Nomenclature of Common Metamorphic Rocks

Transcription

1 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. I 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 sericite. 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 (sericite + some chlorite ± biotite) exceeds 50%. The other most abundant constituent l"ein Vorschlag zur quantitativen und qualitativen Klassifikation der kristallinen Schiefer" (a symposium). Neue Jahrb. Minerals Monatsh. : (1962). 312

2 Nomenclature of Common Metamorphic Rocks 313 is quartz. If the amount of quartz exceeds the amount 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. Sericite-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 provides, 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-

3 314 Appendix ing predominently 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 int e r o boundaries c k i n g of the quartz grains impart a great strength to the rock. Metamorphic quartzites must be distinguished from unmetamorphosed, diagenetically formed quartzites. Fels. Pels 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 Granoblasfife. See p. 247ff. Eclogite. See p 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.

4 Nomenclature of Common Metamorphic Rocks 315 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). ~ : ~ ~ t ~ ; t ~ Carbonate r - ~ ~ ' Albite (Microcline) Sericite (Biotite. Chlorite) Fig. A-I Composition of metamorphic rocks of lower temperature ranges in terms of certain main constituents as indicated in the diagram.

5 316 Appendix c a, b o n < l t ~ q U i J " z i t ~ Carbonate Plagioclase (Microcline ) Muscovite, Biotite Fig. A-2 Composition of metamorphic rocks of higher temperature ranges in terms of certain main constituents as indicated in the diagram. 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 Neues lahrb. Mineral. Monatsh. 1967: Harker, A. 1932, Metamorphism. Methuen, London. Wenk, C Neues lahrb. Mineral. Monatsh. 1963:

6 Index ACF Diagram, 34 ff Acmite, 168 f, 171, 185,196 Actinolite, 39, 70 ff, 146, 163, 170, 172, 181 ff, 189 f, 202, 209, 229 f Adularia, 197 f Aegirine. See Acmite AFK Diagram, 40 ff AFM Diagram, 47 ff Akermanite, 133 f Albite, 44, 69, 84, 87, 195 f, 198 Alkalifeldspar. See K feldspar Allochemical. See Metasomatic Almandine, 40, 50 f, 75 f, 77 f, 84, 89,161 ff, 207 ff, 209 ff, 213 ff, 219 ff, 222 ff, 232, 240, 243 f, 249, 253 f, 255 f, 257 ff, 265, 268 f Amphibolite, 160 ff, 314 Amphibolite facies, 63 ff, 74 ff Analcime, 9, 12, 176, 179 f, 183 Anatexis, 8, 83 ff, 240, 271 f, 273 ff, 289 ff Andalusite, 38, 76, 81 ff, 89, 90 ff, 197 ff, 200, 206 f, 210, 219, 235 f, 243 Andesite, metamorphism of, 160 ff Andradite, 39, 253 Ankerite, 209 Annite, 50 Anorthite, 39,136 ff, 235 f, 237, 242 ff. See also Plagioclase Anthophyllite, 40, 149 ff, 158, 240, 242 ff Antigorite, 148, 158. See also Serpentine Aragonite, 168 f, 171 f, 183 ff, 189 f, 194, 196, 229 f Augite. See Clinopyroxene Basalt, metamorphism of, 160 ff Biotite, 40, 42 f, 50 f, 76, 84,142, 161 ff, 196,201 ff, 205 ff, 208 ff, 213 ff, 217 f, 219, 222 ff, 225, 231, 242 ff, 249, 253 ff, 256 ff, 273, 296 ff Bronzite, 253 f Brucite, 123, 125, 128 ff, 148 ff, 230,234,242 ff Burial metamorphism, 4 ff Calcite, 33, 69, 96, 109 ff, 125 ff, 133 f, 136 ff, 144 f, 165 ff, 191 f, 194, 196,200 ff, 237 ff, 242 ff Carbonate rocks, metamorphism of, 109 ff Cataclastic metamorphism, 2 Chabazite, 180 Charnockitic granolite, 251 f Chlorite, 9, 11,40,50 f, 69 ff, 71 ff, 74 ff, 80, 97,124,142, 146,155 f, 161, 163,165 ff, 181 ff, 197 f, 200 ff, 205 ff, 209 ff, 229 ff, 234 Chloritoid, 38, 50 f, 72, 74 f, 76 ff, 142,197,205 ff, 211 f,234 Chondrodite, 125 Chrysotile, 148 Clays, metamorphism of, 197 ff Clinochlore, 124 f Clinohumite,

