Trondhjemite is a sodium-rich, intrusive igneous rock belonging to the granitoid family. It is classified as a type of tonalite with a high content of sodic plagioclase and minimal or absent potassium feldspar. Trondhjemite is often associated with Archean and Proterozoic terrains and is significant in studying early continental crust formation.
Composition and Mineralogy[]
Trondhjemite is primarily composed of:
- Plagioclase (Na-rich, mainly oligoclase)
- Quartz
- Minor amounts of biotite, amphibole, or pyroxene
- Minimal or absent K-feldspar
- Accessory minerals such as zircon, apatite, titanite, and magnetite
The rock is typically light in colour due to its high quartz and feldspar content.
Classification and Distinction[]
Trondhjemite is a tonalite-trondhjemite-granodiorite (TTG) suite member and is distinguished from other granitoid rocks by its low potassium content. It differs from:
- Tonalite, which may contain more calcic plagioclase and biotite.
- Granodiorite, which contains a higher proportion of potassium feldspar and biotite.
- Granite which is richer in K-feldspar and has a more varied mineral composition.
In the QAPF classification system, trondhjemite is a feldspar-dominated granitoid where plagioclase constitutes at least 90% of the total feldspar, predominantly sodic in composition.[1]
Geological Significance and Formation[]
Trondhjemites provide insights into early Earth's crust formation processes and are commonly found in Archean cratons. Their formation is associated with:
- Partial melting of hydrated basaltic crust at high pressures.
- Magmatic differentiation in subduction zones.
- Accretionary processes in Archean greenstone belts.
Modern trondhjemites are linked to subduction-related magmatism and may serve as analogs for early continental crust generation.[2]
Occurrence and Distribution[]
Trondhjemites are found in several geological settings, including:
- Archean cratons like the Canadian Shield, Greenland, and Western Australia.
- Proterozoic orogenic belts, where they occur as part of ancient mountain-building processes.
- Active subduction zones, where similar rock types are occasionally observed.
The rock is named after Trondheim, Norway, where it was first described.[3]
Economic Importance[]
Although not a primary ore-bearing rock, trondhjemites can be associated with:
- Gold mineralization, particularly in Archean TTG terranes.
- Rare earth element (REE) deposits, where accessory minerals such as zircon and apatite contain economically valuable elements.[4]
Petrogenetic Significance[]
Trondhjemites play a crucial role in understanding:
- Crustal differentiation and evolution in the early Earth.
- Petrogenesis of Archean terrains.
- Geodynamic settings of ancient subduction and oceanic crust melting.[5]
See Also[]
- Tonalite
- Granodiorite
- TTG Suite
- Archean Cratons
- Subduction Zones
References[]
- ↑ Frost, B. R., & Frost, C. D. (2011). Essentials of Igneous and Metamorphic Petrology. Cambridge University Press.
- ↑ Moyen, J.-F. (2011). "The Archean TTG Record: A Review". Earth-Science Reviews, 106(1-2), 1-37.
- ↑ Barker, F. (1979). "Trondhjemites, Dacites, and Related Rocks." Elsevier.
- ↑ Condie, K. C. (1993). "Plate Tectonics and Crustal Evolution." Pergamon Press.
- ↑ Rollinson, H. R. (1993). "Using Geochemical Data: Evaluation, Presentation, Interpretation." Longman Scientific & Technical.