Tonalite is anigneous,plutonic (intrusive)rock, offelsic composition, withphaneritic (coarse-grained) texture.Feldspar is present asplagioclase (typicallyoligoclase orandesine) withalkali 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 andbiotite are common in lesser quantities, while accessoryminerals includeapatite,magnetite andzircon.[3][4]
In older references tonalite is sometimes used as a synonym forquartz diorite. However the currentIUGSclassification defines tonalite as having greater than 20% quartz, whilequartz 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 structurallineament and mountain pass,Tonale Pass, in theItalian andAustrianAlps. The name was first applied byGerhard vom Rath in 1864.[5] The termadamellite was originally applied by A. Cathrein in 1890 toorthoclase-bearing tonalite (likely agranodiorite) at Monte Adamello, Italy, in 1890, but later came to refer toquartz monzonite, and is now a deprecated term.[6]
Trondhjemite is anorthoclase-deficient variety ofsodium-rich tonalite with minorbiotite as the onlymafic mineral, named afterNorway's third largest city,Trondheim.[7]
Tonalites, together withgranodiorites, are characteristic ofcalc-alkalinebatholiths formed abovesubduction zones.[8]
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