7 318 Index Clinoptilolite, 9, 179, 188 Clinopyroxene, 148 f, 156, 161 ff, 225 ff. See also Diopside Coalification, 2, 13 Contact metamorphism, 2 ff, 95 ff, 242 f Cordierite, 38, 50 f, 75 ff, 81, 84, 89,97,197,201,211,213 ff, 217,219 ff, 222 ff, 234, 236, 240,242 ff, 258 ff, 260, 273, 303 ff Corundum, 137 ff Crossite, 40, 142, 168 f Cummingtonite, 39 f, 164 Depth zones, metamorphic, 54 f Diagenesis, 1, 5, 8, 9 ff, 180 Diaphthoresis, 17 Diatexite, 304 f Differentiation, metamorphic, 16 f Diopside, 33, 39, 96,111 ff, 119 ff, 133,146, 156ff, 161 ff, 185, 2 3 9, 2249, f ~ Dolomite, 33, 39, 96,109 ff, 128 ff, 142,200,209,237 ff, 242 ff, 253 Dynamothermal metamorphism, 3 ff, 100 ff Eastonite-Siderophyllite, 50 Eclogite, 86, 256, 264 ff Enderbitic granolite, 251 f Enstatite, 40, 149 ff Epidote, 39, 44, 69 f, 146, 161 ff, 170 f, 173 ff, 181,202,209, 231, 264. See also Zoisite Eutectic, 277 Facies, general, 54 ff Fluid phase, 15 ff, 19 ff Forsterite, 111 ff, 120 ff, 133, 148 ff, 230,233 f, 238, 240, 242 ff, 253 Fugacity, oxygen fugacity, 23 f Gedrite,40 Geobarometer, 143,236,240 ff Geothermometer, 240 ff Gismondine, 180 Glauconite, 9, 210 f Glaucophane, 11,40, 72, 87 f, 162, 168 f, 185, 189 f, 196,230, 264 Gmelinite, 180 Gneiss, nomenclature, 313 ff Grade of metamorphism, 6 f, 61 ff, 63 ff, 288 ff Granite, 271 f, 274, 278, 283, 286, 292,294 ff, 299, 306 ff Granitization, 272,301,309 f Granoblastic texture, 248 ff Granoblastite, 247 ff, 257 Granolite, 245 ff, 257 ff, 260 ff Granulite, 86 f, 245 ff. See also Granolite Graphite, 22 f, 42 Graywacke, metamorphism of, 195 f, 256 Greenschist, Greenstone, 165 ff Greenschist facies, 63 f, 73 f Grossularite, 33, 39,136 ff, 146, 184,209,237,242 ff, 249, 253,256,265,268 f Grunerite, 39 Hedenbergite, 39 Hematite, 206 Heulandite,9, 13, 176ff, 179 Hornblende, 39, 74 f, 156, 160 ff, 232 f, 242 ff, 249,253 ff, 161 ff, 264, 298 Hornfels, Hornfels facies, 56 ff, 97, 242,314 Hydrothermal metamorphism, 2 f Hypersthene, 40, 87,155,162 f, 222,242,245, 2 4 9, f ~ 256 ff, 260 ff, 264 Hypersthene zone, 86 f, 245 ff, Idocrase, 39, 146 Illite, 9,11,71 ff,201 Ilmenite, 298 Isochemical, 16 ff, Isograd, 65 f, 228 ff, 242 ff

