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 leam a few names of rock groups, which are characterized by a certain fabric andjor 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 well-crystallized 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. '''Ein Vorschlag zur quantitativen und qualitativen Klassifikation der kristallinen Schiefer" (a symposium). Neues Jahrb. Minerals Monatsh.: (1962).

2 Nomenclature of Common Metamorphic Rocks 341 In phyllites the amount of phyllosilicates (phengite + some chlorite ± biotite) exceeds 50%. The other most abundant constituent 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. Examples are chloritoid-chlorite-albite phyllite and 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 of 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 aschist 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 aschist. If rocks contain more feldspar, they are designated as gneisses rather than schists. It is true that schists commonly contain less than 20% and gneis ses more than 20% feldspar, but this distinction is generally not valid. The most characteristic difference between schists (or quartz schists) and gneis ses is not the mineralogie al composition but the fabric. This distinction between schistose and gneissic fabric was c1early stated by Wenk (1963): "When hit with a hammer, rocks having aschistose 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 mineralogica~ features. Thus Fritsch et al. (1967) advocated the use ofthe term gneiss for

3 342 Petrogenesis of Metamorphic Rocks a rock with recognizable parallel structure consisting predominently of quartz and feldspar-feldspar amounting commonly to more than 20% and mica to at least 10%. Tw~ 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 ofphyllites, e.g., kyanitestaurolite-garnet-biotite gneiss. Amphibolite. A rock consisting predominantly of hornblende and plagioc1ase, which is produced by metamorphism of basaltic magmatic rocks, tuffs, or marls. The hornblende prisms lie within the plane of schistosity ifthis 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 predominantly of fine- to coarse-grained 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, plagioc1ase fels, calc-silicate 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, which splinters 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 larger porphyroblastic minerals (or rehcs) are frequently 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. 256ff. Eclogite. See p Prefixes Meta-. This prefix designates metamorphosed igneous or sedimentary roeks in which the original fabrie still can be reeognized; e.g., metabasalts, metagraywackes. Others use the prefix "meta-" in a more gen-

4 Nomenclature of Common Metamorphic Rocks 343 eral 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. 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 c1assification of common metamorphic rocks is shown in Figures A-l 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 c1assification shown in Figures A-l and A-2 applies to rocks predominantly 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 Albit, (Microclin.) S.ricit, (Biotit,. Chlorit.) Fig. A-l Composition of metamorphic rocks of lower temperature ranges in terms of certain main constituents as indicated in the diagram.

5 344 Petrogenesis of Metamorphic Rocks Muscolfit Biotit. Fig. A-2 Composition of metamorphic rocks of higher temperature ranges in terms of certain main constituents as indicated in the diagram. lower temperature and Figure A-2 for rocks formed at higher temperature. In higher-grade metamorphic rocks, schists take the place of phyllites, and calc-silicates such as diopside and grossularite, which are not found in rocks oflow temperature, are present, e.g., in marbles (silicate marble). 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. is to be used only ifthe characteristic fabric is developed, irrespective of mineralogical composition. References Fritsch, W. Meixner, H., and Wieseneder, H Neues Jahrb. Mineral. Monatsh. 1967: Harker, A. 1932, Metamorphism. Methuen, London. Wenk, C Neues Jahrb. Mineral. Monatsh. 1963:

