Tonalite

Saturday, June 12, 2010

Tonalite is a plutonic rock, of felsic composition, with phaneritic texture. Feldspar is present as plagioclase (typically oligoclase or andesine) with 10% or less alkali feldspar. Quartz is present as more than 20% of the rock. Amphiboles and pyroxenes are common accessory minerals. Tonalite is sometimes used as a synonym for quartz diorite. Nevetheless the current IUGS classification defines tonalite as having greater than 20% quartz and quartz diorite with from 5 to 20% quartz.

The name tonalite 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.

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