Trondhjemite is a granitic rock of unusual composition. It is a variety of tonalite, i.e., a granitoid in which the dominant feldspar is plagioclase, but it is distinguished by a relatively high SiO2 content (expressed in the rock as abundant quartz) and a high Na/Ca ratio which results in sodic plagioclase with an oligoclase composition. Trondhjemites are occasionally found on mid-ocean ridges but are common in Archean terranes where they form part of the tripartite trondhjemite-tonalite-granodiorite assemblage, also known as TTG. This assemblage constitutes most of the granitoid crust of the oldest continents.
Opinions are divided about the origin of these rocks. Some authors attribute them to the melting of hot Archean subducting oceanic crust, while others prefer melting in the lower parts of granite-greenstone belts.