6 Index ACF Diagram, 35ff, 44ff, 17lff Acmite, 177, 179, 194,205,276 Actinolite, 40, 71ff, 152, 17lff, 174ff, 178, 184, 189ff, 195ff, 212, 24Of, 244,280 Adularia, 187f Aegirine, See Acmite AFK Diagram, 39, 42ff AFM Diagram, 48ff Äkermanite, 136ff Albite, 12,45,75, 89f, 150, 169f, 171ff, 178ff, 187ff, 204, 207, 211, 228, 241,244,280 Alkali feldspar, See K feldspar Allochemical, See Metasomatic Almandine, 41, 5lf, 76, 78, 82, 85, 9Off, 169, 17lff, 217f, 22Off, 224f, 227ff, 23Off, 233ff, 244f, 250, 260, 264ff, 270, 277, 28Of, 284, 316, 32lf,323 Amphibolite, 168ff, 173, 342 Amphibolite facies, 64ff, 75ff Analcime, 9, 12, 188ff Anatexis, 8,65, 85f, 23Of, 251, 283ff, 286ff, 309ff, 316ff, 329ff Andalusite, 40, 78, 82, 90, 9lff, 147, 209f, 218, 227, 230, 246ff, 253 Andesite, metamorphism of, 168ff Andradite, 40, 264, 280f Anhydrite, 2 Ankerite, 219 Annite, 51 Anorthite, 40, 45, 142ff, 147ff, 248, 285. See also Plagioclase Anthophyllite, 41, 154ff, 161, 250, 253f Antigorite, 154ff, 159ff, 244f. See also Serpentine Aragonite, 175ff, 179f, 190, 193, 195, 198f, 205, 240f Basalt, metamorphism of, 169ff Biotite, 41, 43f, 5lff, 85f, 98,147,169, 173, 21lff, 215ff, 219ff, 223, 227ff, 23Off, 234, 242f, 264ff, 268ff, 273, 284, 286, 310, 313, 316, 32Off, 337 Bronzite, 264f Brucite, 127f, 13lf, 154ff, 164f,244f, 253 Burial metamorphism, 4ff, 6 Calcite, 34, 70, 112ff, 126ff, 128ff, 134ff, l4off, 147ff, 170, 176f, 185f, 19Of, 200f, 205, 210, 212, 241, 248ff, 264, 286 Carbonate rocks, metamorphism of, 19f, 11lff Cataclastic metamorphism, 2 Chabazite, 188 Charnockitic granolite, 263f Chlorite, 9, 11,41, 5lf, 7Off, 74ff, 82, 99, 14Of, 147, 162f, 169, 17lff, 18lff, 184ff, 189ff, 195ff, 207, 21Of, 215ff, 219ff, 223, 227f, 24Off,244ff Chloritoid, 40, 5lf, 73, 76ff, 107f, 141, 147, 215ff, 218ff, 22Off, 223, 227, 245f,280 Chondrodite, 127 Chrysotile, 154ff, 159ff Clays, metamorphism of, 205ff Clinochlore, 127f, 152 Clinohumite, 127

