Glossary of terms

folded gneiss, Shetland. Photo, D Fettes

The SCMR has established a glossary of about 1200 terms dealing with metamorphic rocks.

The glossary contains, in alphabetical order, definitions, etymology, type localities and literature references for a comprehensive list of terms related to metamorphic rocks and their structures, including fault rocks and the whole field of impactites.

In the glossary terms are defined under the following categories:

  • Recommended terms are those recommended for use in international papers and books. When used without a further qualification it is assumed that they refer to the definition given in the SCMR glossary.
  • Restricted terms are those not recommended for international use, or if used, such terms have explicitly to be defined, either by referring to the corresponding definition given in the SCMR glossary, or by citing the author's own definition. The SCMR recommends the replacement of restricted terms by recommended terms whenever practical.
  • Unnecessary terms are those not recommended for further use and which should be replaced by recommended rock names.

The glossary has been published in hardcopy along with the specialist recommendations (see home page)

Examples of glossary entries

Barrovian-type facies series:(in honour of G Barrow , 1853–1932, who first described and mapped series in 1893 and 1912, using mineral zones; Grampian Highlands of Scotland, Dalradian supergroup, UK) . Type of facies series characterised by the progressive development of garnet, kyanite from lower to higher grades; andalusite, cordierite and glaucophane are absent. The facies represents medium P/T conditions (see Smulikowski et al., Fig. 3, this vol.) and is common in regional metamorphism . It is equivalent to the medium-P/T type of regional metamorphism of Miyashiro (see baric types of metamorphism ). See also Barrow's zones . (Read, 1952, p.278; Fyfe et al., 1958, p.228; Miyashiro, 1961, p.278; Hietanen, 1967, p.195; Turner, 1981, p.375).

Epizone: (Greek epi , upon and zônê , zone). Zone of low-grade metamorphic rocks characterised by illite Kübler index (KI) mean values less than 0.25 Δº2Θ CuK α. The term was originally proposed by Becke and by Grubenmann to indicate a shallow depth of metamorphism (including contact metamorphism for Grubenmann); as such it was synonymous with epimetamorphic zone . This latter usage is now regarded as unnecessary and at present epizone is mainly used in the context of illite Kübler index investigations. Cf. epimetamorphism . (Grubenmann 1904, p.57;Grubenmann & Niggli 1924, p.374, 397; Lindgren 1933, p.95; Kübler 1967, p.111; Kübler 1968, p.393; Kübler 1984, p.578)

Impact pseudotachylite: (Greek pseudês , false, tachys ,quick, and lithos , stone). Pseudotachylite produced by impact metamorphism . Dyke -like breccia formed by frictional melting in the basement of impact craters , resulting often in irregular vein-like networks. Typically, it contains unshocked and shocked mineral and lithic clasts in a fine-grained aphanitic matrix. See also melt vein . (Shand, 1916, p.198; Dence, 1971, p.5555; Stöffler et al., 1988, p.289, Fig.8, Table2; Reimold, 1995, p.247; Spray, 1998, p.195; Dressler & Reimold, 2004, p.2–36).

P-T- t path: Changing pressure-temperature conditions experienced by a mineral or a rock with time, or a line-curve on a P-T grid showing these changes. (OU; Yardley, 1989, p. 198; Spear, 1993, p.2; Miyashiro, 1994, p. 352).

Pressure shadow: (German Streckungshof ). Area of low strain in a deformed rock that was protected from the maximum compressive stress by its proximity to a rigid body, either a porphyroblast or a porphyroclast . The area of low strain may preserve pre-existing structures or space may be created in which new minerals may grow. The growth of new minerals, typically quartz or calcite, in the low-strain area may resemble a beard particularly if the minerals have a fibrous habit, hence bearded structure . Syn. strain shadow. (Mügge, 1930, p.475; Spry, 1969, p.240; Ramsay & Huber, 1983, p.279; Vernon, 2004, p.477).

Schollen: (German Scholle , clod, flake). In a migmatite , blocks or rafts of paleosome within the neosome ; the structure is similar to agmatite but the neosome is more abundant so that the disrupted blocks float like rafts. (OU; Mehnert, 1968, p.15; Ashworth, 1985, p.3).

Solvus: (Latin solvere , to dissolve). Curved P-T-X line or surface that separates the field of homogeneous solid solution from the field of limited mutual solid solution; Cf. liquidus. (Edgar, 1974, p.21; Bucher & Frey, 1994, p.122).

Tektite: (Greek têktos , molten). Impact glass formed at terrestrial impact craters from melt ejected ballistically and deposited as aerodynamically shaped bodies in a strewn field outside the continuous ejecta blanket . The size of tektites ranges from the submillimetre range ( microtektites , generally found in deep-sea sediments) to the subdecimetre range, rarely to decimetres. (Suess, 1900, p.191; Spencer, 1933a, p.117; Park, 1989b, p.554; Glass, 1990, p.393).