Tonalite

Tonalite is an igneous, plutonic (intrusive) rock, of felsic composition, with phaneritic (coarse-grained) texture. Feldspar is present as plagioclase (typically oligoclase or andesine) with alkali feldspar making up less than 10% of the total feldspar content. Quartz (SiO2) is present as more than 20% of the total quartz-alkali feldspar-plagioclase-feldspathoid (QAPF) content of the rock.[1][2] Amphiboles and biotite are common accessory minerals.[3]

A piece of tonalite on red granite gneiss from Tjörn, Sweden
QAPF diagram with tonalite field highlighted

In older references tonalite is sometimes used as a synonym for quartz diorite. However the current IUGS classification defines tonalite as having greater than 20% quartz, while quartz diorite varies its quartz content from 5 to 20%.[1]

The name is derived from the type locality of tonalites, adjacent to the Tonale Line, a major structural lineament and mountain pass, Tonale Pass, in the Italian and Austrian Alps. The name was first applied by Gerhard vom Rath in 1864.[4] The term adamellite was originally applied by A. Cathrein in 1890 to orthoclase-bearing tonalite (likely a granodiorite) at Monte Adamello, Italy, in 1890, but later came to refer to quartz monzonite, and is now a deprecated term.[5]

Trondhjemite is an orthoclase-deficient variety of sodium-rich tonalite with minor biotite as the only mafic mineral, named after Norway's third largest city, Trondheim.[6]

Tonalites, together with granodiorites, are characteristic of calc-alkaline batholiths formed above subduction zones.[7]

References
  1. Le Bas, M. J.; Streckeisen, A. L. (1991). "The IUGS systematics of igneous rocks". Journal of the Geological Society. 148 (5): 825–833. Bibcode:1991JGSoc.148..825L. CiteSeerX 10.1.1.692.4446. doi:10.1144/gsjgs.148.5.0825. S2CID 28548230.
  2. "Rock Classification Scheme - Vol 1 - Igneous" (PDF). British Geological Survey: Rock Classification Scheme. 1: 1–52. 1999.
  3. Allaby, Michael (2013). "tonalite". A dictionary of geology and earth sciences (Fourth ed.). Oxford: Oxford University Press. ISBN 9780199653065.
  4. Jackson, Julia A., ed. (1997). "tonalite". Glossary of geology (Fourth ed.). Alexandria, Virginia: American Geological Institute. ISBN 0922152349.
  5. Streckeisen, A. (1 March 1976). "To each plutonic rock its proper name". Earth-Science Reviews. 12 (1): 1–33. doi:10.1016/0012-8252(76)90052-0.
  6. Jackson 1997, "trondhjemhite".
  7. Castro, Antonio (September 2013). "Tonalite–granodiorite suites as cotectic systems: A review of experimental studies with applications to granitoid petrogenesis". Earth-Science Reviews. 124: 68–95. doi:10.1016/j.earscirev.2013.05.006.
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