7 346 Petrogenesis of Metamorphic Rocks C1inoptilolite, 9, 187f, 197. See also Heulandite Clinopyroxene, See Diopside Clinozoisite, See Zoisite Coalification, 2, 13 Contact metamorphism, 2ff, 97ff, 133ff, 252f Cordierite, 40, 77ff, 82f, 85, 91ff, 99, 221ff, 223ff, 229, 23Off, 233ff, 245ff, 250, 252f, 27Off, 284, 286, 316,321,323 Corundum, 142 Crossite, 41, 175ff, 194 Cummingtonite, 40, 161, 172f Diagenesis, 1, 5, 8, 9ff, 188f, 207 Diaphtoresis, 17 Differentiation, metamorphic, 17 Diopside, 40,98, l06f, 113ff, 131, 136f, 152, 163ff, 169ff, 187, 264, 267ff, 276 Dolomite, 40, 112ff, 125ff, 132, 140, 147, 154, 164, 210, 219, 249, 264 Dynamothennal metamorphism. 3ff Eastonite-Siderophyllite, 52 Eclogite, 88, 165, 268, 276ff Enderbitic granolite, 263f Enstatite, 41, 154ff, 161ff Epidote, 40, 7Of, 152, 169, 173, 175ff, 178f, 183, 189, 191ff, 196ff, 212, 219f, 242, 276, 280. See also Zoisite Eutectic, 291 Evaporites, 2 Facies, general, 55ff Fluid phase, 15ff, 19ff, I11ff Forsterite, 113ff, 132, 136f, 155ff, 16Off, 243, 245, 249f, 253 Fugacity, oxygen fugacity, 24ff Gamet, See Almandine Gedrite, 41, 322 Geobarometer, 148,.151,246,248, 25Off, 254, 272 Geothermometer, 114,246, 25Off, 272, 281 Gismondine, 188 Glauconite, 9, 211f Glaucophane, 5, 11, 41, 65, 73, 89f, 175ff, 179, 194f, 198f, 205, 24Of, 276,280 Gmelinite, 188 Gneiss, nomenclature, 341f Grade of metamorphism, 6ff, 64ff, 239ff Granite, 281ff, 283ff, 307, 31Off, 314, 316ff,320,323ff,329ff,333ff Granitization, 285, 325, 329, 33Of, 332f Granoblastite, 258f, 264ff Granoblastic texture, 258f, 261f Granolite, Definition etc., 259ff, 264ff, 280f Granulite, 54, 88, 168, 234, 256ff. See also Granolite Graphite, 19ff, 22f, 141,147,209 Graywacke, metamorphism of, 204f, 268, 309ff, 326ff Greenschist, Greenstone, 170ff Greenschist facies, 64f, 74f Grossularite, 40, 142ff, 152, 193, 220, 248, 264, 266f, 277, 280f Grunerite, 40 Hedenbergite, 40, 276 Hematite, 24f, 2i7 Heulandite, 9, 13, 184ff, 187, 189 Hornblende, 40, 75f, 163, 169ff, 174, 244f, 252, 260, 264ff, 273, 276, 279, 284, 316, 322f, 337 Hornfels, Hornfelsfacies, 2, 57ff, 99, 252, 347 Hydrothermal metamorphism, 2f Hypersthene, 41, 88, 252f, 256ff, 260, 264ff, 272f, 276 Hypersthene zone, 87f, 170,234, 256ff Idocrase, 40, 151f Illite, 9, 11, 13, 72f, 206f, 210 Ilmenite, 322 Isochemical, 16ff Isograd, 66ff Jadeite, jadeitic pyroxene, 5,41,65,69, 73,89f, 176f, 190, 194, 198f, 204f, 24Of, 276f, 281

