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Nomenclature of Common Metamorphic Rocks

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Nomenclature of Common Metamorphic Rocks 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 min­ eralogical composition of the rock. The names of magmatic rocks have to be memorized like words of a foreign language. Fortunately, this dif­ ficulty 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 recommend­ able and is to a large extent adopted here. Names of Important Rock Groups Phyllite. Fine-grained and very finely schistose rock, the platy min­ erals 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.: 163-172 (1962). 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 desig­ nated 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 phyllosil­ icates, 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 feld­ spar 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 col­ umns." 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 inter­ locking boundaries and provide, as compared to schists, a better coher­ ence and a coarser fissility to the rock; nevertheless, the fissility in many cases creates an almost perfect plane" (Wenk, 1963). Some prefer a def­ inition of gneiss based not only on fabric but also on mineralogica~ fea­ tures. Thus Fritsch et al. (1967) advocated the use ofthe term gneiss for 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 gray­ wackes, shales, etc. The particular mineralogical composition is indi­ cated according to the same rule as in the case ofphyllites, e.g., kyanite­ staurolite-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 devel­ oped 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 indi­ cated in the usual manner, e.g., muscovite-biotite marble. Quartzite. A rock composed of more than 80% quartz. The inter­ locking boundaries of the quartz grains impart a great strength to the rock. Metamorphic quartzites must be distinguished from unmetamor­ phosed, 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 metamor­ phic 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 equidi­ mensional 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. 276. Prefixes Meta-. This prefix designates metamorphosed igneous or sedimen­ tary roeks in which the original fabrie still can be reeognized; e.g., meta­ basalts, metagraywackes. Others use the prefix "meta-" in a more gen- 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 distinc­ tion between gneiss and schist or phyllite should not be ignored; there­ fore, 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 assign­ ing a name to a rock. The distinction between gneiss and schist or phyl­ lite 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. 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 tempera­ ture. In higher-grade metamorphic rocks, schists take the place of phyl­ lites, 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
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