8 Index 319 Jadeite, jadeitic pyroxene, 40, 69, 72,87 f, 168 f, 183, 185 f, 189 f, 195 f, 229 f, 264, 269 Kaolinite, 136, 142, 191, 198 f K feldspar (including Alkalifeldspar), 81 ff, 97,114,145,196,201 f, 206 f, 209, 219 ff, 240, 242 ff, 249,251,256 ff, 273 ff, 296 ff Kyanite, 38, 76, 81 ff, 89,90 ff, 143,197,200,206 f, 210, 220 ff, 235 f, 243 f, 249, 258, 260,264 Larnite, 131 ff Laumontite, I ff, 38, 72, 87, 136, 167 ff, 177 ff, 191 f, 195, 229 f Lawsonite, II ff, 38, 67 ff, 71 ff, 87,136,167 ff, 195 f, 229 f Levyne, 180 Mafic rocks, metamorphism of, 160 ff Magnesite, 109 f, 123 f, 128 f, 148 ff Magnetite, 23 f, 155, 158,206,298 Margarite, 38, 67 f, 137,142 f, 196, 235 f, 243 f Mariolite, 39 Marl, metamorphism of, 136 ff Majonite, 39 Melilite, 130 ff, 133 Merwinite, 133 f Mesolite, 180 Metamorphism Definition, I f, 8 Dynamothermal, 3 ff Factors of, 15 ff Hydrothermal, 2 f Retrograde, 17 Types, 1 ff Metamorphic grades. See grades of metamorphism Metasomatism, 16,301,306 f, 309 Metatexite, 304 f Mica schist, nomenclature, 312 ff Migmatite, 81, 240, 271 ff,291, 294 f, 298 f, 301 ff Monticellite, 130 ff Montmorillonite, 184, 191, 201 Mordenite, 177, 180 Muscovite, 40, 42, 75 ff, 81 ff,97, 142,145,165 ff, 197,201, 203,209 f, 213 f, 217 f, 219 ff, 234 f, 239 f, 242 ff, 248, 253, 296 Natrolite, 180 Olivine, 148, 158, 162 f. See also Forsterite Omphacite, 168 f, 264 f, 269 Orthopyroxene. See Enstatite and Hypersthene Paragenesis, general 27 ff Paragonite, 11,40,42 f, 69, 142, 196, 198, 200 f, 206 ff Pelites, metamorphism of, 197 ff Periclase, 124, 128 f, 133, 149 ff Phase Rule, 31, 112 Phengite, 11, 40, 42, 71 ff, 79, 165 ff, 197 f, 200 ff, 205 ff, 209. See also Muscovite Phillipsite, 180 Phlogopite, 40,50, 114, 125, 156 Phyllite, nomenclature, 312 ff Pistacite, 39, 70 Plagioclase, 44, 74 f, 78, 82 ff, 142, 144 f, 160 ff, 219 f, 222 f, 233,235 f, 249, 251, 265 f, 273 ff, 296 ff. See also Albite and Anorthite Prehnite, 39,67,71 ff, 88,136 ff, 167,171 ff, 181 ff, 191 f, 195 f Pressure Directed, 23 ff, Fluid, Pt, 18 f Hydrostatic, 18 Load, PI, 18 f Overpressure, 18 f, 24 f Solid, Ps, 19 Units, 18 Pressure divisions of metamorphic grades, 87 ff

9 320 Index Pumpellyite, 39,67 ff, 70 ff, 88, 167 ff, 171 ff, 181 ff, 195 f Pyrope, 40,156,209,265,268 f Pyrophyllite, II, 38, 67, 72, 142 f, 191 f, 197 ff, 205 f, 208 Quartz, 30 ff, 34 ff, 67 ff, 82 ff, III ff, 125 ff, 138 ff, 149 ff, 161,197 ff, 205 ff, 219 f, 235 ff, 242 ff, 251,206,273 ff, 296 ff Rankinite, 131 ff Regional metamorphism, 3 ff, 95, 100 ff Riebeckite, 171,202 Rutile, 161 ff, 192,264 f Sagvandite, 148 Sanidinite facies, 130 ff Saponite, 177 Sapphirine, 156,260 Scapolite, 39,125,253 Schist, nomenclature, 313 ff Serpentine, 129, 148 ff, 229 f, 233 f, 242 ff Sillimanite, 38, 81 ff, 89, 90 ff, 197, 217,219 ff, 240, 243,249, 257 ff, 260,273 Slate, 198 Spessartine, 40, 209, 219, 268 Spilite, 188 f Sphene, 161 ff, 169f, 175, 192, 196,298 Spinel, 125, 156 Sporrite, 131 f Staurolite, 38, 50 f, 74 ff, 81,97, 161,197,208,211 ff, 217 f, 234,236,239,242 ff Stilbite, 180, 188 Stilpnomelane, 40, 43,50 f, 165 ff, 173,196,201 ff, 204 ff, 209, 231 f,234 Subfacies, 58 ff Talc, 40,109, III ff, 148 ff, 158, 229 f, 233 f, 237 ff, 242 ff Tilleyite, 131 ff Tremolite, 33, 39, 96, III ff, 146, 156 ff, 158,237 ff, 242 ff Ultramafic rocks, metamorphism of, 148 ff Vesuvianite. See Idocrase Wairakite, 172, 175 ff, 182 ff, 191, 229 f Wollastonite, 32 f, 39, 96, 125 ff, 131 ff, 136 ff, 243 Zeolite facies, 176 ff, 179 Zoisite, inclusive Orthozoisite and Clinozoisite, 39,44,67 ff, 70 ff, 136 ff, 145, 161 ff, 173, 181 ff, 229 f, 233, 236 f, 242 ff, 255,264 Zones, metamorphic, classic, 100 ff