ff, 193, 196, 134 ff Wollastonite, 20, 40, 97,128 ff, 134 ff, 139 ff, 143 ff, 249 Zeolite facies, 5, 181, 182 ff Zoisite, inclusive Orthozoisite and Clinozoisite, 40, 68 ff, 81,89, 139ff, 145f, 148f, 166, 168ff, 172, 186 ff, 235 f, 239, 243, 274