8 Index 347 Kaolinite, 11, 14Of, 146, 200, 206ff, 210 K feldspar (including alkali feldspar), 43, 53, 83ff, 99, 149f, 206, 211ff, 214ff, 218ff, 23Off, 250, 252f, 260, 265ff, 269ff, 286ff, 290, 309ff, 316 Kyanite, 40, 78, 82ff, 90, 91ff, 147f, 209ff, 218, 227, 246ff, 253, 270, 272, 276f, 279 Larnite, 134ff Laumontite, 2, 5f, I1ff, 40, 65, 89f, 140, 175ff, 180, 182f, 184f, 186ff, 197f, 200, 240ff Lawsonite, 5, I1f, 40, 65, 68ff, 72, 89f, 140, 175ff, 189ff, 191f, 195f, 198, 200, 204f, 240ff Levyne, 188 Lizardite, 154 Mafic rocks, metamorphism of, 168ff Magnesite, ll1ff, 126, 131f, 154ff, 162ff Magnetite, 23f, 154, 166,217,223, 322 Margarite, 40, 69f, 141, 146ff, 150,210, 246ff Mariolite, 40 MarI, metamorphism of, 14Off, 200 Mejonite, 40 Melilite, 133, 136 Merwinite, 133, 136f Mesolite, 188 Metamorphism Beginning of, llf Definition, If, 8 Dynamothermal, 3ff Factors of, 15ff Hydrothermal, 2ff, 184ff Retrograde, 2, 17 Types, Iff, 6f Metamorphic grades. See Grades of metamorphism Metasomatism, 16f, 329, 331f Migmatite, 8, 83, 104, 246, 250, 264, 283ff, 323, 325ff Monticellite, 133, 136 Montmorillonite, 9, 11, 200, 206f, 210 Mordenite, 186, 188 Muscovite, 23, 41, 43f, 76ff, 81, 83ff, 86ff, 99, 147, 149, 172,207,211, 219ff, 223, 227ff, 231, 242f, 245f, 249f, 264, 280 Mylonite, 2 Natrolite, 188 Nontronite, 185 Olivine, See Forsterite Omphacite, 276, 281 Orthopyroxene. See Enstatite, Bronzite, and Hypersthene Paragenesis, general, 28ff Paragonite, 11,41, 43f, 69, 14Of, 147, 151,176,200,207, 21Of, 253, 280 Pelites, metamorphism of, 206ff, 268ff Periclase, 131ff, 136, 155ff Phase Rule, 19,32, 113 Phengite, 11,41,43, 72f, 140,207, 21Off, 219f, 242, 34Of. See also Muscovite Phillipsite, 188 Phlogopite, 41, 52, 116, 127f, 163 Phyllite, nomenclature, 340f Pistacite, 40, 71 Plagioclase, 75f, 83ff, 86, 107f, 143, 147ff, 169ff, 244, 250, 260, 264ff, 277, 286ff, 316, 319ff. See also Albite and Anorthite Prehnite, 11, 40, 65, 68, 72ff, 90, 140, 175, 179ff, 181ff, 189ff, 193, 196f, 200, 204f, 240ff Pressure Directed, 24f Fluid, 18ff Hydrostatic, 18 Load, 18f Overpressure, 18, 25f Solid, 19 Units, 18 Pressure divisions of metamorphic grades, 88ff Pumpellyite, 40, 65, 68ff, 90, 175f, 18lff, 189ff, 196ff, 200f, 204f, 240ff

9 348 Petrogenesis of Metamorphic Rocks Pyrope, 41, 163,220,260,266,277, 280f Pyrophyllite, 11, 40, 73, 14Of, 147, 200f, 206ff, 210, 215ff, 218 Quartz. Too many to be cited here Rankinite, 134ff Reaction-isograd, 66ff Regional metamorphism, 3ff, 102ff Riebeckite, 179, 212 Rutile, 109, 201, 276 Sagvandite, 154 Sanidinite facies, 133ff Sapphirine, 163, 272f Scapolite, 40, 127, 264 Schist, nomenclature, 341 Scolecite, 188 Serpentine, 154ff, 163ff, 24Of, 243, 253 Sillimanite, 40, 83ff, 9Iff, 103ff, 227f, 23Off, 234ff, 250, 253, 265, 270, 272,286,316,32Off Spessartine, 41, 220, 231, 260, 280 Sphene, 169ff, 176ff, 182, 201, 205, 322 Spilite, 197 Spinei, 127, 163 Spurrite, 133ff Staurolite, 4Of, 5If, 76ff, 8If, 99, 103ff, 211,219, 22Iff, 228ff, 247, 249, 253ff Stilbite, 188, 197 Stilpnomelane, 41, 44, 5If, 173f, 176ff, 181,205, 211ff, 219, 242f Tale, 41, lliff, 154ff, 24Of, 250, 253, 254 Thomsonite, 188 Tilleyite, 134ff Tremolite, 40, 98, 107f, 112ff, 152, 163ff,249 Ultramafic rocks, metamorphism of, 154ff Vesuvianite. See Idocrase Wairakite, 89, 181, 183, 184ff, 192, 196f, 200 Wollastonite, 19,29,40,98, 126, 128ff, 134ff, 142ff, 146, 252 Zeolite facies, 5, 184ff, 200 Zoisite, inclusive Orthozoisite and Clinozoisite, 40, 68ff, 74f, 82, 90, 141ff, 145ff, 148ff, 151, 169, 17Iff, 175f, 181, 189ff, 210, 24Off, 247f, 253, 266, 276, 279