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Geology is the Way

Tonalite

Tonalite is a type of plutonic igneous rock containing quartz (20 – 60% of the felsic minerals) and abundant plagioclase. K-feldspar is rare or absent in a tonalite and, when present, it constitutes less than 10% of the feldspars. The composition of plagioclase typically ranges from oligoclase to andesine. Tonalites may contain several mafic minerals, like hornblende, biotite, and/or pyroxene, typically constituting 10 to 40% of the rock.

The name ‘tonalite’ derives from the Tonale Pass in the Adamello Group (Alto Adige, Italy). Trondhjemite and plagiogranite are two synonym terms that can be used for light-colored varieties (less than 10% of mafic minerals) of tonalite. The term ‘trondhjemite’ derives from the Norwegian town of Trondhjem, now Trondheim.

tonalite

Tonalite with white plagioclase, transparent quartz, and black mafic minerals. Most of the mafic minerals consist of hornblende (prismatic), but some biotite (metallic) is also present. Photo Khruner.

tonalite

Tonalite from Adamello, Italy. The white grains consist of plagioclase. Quartz is grey to transparent. The black crystals are amphiboles (hornblende). Photo Parks.it.

Tonalite
Plutonic igneous rock
Felsic minerals:
quartz
plagioclase
alkali feldspar
Mafic minerals:
biotite
hornblende
pyroxene

QAPF classification:
Q = 20 – 60%
Plagioclase/feldspars > 90%
Colored varieties:
• trondhjemite/plagiogranite (M < 10%)
• melatonalite (M > 40%)
Extrusive equivalent: dacite

tonalite in diorite

Tonalite (light-colored, center) intruded in diorite (the darker rocks on the left and the right). Precambrian. Confusion Hill, St. Louis County, northeastern Minnesota, USA. Photo by James St. John.

Trondhjemite

Trondhjemite, as shown above, is a very light-colored variety of tonalite. Sample from Norway. Photo Halvard.

References
Drummond, M. S., Defant, M. J., & Kepezhinskas, P. K. (1996). Petrogenesis of slab-derived trondhjemite–tonalite–dacite/adakite magmas. Earth and Environmental Science Transactions of the Royal Society of Edinburgh87(1-2), 205-215.
Glikson, A. Y. (1979). Early Precambrian tonalite-trondhjemite sialic nuclei. Earth-Science Reviews15(1), 1-73.
Martin, H., Smithies, R. H., Rapp, R., Moyen, J. F., & Champion, D. (2005). An overview of adakite, tonalite–trondhjemite–granodiorite (TTG), and sanukitoid: relationships and some implications for crustal evolution. Lithos79(1-2), 1-24.
Smithies, R. H. (2000). The Archaean tonalite–trondhjemite–granodiorite (TTG) series is not an analogue of Cenozoic adakite. Earth and Planetary Science Letters182(1), 115-125.

        

See also
Tonalite – Alexstrekeisen.it

it_IT Italiano
Igneous Minerals
Igneous Textures
Plutonic Rocks
Igneous Bodies

 

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