Trondhjemite
Light-colored intrusive igneous rock
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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]
| Igneous rock | |
Tonalite-trondhjemite-granodiorite migmatite (left) and trondhjemitic gneiss (right) from northern China | |
| Composition | |
|---|---|
| plagioclase: (albite to andesine), quartz, alkali feldspar |
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
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]
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