Trondhjemite

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.¹² Trondhjemites that occur in the oceanic crust or in ophiolites are usually called plagiogranites.³⁴

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

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