Trondhjemite
| Igneous rock | |
 Tonalite-trondhjemite-granodiorite migmatite (left) and trondhjemitic gneiss (right) from northern China | |
| Composition | |
|---|---|
| plagioclase: (albite to andesine), quartz, alkali feldspar |
Trondhjemites are leucotonalites, a variety of leucocratic tonalite in which the modal mineralogy mostly consists of plagioclase in the form of albite to andesine, >20% quartz, and <10% alkali feldspar.[1][2] Trondhjemites that occur in the oceanic crust or in ophiolites are usually called plagiogranites.[3][4]
Trondhjemites are common in Archean terranes, occurring in conjunction with tonalite and granodiorite as the TTG (tonalite-trondhjemite-granodiorite) orthogneiss suite.[5] Trondhjemite dikes also commonly form part of the sheeted dike complex of an ophiolite.[4]
The rock type was first described by Victor Goldschmidt in 1916. The name of the rock type is derived from the city of Trondheim, Norway.[1] Despite the name and locality, the Trondheim trondhjemites are not considered ideal type specimen because greenschist metamorphism has erased their original mineralogy.[2] Other well-known trondhjemite localities include the Rio Brazos quartz-eye trondhjemite in the Brazos Mountains, New Mexico,[6] the Twilight Gneiss of the Needle Mountains, Colorado, trondhjemites of the Wind River Range, Wyoming, the Trinity Alps of California, and areas near Riggins, Idaho.[2]
Formation
[edit]Generation of trondhjemite-tonalitic magmas are understood be caused by fractionation of wet basaltic and/or dry low-K tholeiitic-andesitic magmas caused by partial melting of quartz eclogites, amphibolites or gabbros.[2]
Notes
[edit]- 1 2 Jackson 1997, "trondjhemite".
- 1 2 3 4 Barker 1979.
- ↑ Jackson 1997, "plagiogranite".
- 1 2 Philpotts & Ague 2009, pp. 367, 372.
- ↑ Philpotts & Ague 2009, p. 398.
- ↑ Barker, Fred; Peterman, Z. E.; Henderson, W. T.; Hildreth, R. E. (1974). "Rubidium-strontium dating of the trondhjemites of Rio Brazos, New Mexico and of the Kroenke Granodiorite, Colorado" (PDF). Journal of Research of the U.S. Geological Survey. 2 (6). U.S. Geological Survey: 705–709.
References
[edit]- Jackson, Julia A., ed. (1997). Glossary of geology (4th ed.). Alexandria, Virginia: American Geological Institute. ISBN 0922152349.
- Philpotts, Anthony R.; Ague, Jay J. (2009). Principles of igneous and metamorphic petrology (2nd ed.). Cambridge, UK: Cambridge University Press. ISBN 9780521880060.
- Barker, Fred (1979). Developments in Petrology: Trondhjemites, Dacites, and Related Rocks. Elsevier. ISBN 0-444-41765-6.
Further reading
[edit]- Best, Myron G. (2002) Igneous and Metamorphic Petrology, Blackwell Publishing, 2nd ed. ISBN 1-4051-0588-7
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